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Cytochrome c nitrite reductase from the bacterium Geobacter lovleyi represents a new NrfA subclass

Open AccessPublished:June 09, 2020DOI:https://doi.org/10.1074/jbc.RA120.013981
      Cytochrome c nitrite reductase (NrfA) catalyzes the reduction of nitrite to ammonium in the dissimilatory nitrate reduction to ammonium (DNRA) pathway, a process that competes with denitrification, conserves nitrogen, and minimizes nutrient loss in soils. The environmental bacterium Geobacter lovleyi has recently been recognized as a key driver of DNRA in nature, but its enzymatic pathway is still uncharacterized. To address this limitation, here we overexpressed, purified, and characterized G. lovleyi NrfA. We observed that the enzyme crystallizes as a dimer but remains monomeric in solution. Importantly, its crystal structure at 2.55-Å resolution revealed the presence of an arginine residue in the region otherwise occupied by calcium in canonical NrfA enzymes. The presence of EDTA did not affect the activity of G. lovleyi NrfA, and site-directed mutagenesis of this arginine reduced enzymatic activity to <3% of the WT levels. Phylogenetic analysis revealed four separate emergences of Arg-containing NrfA enzymes. Thus, the Ca2+-independent, Arg-containing NrfA from G. lovleyi represents a new subclass of cytochrome c nitrite reductase. Most genera from the exclusive clades of Arg-containing NrfA proteins are also represented in clades containing Ca2+-dependent enzymes, suggesting convergent evolution.
      The global nitrogen cycle is an indispensable biogeochemical process, as nitrogen is a fundamental component of every living organism (
      • Lehnert N.
      • Dong H.T.
      • Harland J.B.
      • Hunt A.P.
      • White C.J.
      Reversing nitrogen fixation.
      ). One of its critical pathways is the dissimilatory nitrate reduction to ammonium (DNRA), a respiratory process that involves the reduction of nitrate to nitrite followed by the conversion of nitrite to ammonium (
      • Tiedje J.M.
      Ecology of denitrification and dissimilatory nitrate reduction to ammonium.
      ). In contrast to denitrification, the DNRA process does not release nitrogen as gaseous compounds (e.g. N2 and the greenhouse gas N2O), thereby conserving nitrogen in the ecosystem and minimizing nutrient loss (
      • Fernandes S.O.
      • Bonin P.C.
      • Michotey V.D.
      • Garcia N.
      • Lokabharathi P.A.
      Nitrogen-limited mangrove ecosystems conserve N through dissimilatory nitrate reduction to ammonium.
      ,
      • Huygens D.
      • Rütting T.
      • Boeckx P.
      • Van Cleemput O.
      • Godoy R.
      • Müller C.
      Soil nitrogen conservation mechanisms in a pristine south Chilean Nothofagus forest ecosystem.
      ,
      • Putz M.
      • Schleusner P.
      • Rütting T.
      • Hallin S.
      Relative abundance of denitrifying and DNRA bacteria and their activity determine nitrogen retention or loss in agricultural soil.
      ). The second step in the DNRA pathway, the conversion of nitrite to ammonium, is a six-electron, eight-proton process catalyzed by cytochrome c nitrite reductase, also known as NrfA (
      • Simon J.
      Enzymology and bioenergetics of respiratory nitrite ammonification.
      ,
      • Einsle O.
      • Messerschmidt A.
      • Huber R.
      • Kroneck P.M.
      • Neese F.
      Mechanism of the six-electron reduction of nitrite to ammonia by cytochrome c nitrite reductase.
      ). In all available crystal structures to date (
      • Einsle O.
      • Messerschmidt A.
      • Stach P.
      • Bourenkov G.P.
      • Bartunik H.D.
      • Huber R.
      • Kroneck P.M.
      Structure of cytochrome c nitrite reductase.
      ,
      • Einsle O.
      • Stach P.
      • Messerschmidt A.
      • Simon J.
      • Kröger A.
      • Huber R.
      • Kroneck P.M.
      Cytochrome c nitrite reductase from Wolinella succinogenes: structure at 1.6 Å resolution, inhibitor binding, and heme-packing motifs.
      ,
      • Bamford V.A.
      • Angove H.C.
      • Seward H.E.
      • Thomson A.J.
      • Cole J.A.
      • Butt J.N.
      • Hemmings A.M.
      • Richardson D.J.
      Structure and spectroscopy of the periplasmic cytochrome c nitrite reductase from Escherichia coli.
      ,
      • Youngblut M.
      • Judd E.T.
      • Srajer V.
      • Sayyed B.
      • Goelzer T.
      • Elliott S.J.
      • Schmidt M.
      • Pacheco A.A.
      Laue crystal structure of Shewanella oneidensis cytochrome c nitrite reductase from a high-yield expression system.
      ,
      • Rodrigues M.L.
      • Oliveira T.F.
      • Pereira I.A.
      • Archer M.
      X-ray structure of the membrane-bound cytochrome c quinol dehydrogenase NrfH reveals novel haem coordination.
      ,
      • Cunha C.A.
      • Macieira S.
      • Dias J.M.
      • Almeida G.
      • Goncalves L.L.
      • Costa C.
      • Lampreia J.
      • Huber R.
      • Moura J.J.
      • Moura I.
      • Romão M.J.
      Cytochrome c nitrite reductase from Desulfovibrio desulfuricans ATCC 27774. The relevance of the two calcium sites in the structure of the catalytic subunit (NrfA).
      ), the enzyme is a dimer and contains five heme c cofactors per monomer with a Ca2+ ion located at the distal side of the heme-1 active site. Although the exact role of the calcium ion is uncertain, it is proposed to be involved in proton delivery (
      • Bykov D.
      • Neese F.
      Six-electron reduction of nitrite to ammonia by cytochrome c nitrite reductase: insights from density functional theory studies.
      ) and structural stability (
      • Cunha C.A.
      • Macieira S.
      • Dias J.M.
      • Almeida G.
      • Goncalves L.L.
      • Costa C.
      • Lampreia J.
      • Huber R.
      • Moura J.J.
      • Moura I.
      • Romão M.J.
      Cytochrome c nitrite reductase from Desulfovibrio desulfuricans ATCC 27774. The relevance of the two calcium sites in the structure of the catalytic subunit (NrfA).
      ), and/or to contribute to a positive electrostatic potential that is essential for substrate delivery to the active site (
      • Cunha C.A.
      • Macieira S.
      • Dias J.M.
      • Almeida G.
      • Goncalves L.L.
      • Costa C.
      • Lampreia J.
      • Huber R.
      • Moura J.J.
      • Moura I.
      • Romão M.J.
      Cytochrome c nitrite reductase from Desulfovibrio desulfuricans ATCC 27774. The relevance of the two calcium sites in the structure of the catalytic subunit (NrfA).
      ,
      • Bykov D.
      • Neese F.
      Six-electron reduction of nitrite to ammonia by cytochrome c nitrite reductase: insights from density functional theory studies.
      ).
      Geobacter lovleyi has emerged as a model representative of environmental bacteria that drives DNRA in nature (
      • Reguera G.
      • Kashefi K.
      The electrifying physiology of Geobacter bacteria, 30 years on.
      ). This organism has been shown to dominate a microbial community in an enrichment culture grown under limiting nitrate (
      • Van Den Berg E.M.
      • Van Dongen U.
      • Abbas B.
      • Van Loosdrecht M.C.
      Enrichment of DNRA bacteria in a continuous culture.
      ,
      • van den Berg E.M.
      • Rombouts J.L.
      • Kuenen J.G.
      • Kleerebezem R.
      • van Loosdrecht M.C.
      Role of nitrite in the competition between denitrification and DNRA in a chemostat enrichment culture.
      ,
      • van den Berg E.M.
      • Elisário M.P.
      • Kuenen J.G.
      • Kleerebezem R.
      • van Loosdrecht M.C.
      Fermentative bacteria influence the competition between denitrifiers and DNRA bacteria.
      ), a condition that enhances DNRA over denitrification. The switch from nitrate to nitrite as the electron acceptor in the enrichment culture did not change the dominant ribotype, suggesting that the DNRA pathway of G. lovleyi can efficiently use both substrates (
      • van den Berg E.M.
      • Rombouts J.L.
      • Kuenen J.G.
      • Kleerebezem R.
      • van Loosdrecht M.C.
      Role of nitrite in the competition between denitrification and DNRA in a chemostat enrichment culture.
      ). This physiology was unexpected, given that Geobacter species are generally known to be reducers of iron oxide minerals (
      • Reguera G.
      • Kashefi K.
      The electrifying physiology of Geobacter bacteria, 30 years on.
      ).
      In this paper, we report a NrfA subclass from G. lovleyi SZ. The isolated enzyme shares 31% or lower identity to the previously crystallized NrfAs. In contrast to the monomer-dimer equilibrium displayed by its homolog from Escherichia coli (
      • Clarke T.A.
      • Cole J.A.
      • Richardson D.J.
      • Hemmings A.M.
      The crystal structure of the pentahaem c-type cytochrome NrfB and characterization of its solution-state interaction with the pentahaem nitrite reductase NrfA.
      ), the G. lovleyi NrfA remains a monomer in solution, enabling homogeneous samples for future mechanistic investigations. Notably, the crystal structure of G. lovleyi NrfA does not contain a calcium ion near the heme-1 active site, as in canonical NrfA enzymes. Instead, the enzyme contains an arginine in the region occupied by calcium in other crystallized homologs. Phylogenetic analysis and structure comparison provide insights into the function of the calcium ion in Ca2+-dependent NrfA homologs and adaptive responses of some environmental DNRA bacteria for Ca2+-independent nitrite reduction.

      Results

      G. lovleyi NrfA was heterologously overexpressed in S. oneidensis

      We explored several strategies to overexpress G. lovleyi NrfA, ultimately selecting S. oneidensis MR-1 as the most efficient heterologous host. S. oneidensis expresses c-type heme proteins under both aerobic and anaerobic conditions (
      • Gao H.
      • Barua S.
      • Liang Y.
      • Wu L.
      • Dong Y.
      • Reed S.
      • Chen J.
      • Culley D.
      • Kennedy D.
      • Yang Y.
      • He Z.
      • Nealson K.H.
      • Fredrickson J.K.
      • Tiedje J.M.
      • Romine M.
      • et al.
      Impacts of Shewanella oneidensis c-type cytochromes on aerobic and anaerobic respiration.
      ), and strategies for overexpressing these proteins are well developed (
      • Shi L.
      • Lin J.T.
      • Markillie L.M.
      • Squier T.C.
      • Hooker B.S.
      Overexpression of multi-heme c-type cytochromes.
      ). Purification by Strep-tag II affinity chromatography followed by size-exclusion chromatography yielded 6–8 mg of highly purified WT NrfA per liter of culture, as indicated by denaturing gel electrophoresis (Fig. S1). The mass of the parent ion deduced from MS (electrospray ionization) was 56,196 Da, consistent with the mass of the polypeptide chain plus the five heme cofactors but no calcium ion. The metal content was further confirmed by inductively coupled plasma-optical emission spectroscopy (ICP-OES) (Table S1). The UV-Vis and EPR spectra (Fig. S2 and S3) are consistent with previously characterized NrfA enzymes (
      • Bamford V.A.
      • Angove H.C.
      • Seward H.E.
      • Thomson A.J.
      • Cole J.A.
      • Butt J.N.
      • Hemmings A.M.
      • Richardson D.J.
      Structure and spectroscopy of the periplasmic cytochrome c nitrite reductase from Escherichia coli.
      ,
      • Ali M.
      • Stein N.
      • Mao Y.
      • Shahid S.
      • Schmidt M.
      • Bennett B.
      • Pacheco A.A.
      Trapping of a putative intermediate in the cytochrome c nitrite reductase (ccNiR)-catalyzed reduction of nitrite: implications for the ccNiR reaction mechanism.
      ,
      • Pereira I.A.C.
      • Legall J.
      • Xavier A.V.
      • Teixeira M.
      Characterization of a heme c nitrite reductase from a non-ammonifying microorganism, Desulfovibrio vulgaris Hildenborough.
      ,
      • Pereira I.C.
      • Abreu I.A.
      • Xavier A.V.
      • LeGall J.
      • Teixeira M.
      Nitrite reductase from Desulfovibrio desulfuricans (ATCC 27774)–a heterooligomer heme protein with sulfite reductase activity.
      ).

      Dynamic light scattering indicated that G. lovleyi NrfA remains a monomer in solution

      Characterization of G. lovleyi NrfA by size-exclusion chromatography suggested that the enzyme is a monomer in solution (Fig. S4), similar to what was observed previously for some NrfA homologs (
      • Liu M.C.
      • Bakel B.W.
      • Liu M.Y.
      • Dao T.N.
      Purification of Vibrio fischeri nitrite reductase and its characterization as a hexaheme c-type cytochrome.
      ,
      • Kajie S.
      • Anraku Y.
      Purification of a hexaheme cytochrome c552 from Escherichia coli K 12 and its properties as a nitrite reductase.
      ,
      • Schumacher W.
      • Kroneck P.M.H.
      Dissimilatory hexaheme c nitrite reductase of “Spirillum” strain 5175: purification and properties.
      ,
      • Schumacher W.
      • Hole U.
      • Kroneck P.M.
      Ammonia-forming cytochrome c nitrite reductase from Sulfurospirillum deleyianum is a tetraheme protein: new aspects of the molecular composition and spectroscopic properties.
      ). A complicating factor with this approach, however, is the dilution that naturally occurs during chromatography, making it challenging to assess the dependence of the oligomerization state on enzyme concentration. To bypass this limitation, we used dynamic light scattering (DLS) to measure the hydrodynamic radius (RH) of the enzyme in solution at the concentration of interest (
      • Nobbmann U.
      • Connah M.
      • Fish B.
      • Varley P.
      • Gee C.
      • Mulot S.
      • Chen J.
      • Zhou L.
      • Lu Y.
      • Shen F.
      • Yi J.
      • Harding S.E.
      Dynamic light scattering as a relative tool for assessing the molecular integrity and stability of monoclonal antibodies.
      ,
      • Borgstahl G.E.
      How to use dynamic light scattering to improve the likelihood of growing macromolecular crystals.
      ). The hydrodynamic radius measured using this technique reflects the average contribution from each oligomer. For G. lovleyi NrfA, the theoretical monomer and dimer RH is 3.3 nm and 4.5 nm, respectively. These calculated radii are in close agreement with those estimated for globular proteins of similar size (
      • Stetefeld J.
      • McKenna S.A.
      • Patel T.R.
      Dynamic light scattering: a practical guide and applications in biomedical sciences.
      ). The consistency of the radius measured at increasing concentrations (Table 1) indicated that G. lovleyi NrfA remains monomeric in solution even at very high concentrations. This result was consistent with the polydispersity index measured for each sample. A sample containing only a single species is expected to have a polydispersity index of less than 15% (
      • Borgstahl G.E.
      How to use dynamic light scattering to improve the likelihood of growing macromolecular crystals.
      ).
      Table 1Data from dynamic light scattering
      Concentration (µm)RH (nm)Polydispersity index (%)
      243.411.9
      523.610.9
      1053.411.9
      1653.411.9
      1753.412.5
      2663.313.6
      3273.14.3

      The crystal structure of G. lovleyi NrfA revealed key structural differences

      Although G. lovleyi NrfA is a monomer in solution, it crystallized as a dimer, consistent with other structurally characterized NrfA homologs. Alignment of the G. lovleyi NrfA crystal structure with that from Sulfurospirillum deleyianum (PDB entry 1QDB), its closest structurally characterized homolog (Fig. S5), revealed a C-α root mean square deviation (RMSD) of 2.67 (PyMOL, Schrödinger, Inc.). The crystal, which resolved at 2.55 Å, belongs to the primitive monoclinic space group P212121 with unit cell a = 110.9, b = 144.6, and c = 234.9 (Table 2). The dimer interface is dominated by three α-helices from each monomer, and it is primarily stabilized by a strong electrostatic contribution from two salt bridges (Fig. 1A) as well as H-bonding interactions. The structure revealed four bis-His coordinated hemes as well as a Lys-ligated heme (heme-1) that constitutes the active site. The hemes from this structure aligned well with the hemes from S. deleyianum NrfA (Fig. 1B). The substrate binding site at the distal side of the active-site heme-1 is occupied by sulfate (efforts to obtain high-quality crystals with nitrite bound were unsuccessful), which is stabilized by the active-site residues Tyr-221, His-278, and Arg-122, as well as a water molecule (Fig. 2A). These active-site residues are strictly conserved in all NrfA enzymes characterized to date.
      Table 2Crystallographic statistics
      ParameterValue(s) for G. lovleyi NrfA-1
      Data collection
       BeamlineLS-CAT (21-ID-D)
       Wavelength (Å)1.72
       Space groupP212121
       Unit cell
           a, b, c (Å)110.9 144.6 234.9
           α, β, γ ()90.0, 90.0, 90.0
       Resolution
      Highest resolution shell is shown in parentheses.
      (Å)
      29.36–2.55 (2.64–2.55)
       Redundancy
      Highest resolution shell is shown in parentheses.
      5.5 (5.3)
       Completeness
      Highest resolution shell is shown in parentheses.
      (%)
      98.9 (98.8)
      II
      Highest resolution shell is shown in parentheses.
      4.8 (1.8)
      Rmerge
      Highest resolution shell is shown in parentheses.
      ,
      Rmerge=∑hkl∑j|Ij(hkl)−<I(hkl)>|/∑hkl∑jIj(hkl), where I is the intensity of reflection.
      0.20 (1.66)
      Rpim
      Highest resolution shell is shown in parentheses.
      ,
      Rpim=∑hkl[1/(N−1)]1/2∑j|Ij(hkl)−<I(hkl)>|/∑hkl∑jIj/(hkl), where N is the redundancy of the dataset.
      0.11 (0.77)
       CC1/20.36
      Average CC1/2 was not reported by the version of HKL2000 used in this work.
      Refinement
       No. of unique reflections122,238
       No. of atoms23,376
       Protein (n)21,161
       Heme (n)1290
       Sulfate (n)30
      Rwork/Rfree
      Rwork=∑hkl||Fobs|−|Fcalc||/∑hkl|Fobs|, where Fobs and Fcalc is the observed and the calculated structure factor, respectively. Rfree is the cross-validation R factor for the test set of reflections (5% of the total) omitted from model refinement.
      0.19/0.24
       Wilson B-factor (Å2)34.2
       B-factor (Å2)
           Protein39.7
           Heme31.6
           Sulfate35.7
           Solvent33.9
       RMSD
           Bond length (Å)0.010
           Bond angle ()1.21
       Ramachandran plot (%)
           Favored96.0
           Allowed3.5
           Outliers0.5
      a Highest resolution shell is shown in parentheses.
      b Rmerge=hklj|Ij(hkl)<I(hkl)>|/hkljIj(hkl), where I is the intensity of reflection.
      c Rpim=hkl[1/(N1)]1/2j|Ij(hkl)<I(hkl)>|/hkljIj/(hkl), where N is the redundancy of the dataset.
      d Average CC1/2 was not reported by the version of HKL2000 used in this work.
      e Rwork=hkl||Fobs||Fcalc||/hkl|Fobs|, where Fobs and Fcalc is the observed and the calculated structure factor, respectively. Rfree is the cross-validation R factor for the test set of reflections (5% of the total) omitted from model refinement.
      Figure thumbnail gr1
      Figure 1A, crystal structure of G. lovleyi NrfA dimer shown with its hemes and salt bridges (positively and negatively charged residues in blue and red, respectively) at the dimer interface. B, alignment of heme groups from S. deleyianum NrfA (PDB entry 1QDB) and G. lovleyi. The orientation of the heme group is the same as in panel A but without the protein backbone.
      Figure thumbnail gr2
      Figure 2G. lovleyi (A) and S. deleyianum (PDB entry ) (B) NrfA active sites. A red sphere represents a water molecule, and a yellow dash represents either an H-bond or a salt bridge. The G. lovleyi NrfA active-site difference density map is provided in .
      A key structural difference between G. lovleyi NrfA and the previously crystallized homologs can be found at the active-site region. Specifically, no calcium was found near the heme-1 active site in G. lovleyi NrfA. Instead, the active site contains an arginine residue (Arg-277) in the region that would otherwise be occupied by a calcium ion in other NrfA homologs (Fig. 2). Arg-277 forms a salt bridge with Glu-263 as well as H-bonds to both the side chain and amide oxygen atoms of the active-site tyrosine (Tyr-221). In addition, there is a π-cation interaction between Arg-277 and Tyr-221. In the Ca2+-containing homologs, the amide oxygen of this same tyrosine is ligated to the calcium (Fig. 2B). In contrast to the presence of two well-ordered H2O molecules coordinated to the calcium ion in the Ca2+-containing homologs, the presence of a water molecule in the vicinity of Arg-277 in G. lovleyi NrfA could not be definitively ascertained (i.e. only two out of six monomers revealed a well-ordered water molecule H-bonded to Arg-277). Interestingly, the positive and negative charges at both the Ca2+ site and the Arg site are more or less balanced. To achieve this, the calcium is coordinated by Glu-216, its two H2O ligands are H-bonded to Asp-267 (Fig. 2B), and its lysine ligand is H-bonded to Asp-400 (not shown). In G. lovleyi NrfA, however, charge balance is achieved by Arg-277 forming a salt bridge to Glu-263 (Fig. 2A). It is important to note that except for Desulfovibrio desulfuricans, all NrfA enzymes crystallographically characterized thus far, including G. lovleyi NrfA, were purified and crystallized without the addition of calcium to any of the buffers (
      • Einsle O.
      • Messerschmidt A.
      • Stach P.
      • Bourenkov G.P.
      • Bartunik H.D.
      • Huber R.
      • Kroneck P.M.
      Structure of cytochrome c nitrite reductase.
      ,
      • Einsle O.
      • Stach P.
      • Messerschmidt A.
      • Simon J.
      • Kröger A.
      • Huber R.
      • Kroneck P.M.
      Cytochrome c nitrite reductase from Wolinella succinogenes: structure at 1.6 Å resolution, inhibitor binding, and heme-packing motifs.
      ,
      • Bamford V.A.
      • Angove H.C.
      • Seward H.E.
      • Thomson A.J.
      • Cole J.A.
      • Butt J.N.
      • Hemmings A.M.
      • Richardson D.J.
      Structure and spectroscopy of the periplasmic cytochrome c nitrite reductase from Escherichia coli.
      ,
      • Youngblut M.
      • Judd E.T.
      • Srajer V.
      • Sayyed B.
      • Goelzer T.
      • Elliott S.J.
      • Schmidt M.
      • Pacheco A.A.
      Laue crystal structure of Shewanella oneidensis cytochrome c nitrite reductase from a high-yield expression system.
      ,
      • Rodrigues M.L.
      • Oliveira T.F.
      • Pereira I.A.
      • Archer M.
      X-ray structure of the membrane-bound cytochrome c quinol dehydrogenase NrfH reveals novel haem coordination.
      ,
      • Cunha C.A.
      • Macieira S.
      • Dias J.M.
      • Almeida G.
      • Goncalves L.L.
      • Costa C.
      • Lampreia J.
      • Huber R.
      • Moura J.J.
      • Moura I.
      • Romão M.J.
      Cytochrome c nitrite reductase from Desulfovibrio desulfuricans ATCC 27774. The relevance of the two calcium sites in the structure of the catalytic subunit (NrfA).
      ). In addition, our attempts to crystallize G. lovleyi NrfA in the presence of calcium yielded poorly diffracting crystals.
      Using CAVER software (
      • Chovancova E.
      • Pavelka A.
      • Benes P.
      • Strnad O.
      • Brezovsky J.
      • Kozlikova B.
      • Gora A.
      • Sustr V.
      • Klvana M.
      • Medek P.
      • Biedermannova L.
      • Sochor J.
      • Damborsky J.
      CAVER 3.0: a tool for the analysis of transport pathways in dynamic protein structures.
      ), we identified a tunnel connecting the putative substrate and product channels (Fig. 3). The substrate channel in G. lovleyi NrfA is in a location similar to that in the S. deleyianum NrfA protein (
      • Einsle O.
      • Messerschmidt A.
      • Stach P.
      • Bourenkov G.P.
      • Bartunik H.D.
      • Huber R.
      • Kroneck P.M.
      Structure of cytochrome c nitrite reductase.
      ), and it is highly cationic because of the presence of several positively charged amino acid residues in the vicinity, i.e. Lys-124, Lys-225, Arg-277, and Arg-122. Whereas Arg-122 is strictly conserved in all NrfA proteins and Arg-277 is strictly conserved in Arg-containing NrfA proteins, Lys-124 and Lys-225 are highly conserved in Arg-containing but not in Ca2+-containing NrfA homologs. Conversely, the product channel in G. lovleyi NrfA diverges from that of S. deleyianum. In S. deleyianum, the product channel hovers above heme-4 while in G. lovleyi it is near heme-3. Furthermore, the G. lovleyi NrfA product channel is surrounded with negatively charged carboxylates from heme-1 and heme-3 as well as from amino acid residues Glu-101 and Asp-114. These residues guide the exit of the positively charged ammonium ion.
      Figure thumbnail gr3
      Figure 3G. lovleyi NrfA monomer shown with a putative channel for substrate entry and product exit.

      G. lovleyi NrfA displayed a kinetic behavior similar to that of previously characterized homologs

      In NrfA assays, dithionite-reduced methyl viologen was used as an artificial electron donor to drive the enzymatic reduction of nitrite. The disappearance of the intense blue color from the reduced methyl viologen was monitored to measure the activity of the enzyme. Despite lacking the calcium ion, G. lovleyi NrfA demonstrates kinetic behavior similar to that of previously characterized homologs. It displays zero-order kinetics with respect to the concentration of methyl viologen, first-order kinetics with respect to the concentration of the enzyme, and hyperbolic dependence of the rate with respect to the concentration of nitrite (Fig. 4). The KM and the kcat values obtained from the Michaelis-Menten curve were 27 ± 2 μm NO2 and 1,291 ± 34 μmol NO2 min−1 mg−1 enzyme, respectively. The G. lovleyi NrfA KM is similar to that reported for NrfA proteins purified from E. coli (
      • Bamford V.A.
      • Angove H.C.
      • Seward H.E.
      • Thomson A.J.
      • Cole J.A.
      • Butt J.N.
      • Hemmings A.M.
      • Richardson D.J.
      Structure and spectroscopy of the periplasmic cytochrome c nitrite reductase from Escherichia coli.
      ) and S. oneidensis (
      • Youngblut M.
      • Judd E.T.
      • Srajer V.
      • Sayyed B.
      • Goelzer T.
      • Elliott S.J.
      • Schmidt M.
      • Pacheco A.A.
      Laue crystal structure of Shewanella oneidensis cytochrome c nitrite reductase from a high-yield expression system.
      ). Whereas the kcat calculated here is higher than that reported for some other NrfA enzymes (
      • Bamford V.A.
      • Angove H.C.
      • Seward H.E.
      • Thomson A.J.
      • Cole J.A.
      • Butt J.N.
      • Hemmings A.M.
      • Richardson D.J.
      Structure and spectroscopy of the periplasmic cytochrome c nitrite reductase from Escherichia coli.
      ,
      • Youngblut M.
      • Judd E.T.
      • Srajer V.
      • Sayyed B.
      • Goelzer T.
      • Elliott S.J.
      • Schmidt M.
      • Pacheco A.A.
      Laue crystal structure of Shewanella oneidensis cytochrome c nitrite reductase from a high-yield expression system.
      ,
      • Pereira I.A.C.
      • Legall J.
      • Xavier A.V.
      • Teixeira M.
      Characterization of a heme c nitrite reductase from a non-ammonifying microorganism, Desulfovibrio vulgaris Hildenborough.
      ,
      • Liu M.C.
      • Bakel B.W.
      • Liu M.Y.
      • Dao T.N.
      Purification of Vibrio fischeri nitrite reductase and its characterization as a hexaheme c-type cytochrome.
      ,
      • Stach P.
      • Einsle O.
      • Schumacher W.
      • Kurun E.
      • Kroneck P.
      Bacterial cytochrome c nitrite reductase: new structural and functional aspects.
      ,
      • Clarke T.A.
      • Mills P.C.
      • Poock S.R.
      • Butt J.N.
      • Cheesman M.R.
      • Cole J.A.
      • Hinton J.C.
      • Hemmings A.M.
      • Kemp G.
      • Söderberg C.A.
      • Spiro S.
      • Van Wonderen J.
      • Richardson D.J.
      Escherichia coli cytochrome c nitrite reductase NrfA.
      ,
      • Lukat P.
      • Rudolf M.
      • Stach P.
      • Messerschmidt A.
      • Kroneck P.M.
      • Simon J.
      • Einsle O.
      Binding and reduction of sulfite by cytochrome c nitrite reductase.
      ,
      • Lockwood C.W.J.
      • Burlat B.
      • Cheesman M.R.
      • Kern M.
      • Simon J.
      • Clarke T.A.
      • Richardson D.J.
      • Butt J.N.
      Resolution of key roles for the distal pocket histidine in cytochrome c nitrite reductases.
      ), it is well known that these values are heavily dependent on the exact reaction conditions (
      • Clarke T.A.
      • Mills P.C.
      • Poock S.R.
      • Butt J.N.
      • Cheesman M.R.
      • Cole J.A.
      • Hinton J.C.
      • Hemmings A.M.
      • Kemp G.
      • Söderberg C.A.
      • Spiro S.
      • Van Wonderen J.
      • Richardson D.J.
      Escherichia coli cytochrome c nitrite reductase NrfA.
      ).
      Figure thumbnail gr4
      Figure 4The Michaelis-Menten curve was generated using 1 nm WT enzyme at various nitrite concentrations. The Vmax (1.21 ± 0.02 μm NO2 s−1), kcat (1,291 ± 34 μmol NO2 min−1 mg−1), and KM (27 μm NO2) were calculated by fitting the Michaelis-Menten curve using the Michaelis-Menten function in Origin(Pro). Inset, activity assay for mutants and WT enzymes using 1 mm nitrite. The rate of reaction doubled when the WT enzyme concentration was doubled. The rates obtained for the mutants are no more than 3% of the WT enzyme of the same concentration. Within error, there is no difference in either the curves or the rates between the R277K and R277Q variants. The addition of 2 mm Ca2+ did not rescue the activity of the mutants. No oxidation of reduced methyl viologen was observed in the absence of the enzyme.
      Notably, G. lovleyi NrfA is unaffected by the absence of added calcium or the presence of 1 mm EDTA in the reaction buffer (Fig. S7). This is in contrast to the catalytic behavior displayed by its homolog from S. deleyianum, where the removal of its calcium by EDTA treatment reduced the activity of the enzyme by 50% (
      • Stach P.
      • Einsle O.
      • Schumacher W.
      • Kurun E.
      • Kroneck P.
      Bacterial cytochrome c nitrite reductase: new structural and functional aspects.
      ). This further highlights that G. lovleyi NrfA catalyzes nitrite reduction in a Ca2+-independent manner. To test the importance of the arginine residue located in the region otherwise occupied by calcium in other NrfA homologs, we generated enzyme variants carrying conservative (R277K and R277Q) and radical (R277A) substitutions. Each of the enzyme variants contains all five hemes, as deduced by MS (56,107 Da for R277A and 56,164 Da for R277K and R277Q). The rates obtained for these variants were significantly lower than that of the WT enzyme (Fig. 4, inset). These data suggest that the arginine residue in G. lovleyi NrfA is critical to catalysis and plays a role similar to that of the calcium ion in other homologs.

      Phylogenetic analysis revealed a diverse evolutionary path of Arg-containing NrfA proteins

      We identified a wide taxonomic diversity of bacteria with NrfA proteins carrying the critical arginine residue (Arg-277) of the G. lovleyi NrfA (Fig. 5), suggesting a novel subclass of Arg-containing, Ca2+-independent NrfA enzymes. To gain insights into the phylogenetic diversity of Arg-containing NrfA proteins, we constructed a phylogenetic tree from a refined alignment of 445 bacterial NrfA proteins. These analyses reconstructed the previously reported phylogenetic clustering of NrfA proteins in 18 distinct clades (designated A to R) (
      • Welsh A.
      • Chee-Sanford J.C.
      • Connor L.M.
      • Löffler F.E.
      • Sanford R.A.
      Refined NrfA phylogeny improves PCR-based nrfA gene detection.
      ), which groups the Geobacter NrfA proteins in clade I (Fig. 5). This clade is polyphyletic and includes NrfA proteins from genera (Desulfuromonas, Pelobacter, and Geobacter) in the two families (Geobacteraceae and Desulfuromonadaceae) described in the order Desulfuromonadales (
      • Reguera G.
      • Kashefi K.
      The electrifying physiology of Geobacter bacteria, 30 years on.
      ). The clade also includes strictly anaerobic genera from the orders Clostridiales (Desulfuromaculum and Dethiobacter), Nitrospirales (Thermodesulfovibrio), Thermodesulfobacteriales (Thermodesulfobacterium), Desulfovibrionales (Desulfovibrio), and Syntrophobacterales (Syntrophobacter) (Fig. 5). Despite its taxonomic diversity, the clade only contains NrfA proteins carrying the conserved arginine residue (Arg-277) found in the G. lovleyi NrfA (NrfA-1Glov in Fig. 5), suggesting they all evolved from a deep-branching common ancestor. Clade I also contains a second NrfA paralog of G. lovleyi (NrfA-2Glov). This paralog shares 72% similarity at the amino acid level with the NrfA-1Glov enzyme. However, the second paralog branched separately, clustering together with other Geobacter NrfA proteins.
      Figure thumbnail gr5
      Figure 5Bayesian phylogenetic tree of NrfA proteins (n = 445) showing collapsed phylogenies for taxonomic orders represented in each of the 18 NrfA clades (designated A to R on the right), as reported by Welsh et al. (
      • Welsh A.
      • Chee-Sanford J.C.
      • Connor L.M.
      • Löffler F.E.
      • Sanford R.A.
      Refined NrfA phylogeny improves PCR-based nrfA gene detection.
      ). The collapsed outgroup cluster (comprised of the genera Dethiobacter, Geobacter, Pelobacter, Caldimicrobium, Thioalkalivibrio, Desulfurivibrio, Calditerrivibrio, Mucispirillum, Suterella, and Parasuterella) is shown on top. The scale bar shows substitutions per site. Clade I, which contains only Arg-containing NrfA proteins, also includes the two NrfA paralogs of G. lovleyi: NrfA-1Glov and NrfA-2Glov (GLOV_RS01015). A tree showing the uncollapsed phylogenies is available in .
      The phylogenetic analyses also provided evidence for the separate emergence of Arg-containing NrfA proteins within clades R, D, and N (Fig. 5), consistent with convergent evolution. These clades are hybrid, comprising both Arg-containing and Ca2+-binding NrfA proteins. Thus, in clade R, we identified Arg-containing NrfA proteins in the actinobacterial order Eggerthellales (Slackia and Cryptobacterium, within the family Eggerthellaceae), but the clade also includes a cluster of Eggerthallales with canonical Ca2+-binding NrfA proteins. Clade D, on the other hand, only includes proteins from the order Alteromonadales (Shewanella and Ferrimonas), but the presence of the two NrfA subclasses splits the clade into two separate clusters. This clade provides examples of both the presence of canonical Ca2+-dependent NrfA proteins and the emergence of Arg-dependent NrfA proteins within the same species (e.g. Shewanella sediminis, Shewanella piezotolerans, and Ferrimonas balearica). The genus Shewanella is also represented in clade C, a phylogenetically distinct group of Ca2+-dependent enzymes represented by the NrfA protein from Shewanella oneidensis, for which a structure is available (
      • Youngblut M.
      • Judd E.T.
      • Srajer V.
      • Sayyed B.
      • Goelzer T.
      • Elliott S.J.
      • Schmidt M.
      • Pacheco A.A.
      Laue crystal structure of Shewanella oneidensis cytochrome c nitrite reductase from a high-yield expression system.
      ). Clade N is also hybrid, containing the two types of NrfA subclasses in two genera within the orders Clostridiales (Desulfitobacterium) and Thermoanaerobacterales (Carboxydothermus). Desulfitobacterium, for example, provides a clear example of NrfA diversification within the same genus, with most species containing the Ca2+-binding protein but one member (Desulfitobacterium dichlororeducens) evolving the Arg-dependent protein (Fig. S9). Even within the same species (e.g. Ferrimonas balearica in clade D), there is the emergence of the Arg motif in phylogenetically distinct NrfA proteins.

      Discussion

      G. lovleyi, first isolated from creek sediment samples cultivated under strict anaerobiosis using acetate-amended enrichment culture (
      • Sung Y.
      • Fletcher K.E.
      • Ritalahti K.M.
      • Apkarian R.P.
      • Ramos-Hernández N.
      • Sanford R.A.
      • Mesbah N.M.
      • Löffler F.E.
      Geobacter lovleyi sp. nov. strain SZ, a novel metal-reducing and tetrachloroethene-dechlorinating bacterium.
      ), can couple the oxidation of acetate to the reduction of U6+, Fe3+, Mn4+, trichloroethene, tetrachloroethane, and, most relevant to this paper, NO3 and NO2 (
      • Reguera G.
      • Kashefi K.
      The electrifying physiology of Geobacter bacteria, 30 years on.
      ). This reaction is most likely catalyzed by NrfA-1, the only one of two paralogs that is encoded in an operon with the putative membrane-bound redox partner NrfH (
      • Reguera G.
      • Kashefi K.
      The electrifying physiology of Geobacter bacteria, 30 years on.
      ). We successfully expressed NrfA-1 in Shewanella oneidensis MR-1 and purified it in the mature form. The enzyme had the same heme content (Table S1), similar UV-Vis and EPR spectroscopic signatures (Figs. S2 and S3), and similar kinetics (Fig. 4) compared to those of previously characterized NrfA homologs.
      G. lovleyi NrfA, like all NrfA enzymes crystallized thus far (
      • Einsle O.
      • Messerschmidt A.
      • Stach P.
      • Bourenkov G.P.
      • Bartunik H.D.
      • Huber R.
      • Kroneck P.M.
      Structure of cytochrome c nitrite reductase.
      ,
      • Einsle O.
      • Stach P.
      • Messerschmidt A.
      • Simon J.
      • Kröger A.
      • Huber R.
      • Kroneck P.M.
      Cytochrome c nitrite reductase from Wolinella succinogenes: structure at 1.6 Å resolution, inhibitor binding, and heme-packing motifs.
      ,
      • Bamford V.A.
      • Angove H.C.
      • Seward H.E.
      • Thomson A.J.
      • Cole J.A.
      • Butt J.N.
      • Hemmings A.M.
      • Richardson D.J.
      Structure and spectroscopy of the periplasmic cytochrome c nitrite reductase from Escherichia coli.
      ,
      • Youngblut M.
      • Judd E.T.
      • Srajer V.
      • Sayyed B.
      • Goelzer T.
      • Elliott S.J.
      • Schmidt M.
      • Pacheco A.A.
      Laue crystal structure of Shewanella oneidensis cytochrome c nitrite reductase from a high-yield expression system.
      ,
      • Rodrigues M.L.
      • Oliveira T.F.
      • Pereira I.A.
      • Archer M.
      X-ray structure of the membrane-bound cytochrome c quinol dehydrogenase NrfH reveals novel haem coordination.
      ,
      • Cunha C.A.
      • Macieira S.
      • Dias J.M.
      • Almeida G.
      • Goncalves L.L.
      • Costa C.
      • Lampreia J.
      • Huber R.
      • Moura J.J.
      • Moura I.
      • Romão M.J.
      Cytochrome c nitrite reductase from Desulfovibrio desulfuricans ATCC 27774. The relevance of the two calcium sites in the structure of the catalytic subunit (NrfA).
      ), has a homodimeric structure. This dimeric structure is often assumed to be functional in vivo, and it is quite possible that intermolecular electron transfer occurs at the dimer interface through heme-5 (
      • Youngblut M.
      • Judd E.T.
      • Srajer V.
      • Sayyed B.
      • Goelzer T.
      • Elliott S.J.
      • Schmidt M.
      • Pacheco A.A.
      Laue crystal structure of Shewanella oneidensis cytochrome c nitrite reductase from a high-yield expression system.
      ,
      • Youngblut M.
      • Pauly D.J.
      • Stein N.
      • Walters D.
      • Conrad J.A.
      • Moran G.R.
      • Bennett B.
      • Pacheco A.A.
      Shewanella oneidensis cytochrome c nitrite reductase (ccNiR) does not disproportionate hydroxylamine to ammonia and nitrite, despite a strongly favorable driving force.
      ). It is not uncommon, however, for NrfA proteins isolated from the periplasmic soluble fractions to elute as monomers by size-exclusion chromatography (
      • Liu M.C.
      • Bakel B.W.
      • Liu M.Y.
      • Dao T.N.
      Purification of Vibrio fischeri nitrite reductase and its characterization as a hexaheme c-type cytochrome.
      ,
      • Kajie S.
      • Anraku Y.
      Purification of a hexaheme cytochrome c552 from Escherichia coli K 12 and its properties as a nitrite reductase.
      ,
      • Schumacher W.
      • Kroneck P.M.H.
      Dissimilatory hexaheme c nitrite reductase of “Spirillum” strain 5175: purification and properties.
      ,
      • Schumacher W.
      • Hole U.
      • Kroneck P.M.
      Ammonia-forming cytochrome c nitrite reductase from Sulfurospirillum deleyianum is a tetraheme protein: new aspects of the molecular composition and spectroscopic properties.
      ), as we observed for G. lovleyi NrfA. A DLS experiment on purified E. coli NrfA performed at various concentrations indicated, however, that this homolog exists in a monomer-dimer equilibrium, even at concentrations as low as 9 μm (
      • Clarke T.A.
      • Cole J.A.
      • Richardson D.J.
      • Hemmings A.M.
      The crystal structure of the pentahaem c-type cytochrome NrfB and characterization of its solution-state interaction with the pentahaem nitrite reductase NrfA.
      ). This suggests that the enzyme is likely a monomer at dilute concentrations but dimerizes at higher concentrations. We performed similar DLS experiments on G. lovleyi NrfA, and our results indicate that the enzyme remains monomeric even at concentrations as high as 300 μm. This suggests that the G. lovleyi NrfA has a dimer dissociation constant, Kd, that is significantly higher than that measured for its homologs (
      • Youngblut M.
      • Judd E.T.
      • Srajer V.
      • Sayyed B.
      • Goelzer T.
      • Elliott S.J.
      • Schmidt M.
      • Pacheco A.A.
      Laue crystal structure of Shewanella oneidensis cytochrome c nitrite reductase from a high-yield expression system.
      ,
      • Clarke T.A.
      • Mills P.C.
      • Poock S.R.
      • Butt J.N.
      • Cheesman M.R.
      • Cole J.A.
      • Hinton J.C.
      • Hemmings A.M.
      • Kemp G.
      • Söderberg C.A.
      • Spiro S.
      • Van Wonderen J.
      • Richardson D.J.
      Escherichia coli cytochrome c nitrite reductase NrfA.
      ,
      • Judd E.T.
      • Youngblut M.
      • Pacheco A.A.
      • Elliott S.J.
      Direct electrochemistry of Shewanella oneidensis cytochrome c nitrite reductase: evidence of interactions across the dimeric interface.
      ,
      • Angove H.C.
      • Cole J.A.
      • Richardson D.J.
      • Butt J.N.
      Protein film voltammetry reveals distinctive fingerprints of nitrite and hydroxylamine reduction by a cytochrome c nitrite reductase.
      ). This is a significant finding, because in vitro mechanistic studies are often interpreted with the assumption that NrfA is a functional dimer based on the crystal structure (Fig. 1). The fact that G. lovleyi NrfA remains in the monomeric state even at very high protein concentrations suggests that the situation is more complex.
      Although G. lovleyi NrfA-1 crystallizes as a homodimer and displays a fold and heme arrangement similar to those of previously crystallized homologs (
      • Einsle O.
      • Messerschmidt A.
      • Stach P.
      • Bourenkov G.P.
      • Bartunik H.D.
      • Huber R.
      • Kroneck P.M.
      Structure of cytochrome c nitrite reductase.
      ,
      • Einsle O.
      • Stach P.
      • Messerschmidt A.
      • Simon J.
      • Kröger A.
      • Huber R.
      • Kroneck P.M.
      Cytochrome c nitrite reductase from Wolinella succinogenes: structure at 1.6 Å resolution, inhibitor binding, and heme-packing motifs.
      ,
      • Bamford V.A.
      • Angove H.C.
      • Seward H.E.
      • Thomson A.J.
      • Cole J.A.
      • Butt J.N.
      • Hemmings A.M.
      • Richardson D.J.
      Structure and spectroscopy of the periplasmic cytochrome c nitrite reductase from Escherichia coli.
      ,
      • Youngblut M.
      • Judd E.T.
      • Srajer V.
      • Sayyed B.
      • Goelzer T.
      • Elliott S.J.
      • Schmidt M.
      • Pacheco A.A.
      Laue crystal structure of Shewanella oneidensis cytochrome c nitrite reductase from a high-yield expression system.
      ,
      • Rodrigues M.L.
      • Oliveira T.F.
      • Pereira I.A.
      • Archer M.
      X-ray structure of the membrane-bound cytochrome c quinol dehydrogenase NrfH reveals novel haem coordination.
      ,
      • Cunha C.A.
      • Macieira S.
      • Dias J.M.
      • Almeida G.
      • Goncalves L.L.
      • Costa C.
      • Lampreia J.
      • Huber R.
      • Moura J.J.
      • Moura I.
      • Romão M.J.
      Cytochrome c nitrite reductase from Desulfovibrio desulfuricans ATCC 27774. The relevance of the two calcium sites in the structure of the catalytic subunit (NrfA).
      ), it lacks the calcium ion near the heme-1 active site seen in the canonical NrfA enzymes. Instead, we identified a conserved arginine residue (Arg-277) that is critical for enzyme activity. The calcium ion associated with other NrfA proteins has been proposed to participate in the proper positioning of two strictly conserved active-site residues, histidine and tyrosine. Cunha et al (
      • Cunha C.A.
      • Macieira S.
      • Dias J.M.
      • Almeida G.
      • Goncalves L.L.
      • Costa C.
      • Lampreia J.
      • Huber R.
      • Moura J.J.
      • Moura I.
      • Romão M.J.
      Cytochrome c nitrite reductase from Desulfovibrio desulfuricans ATCC 27774. The relevance of the two calcium sites in the structure of the catalytic subunit (NrfA).
      ). demonstrated that the NrfA active site is architecturally similar to that of peroxidase classes II and III, which also contain a calcium ion in a similar location in the heme active site. The calcium ion in NrfA and both class II and class III peroxidases are structurally coupled to an active-site histidine. Release of the calcium ion in both Phanerochaete chrysosporium lignin and manganese peroxidases resulted in the histidine binding to the heme and deactivating the enzyme (
      • George S.J.
      • Kvaratskhelia M.
      • Dilworth M.J.
      • Thorneley R.N.
      Reversible alkaline inactivation of lignin peroxidase involves the release of both the distal and proximal site calcium ions and bishistidine co-ordination of the haem.
      ,
      • Sutherland G.R.
      • Aust S.D.
      Thermodynamics of binding of the distal calcium to manganese peroxidase.
      ). This led Cunha et al. to hypothesize that the calcium ion in NrfA plays a similar role and prevents the active-site histidine from binding to the heme-1 iron (
      • Cunha C.A.
      • Macieira S.
      • Dias J.M.
      • Almeida G.
      • Goncalves L.L.
      • Costa C.
      • Lampreia J.
      • Huber R.
      • Moura J.J.
      • Moura I.
      • Romão M.J.
      Cytochrome c nitrite reductase from Desulfovibrio desulfuricans ATCC 27774. The relevance of the two calcium sites in the structure of the catalytic subunit (NrfA).
      ). This is achieved by coordination of the calcium to the side chain oxygen of the conserved glutamine residue that is contiguous to the histidine (Fig. 2B). The Arg-277 in G. lovleyi NrfA, which is extensibly H-bonded to Tyr-221 and Glu-263 (Fig. 2A), is contiguous to the active-site histidine, His-278. This H-bonding network prevents the active-site histidine from binding to the heme-1 iron, similar to what was suggested for the Ca2+-binding NrfAs. The <3% activity of the R277K amino acid variant relative to the WT enzyme (Fig. 4) may be because of the inability of the lysine to form the same H-bonding network as the arginine. Additionally, the calcium ion may also be required for the proper positioning of an active-site Tyr residue. In Ca2+-binding NrfA enzymes, the carbonyl oxygen of this tyrosine is ligated to calcium, presumably ensuring proper positioning. In G. lovleyi NrfA, Arg-277 is H-bonded to this tyrosine (Tyr-221) and ensures proper orientation.
      Another interesting feature in the Ca2+-binding NrfA enzyme is the presence of a strictly conserved lysine residue whose amide oxygen is a ligand to calcium. The lysine ligand in a Ca2+-binding NrfA may be involved in the delivery of nitrite to the active site through electrostatic interactions either by itself or in tandem with calcium. In G. lovleyi NrfA, no lysine is observed in the vicinity of Arg-277, suggesting that any role played by the lysine in a Ca2+-dependent NrfA is presumably assumed by Arg-277. Previously, the calcium ion was believed to be involved in the delivery of catalytic protons through its two water ligands (
      • Bykov D.
      • Neese F.
      Six-electron reduction of nitrite to ammonia by cytochrome c nitrite reductase: insights from density functional theory studies.
      ). In the asymmetric unit of the G. lovleyi crystal structure, however, only two of six monomers show an ordered water molecule H-bonded to Arg-277. This could mean that G. lovleyi NrfA has a different pathway for proton delivery, although given the resolution of the structure, it is impossible to make a definitive conclusion.
      Interestingly, many Ca2+-dependent proteins are known to have Ca2+-independent homologs. Some examples include dysferlin (
      • Fuson K.
      • Rice A.
      • Mahling R.
      • Snow A.
      • Nayak K.
      • Shanbhogue P.
      • Meyer A.G.
      • Redpath G.M.
      • Hinderliter A.
      • Cooper S.T.
      • Sutton R.B.
      Alternate splicing of dysferlin C2A confers Ca2+-dependent and Ca2+-independent binding for membrane repair.
      ), phosphatidylinositol-specific phospholipase C (
      • Heinz D.W.
      • Ryan M.
      • Bullock T.L.
      • Griffith O.H.
      Crystal structure of the phosphatidylinositol-specific phospholipase C from Bacillus cereus in complex with myo-inositol.
      ,
      • Roberts M.F.
      • Khan H.M.
      • Goldstein R.
      • Reuter N.
      • Gershenson A.
      Search and subvert: minimalist bacterial phosphatidylinositol-specific phospholipase C enzymes.
      ), C-type lectins (
      • Unno H.
      • Itakura S.
      • Higuchi S.
      • Goda S.
      • Yamaguchi K.
      • Hatakeyama T.
      Novel Ca2+-independent carbohydrate recognition of the c-type lectins, SPL-1 and SPL-2, from the bivalve Saxidomus purpuratus.
      ,
      • Shi Y.
      • Zhao X.
      • Wang Z.
      • Shao Y.
      • Zhang W.
      • Bao Y.
      • Li C.
      Novel Ca2+-independent c-type lectin involved in immune defense of the razor clam Sinonovacula constricta.
      ), and protein kinase Cϵ (
      • Ochoa W.F.
      • Garcia-Garcia J.
      • Fita I.
      • Corbalan-Garcia S.
      • Verdaguer N.
      • Gomez-Fernandez J.C.
      Structure of the C2 domain from novel protein kinase Cϵ. A membrane binding model for Ca2+-independent C2 domains.
      ). The obvious recurring theme in these Ca2+-independent homologs is the appearance of either lysine or arginine as calcium replacement residues, presumably assuming the role of the calcium ion. For example, in the Ca2+-independent homolog of phosphatidylinositol-specific phospholipase C, an arginine was found to be involved in lowering the pKa of the inositol C2 hydroxyl group and in the activation of the phosphate group, similar to the function played by the calcium ion in the Ca2+-dependent homolog (
      • Roberts M.F.
      • Khan H.M.
      • Goldstein R.
      • Reuter N.
      • Gershenson A.
      Search and subvert: minimalist bacterial phosphatidylinositol-specific phospholipase C enzymes.
      ).
      The Arg-dependent homologs of the Ca2+-dependent proteins could have emerged as an adaptive response to the scarcity of bioavailable calcium in their habitats. Microbial metabolic activities, including ammonification and precipitation of calcium with carbonates, reduce their bioavailability (
      • Zhu T.
      • Dittrich M.
      Carbonate precipitation through microbial activities in natural environment, and their potential in biotechnology: a review.
      ). This limitation exerts selective pressure to evolve proteins with arginine or lysine residues as a replacement for calcium. The strict conservation of arginine in the Ca2+-independent subclass of NrfA enzymes points to a critical role for this residue in roles otherwise assumed by lysine or calcium in other enzymes. Consistent with this, replacing the active-site arginine with a lysine (R277K) reduced the nitrite reductase activity of the G. lovleyi NrfA to levels comparable with a radical substitution (R277A). This suggests a higher fitness conferred by this substitution relative to Lys (
      • Rey C.
      • Lanore V.
      • Veber P.
      • Guéguen L.
      • Lartillot N.
      • Sémon M.
      • Boussau B.
      Detecting adaptive convergent amino acid evolution.
      ,
      • Christin P.A.
      • Weinreich D.M.
      • Besnard G.
      Causes and evolutionary significance of genetic convergence.
      ). This helps define diagnostic features of NrfA proteins of the Ca2+-dependent and -independent subclasses. Welsh et al. (
      • Welsh A.
      • Chee-Sanford J.C.
      • Connor L.M.
      • Löffler F.E.
      • Sanford R.A.
      Refined NrfA phylogeny improves PCR-based nrfA gene detection.
      ) defined NrfA enzymes as pentaheme nitrite reductases containing a conserved KXQH or KXRH motif between the third and fourth heme-binding sites. Our phylogenetic analyses (Fig. 5) and amino acid sequence alignment (Fig. S8) identified KXQH as specific to Ca2+-dependent enzymes. This motif has three conserved residues of the active site (Lys-279, Gln-281, and His-282 in S. deleyianum; Fig. 2B). These residues likely are critical not only in calcium ion coordination (Lys-279 and Gln-281) (
      • Einsle O.
      • Messerschmidt A.
      • Stach P.
      • Bourenkov G.P.
      • Bartunik H.D.
      • Huber R.
      • Kroneck P.M.
      Structure of cytochrome c nitrite reductase.
      ) but also for the maintenance of high substrate binding affinity (Gln-281) (
      • Clarke T.A.
      • Kemp G.L.
      • Van Wonderen J.H.
      • Doyle R.M.A.
      • Cole J.A.
      • Tovell N.
      • Cheesman M.R.
      • Butt J.N.
      • Richardson D.J.
      • Hemmings A.M.
      Role of a conserved glutamine residue in tuning the catalytic activity of Escherichia coli cytochrome c nitrite reductase.
      ) and for the proper orientation of nitrite for N-O bond cleavage (His-282) (
      • Lockwood C.W.J.
      • Burlat B.
      • Cheesman M.R.
      • Kern M.
      • Simon J.
      • Clarke T.A.
      • Richardson D.J.
      • Butt J.N.
      Resolution of key roles for the distal pocket histidine in cytochrome c nitrite reductases.
      ).
      We found, however, that the distinctive feature of Ca2+-independent enzymes is the presence of an XXRH motif with the conserved active-site histidine (His-278 in G. lovleyi) and arginine residue (Arg-277 in G. lovleyi) that assumes the roles of the calcium ion in these enzymes. This motif evolved separately in 4 clades of NrfA proteins, branching clades R, D, and N into the Ca2+-dependent and Ca2+-independent subclasses and establishing clade I as a phylogenetically diverse group of Arg-containing NrfA proteins represented by G. lovleyi NrfA (Fig. 5). The unifying feature of all the bacteria in clade I is that they are obligate anaerobes with metal, sulfur, and/or sulfate reductive metabolisms (
      • Reguera G.
      • Kashefi K.
      The electrifying physiology of Geobacter bacteria, 30 years on.
      ,
      • Castro H.F.
      • Williams N.H.
      • Ogram A.
      Phylogeny of sulfate-reducing bacteria.
      ). Some of these organisms use metals as electron acceptors for respiration in reactions that can be inhibited by calcium (
      • Brooks S.C.
      • Fredrickson J.K.
      • Carroll S.L.
      • Kennedy D.W.
      • Zachara J.M.
      • Plymale A.E.
      • Kelly S.D.
      • Kemner K.M.
      • Fendorf S.
      Inhibition of bacterial U(VI) reduction by calcium.
      ). Their oxidative metabolisms also generate carbon dioxide, which favors carbonate precipitation and calcium removal from their habitats (
      • Zhu T.
      • Dittrich M.
      Carbonate precipitation through microbial activities in natural environment, and their potential in biotechnology: a review.
      ). Thus, the emergence of Ca2+-independent NrfA proteins provided these microorganisms a competitive advantage to respire nitrate and nitrite despite the limited bioavailability of the calcium ion.

      Experimental procedures

      Protein expression and purification

      The plasmid used for the overexpression of the enzyme was synthesized by GenScript (Piscataway, NJ). Briefly, the native signal sequence (MRSKLLVPLVAAALCGAAITAC) at the N terminus of the WT G. lovleyi NrfA-1 (GLOV_RS05135) paralog was replaced with pelB, and the Strep-tag II was added at the C terminus. The corresponding gene construct was codon optimized and inserted into pBAD202/D-TOPO (Invitrogen) downstream from the ribosome binding site by 11 noncoding base pairs. The resulting plasmid was then used for site-directed mutagenesis (Phusion DNA polymerase, New England Biolab) to generate the R277K, R277A, and R277Q NrfA variants using appropriate primers. The WT NrfA and its variant plasmids were then individually transformed into the overexpression host, Shewanella oneidensis MR-1 (
      • Shi L.
      • Lin J.T.
      • Markillie L.M.
      • Squier T.C.
      • Hooker B.S.
      Overexpression of multi-heme c-type cytochromes.
      ). Enzyme overexpression was performed by inoculating 10 ml of an overnight culture into a liter of Terrific broth supplemented with kanamycin to a final concentration of 50 µg L−1. The resulting culture was incubated at 30 °C with constant shaking at 160 rpm until an optical density of 0.6 was reached. The culture was then induced with arabinose to a final concentration of 0.02% and incubated for an additional 12 h before harvesting. The cells were harvested by centrifugation, resuspended in 50 ml of 100 mm Tris buffer at pH 7.5, flash-frozen in liquid nitrogen, and stored at −80 °C until further use.
      Purification of the enzyme was accomplished by thawing the frozen 50-ml cell suspension and immediately adding 250 units of Benzonase (Millipore Sigma), 2 tablets of dissolved cOmpleteTM, Mini, EDTA-free protease inhibitor mixture (Roche), and 150 kU of rLysozyme (Millipore Sigma). The cell lysate was then allowed to incubate for 4 h at room temperature with constant stirring, after which the suspension was stored at 4 °C overnight. The next day, the cell suspension was centrifuged at 47,000 relative centrifugal force for 30 min to separate the cell membrane from the lysate. The ionic strength of the lysate was increased by the addition of NaCl to a final concentration of 150 mm, and the solution was passed through a 0.22-μm filter. The protein was then purified by affinity chromatography using a Strep-Tactin XT high-capacity column (IBA Lifesciences). Following protein binding, the column was washed with 12 column volumes of 100 mm Tris buffer containing 150 mm NaCl at pH 8.0. The bound enzyme was eluted using 50 mm biotin. The enzyme was further purified by size-exclusion chromatography (Enrich SEC 650, Bio-Rad) at a flow rate of 0.5 ml/min using 50 mm HEPES with 150 mm NaCl at pH 7.0. Fractions within 80% of the elution band were collected, and the purity was assessed by SDS-PAGE. The molecular weight of the enzyme was determined by MS.

      Extinction coefficient

      Duplicate samples of the WT enzyme were submitted to the Proteomics Core Facility at the UC–Davis Genome Center (Davis, CA) for amino acid analysis. Immediately before sending the samples, the absorbance at 410 nm (λmax) was determined. From the measured absorbance and the enzyme monomer concentration obtained by amino acid analysis, the extinction coefficient was calculated to be 547 mm−1 cm−1. The monomeric concentration of samples used for the succeeding experiments was determined spectrophotometrically using this extinction coefficient.

      ICP-OES

      A 120-μl sample of 208 μm enzyme was digested in 45% nitric acid (≥99.999% purity based on trace metal) at 50 °C overnight. The ICP-OES sample was prepared by diluting the digested sample with deionized water to 5 ml. A solution for blank measurement was prepared in the same manner, except that sample buffer was used instead of the enzyme. Calcium and iron standards with concentrations between 0.1 and 2.0 ppm were prepared in 2% nitric acid. Metal analysis was performed using a Varian 710-ES Axial ICP-OES at a flow rate of 10 rpm. Quantitative analysis of calcium was monitored at 317.933, 393.366, and 396.847 nm, whereas iron was monitored at 234.350, 239.563, and 259.940 nm. Each measured value was an average of three replicates.

      DLS

      A stock solution of 300 μm NrfA was prepared, and the concentrated enzyme was centrifuged at 17,000 relative centrifugal force for 30 min to ensure the removal of any particulate matter. Samples with concentrations between 25 and 300 μm were then prepared by serial dilution, after which the exact concentrations of these samples were determined spectrophotometrically. DLS was performed using DynaPro Nanostar (Wyatt Technology) initially offset using the same buffer as that used for the enzyme. Each measurement was an average of 20 acquisitions with 15 s per acquisition and performed at 25 °C using a 658-nm wavelength. The enzyme's theoretical monomer and dimer RH values were calculated using DYNAMICS V7 (Wyatt Technology) given the monomeric molecular weight of 56,196 Da.

      Crystal structure determination

      The enzyme in HEPES buffer (pH 7.0) containing 150 mM NaCl and 30 mm ammonium sulfate was concentrated to 10 mg/ml. The protein was crystallized by sitting-drop vapor diffusion using the crystallization buffer (200 mm sodium malonate and 15% PEG 3350 at pH 6.5) with a protein:buffer ratio of 2:1 (v/v). The crystals were allowed to grow to full size for 2–3 weeks at 4 °C and cryoprotected using 20–30% glycerol prior to flash freezing in liquid N2. Diffraction data were collected at 1.72 Å (slightly shorter than the K edge of Fe at 1.74 Å) at the 21-ID-D beamline of the Advance Photon Source (APS) at Argonne National Laboratory. The data sets were indexed, integrated, and scaled using the HKL2000 suite (
      • Otwinowski Z.
      • Minor W.
      Processing of X-ray diffraction data collected in oscillation mode.
      ). The initial trial to solve the phasing problem using molecular replacement failed, and the structure of G. lovelyi NfrA was eventually solved by single-wavelength anomalous dispersion using Fe anomalous signals in Phenix.AutoSol (
      • Liebschner D.
      • Afonine P.V.
      • Baker M.L.
      • Bunkoczi G.
      • Chen V.B.
      • Croll T.I.
      • Hintze B.
      • Hung L.W.
      • Jain S.
      • McCoy A.J.
      • Moriarty N.W.
      • Oeffner R.D.
      • Poon B.K.
      • Prisant M.G.
      • Read R.J.
      • et al.
      Macromolecular structure determination using X-rays, neutrons and electrons: recent developments in Phenix.
      ) at 2.7 Å. Nearly all the amino acid residues were built using Phenix.AutoBuild. This initial model was then used as a search template in molecular replacement to solve the structure at 2.55 Å in CCP4 (
      • Winn M.D.
      • Ballard C.C.
      • Cowtan K.D.
      • Dodson E.J.
      • Emsley P.
      • Evans P.R.
      • Keegan R.M.
      • Krissinel E.B.
      • Leslie A.G.
      • McCoy A.
      • McNicholas S.J.
      • Murshudov G.N.
      • Pannu N.S.
      • Potterton E.A.
      • Powell H.R.
      • et al.
      Overview of the CCP4 suite and current developments.
      ) using the first half of the diffraction data with less radiation damage. Model building and refinement were conducted iteratively in Coot (
      • Emsley P.
      • Lohkamp B.
      • Scott W.G.
      • Cowtan K.
      Features and development of Coot.
      ) and Phenix.refine to manually correct geometry and insert both the hemes and sulfate. In the final model (Table 2), one asymmetric unit contained three homodimers, and each monomer includes residues 35–480, five hemes, and one sulfate ion in the active site.

      Activity assay

      The activity assay was performed as previously described (
      • Clarke T.A.
      • Mills P.C.
      • Poock S.R.
      • Butt J.N.
      • Cheesman M.R.
      • Cole J.A.
      • Hinton J.C.
      • Hemmings A.M.
      • Kemp G.
      • Söderberg C.A.
      • Spiro S.
      • Van Wonderen J.
      • Richardson D.J.
      Escherichia coli cytochrome c nitrite reductase NrfA.
      ), with minor modifications. Briefly, 50 mm HEPES buffer at pH 7.0 containing 0.8 mm methyl viologen was prepared (with or without 2 mm calcium chloride for R277A, R277Q, and R277K activity assays) and degassed under vacuum for 30 min. Deionized water and a solution of 250 mm EDTA were also degassed. Inside an anaerobic chamber (Coy Laboratory Products), solutions of 10 mm dithionite and 1 m nitrite were prepared using degassed water and buffer, respectively. A 1 μm sample of enzyme was also prepared by diluting a concentrated sample with an appropriate volume of buffer, and the sample was incubated for at least 30 min inside the chamber. The exact concentration of the enzyme was then determined spectrophotometrically. A 3-ml reaction solution consisting of 0.8 mm methyl viologen (buffer), 0.04–0.06 mm dithionite, and 0.5–2.0 nm enzyme was prepared in a crimped-top modified cuvette. In assays where EDTA was required, EDTA was added to a final concentration of 1 mm. The desired amount of nitrite (10 μm to 1 mm) was then added to the side of the cuvette above the solution to avoid initiating catalysis. The cuvette was sealed with a rubber septum, tightly secured with a crimped aluminum lid, and brought outside the chamber for the kinetic analysis using a spectrophotometer initially baselined using the same buffer. Catalysis was initiated by quickly inverting the cuvette three times to introduce the nitrite previously added to the side of the cuvette. The absorbance at each time point was converted to the concentration of the methyl viologen using ε600 = 13,700 m−1 cm−1, and the rate of decrease of the methyl viologen was used to calculate the rate of the reaction. KM and Vmax values were determined by fitting the Michaelis-Menten curve using the Michaelis-Menten function in Origin [Origin(Pro), version 2019b, OriginLab Corporation, Northampton, MA, USA].

      Phylogenetic analysis

      Phylogenetic analysis was performed in the same manner as that previously described (
      • Welsh A.
      • Chee-Sanford J.C.
      • Connor L.M.
      • Löffler F.E.
      • Sanford R.A.
      Refined NrfA phylogeny improves PCR-based nrfA gene detection.
      ). Approximately 13,400 NrfA amino acid sequences contributed by Joel Klappenbach in FUNGENE (RRID:SCR_018749) were retrieved, excluding sequences from environmental samples as well as sequences from unknown species. The number of sequences was further trimmed to 13,200 by aligning the sequences and retaining only the ones that had the conserved CXXCK and CXXCH heme-binding motifs as well as the conserved His and Tyr active-site residues. The sequences were segregated into two groups: the arginine-bearing and the putative Ca2+-bearing groups (Fig. S8). Each group was then individually aligned using MUSCLE (
      • Edgar R.C.
      MUSCLE: multiple sequence alignment with high accuracy and high throughput.
      ) and clustered at 98% identity using UCLUST's cluster_fast command (
      • Edgar R.C.
      Search and clustering orders of magnitude faster than BLAST.
      ). The two groups were then realigned against each other to generate a single multiple-sequence alignment consisting of 445 sequences. Bayesian phylogenetic analysis for the 445 sequences was performed in Beauti/Beast v1.10.4 (
      • Suchard M.A.
      • Lemey P.
      • Baele G.
      • Ayres D.L.
      • Drummond A.J.
      • Rambaut A.
      Bayesian phylogenetic and phylodynamic data integration using BEAST 1.10.
      ) using the LG + G + I model, which was determined using MEGA v10.1 maximum likelihood model selection (
      • Kumar S.
      • Stecher G.
      • Li M.
      • Knyaz C.
      • Tamura K.
      MEGA X: molecular evolutionary genetics analysis across computing platforms.
      ). For this model, a single chain of Markov chain Monte Carlo was run for 10,000,000 generations and sampled at every 10,000th generation. With a burn-in of the first 1,000,000 generations, effective sample size values of >200 were achieved for all statistical parameters. This burn-in was used in TreeAnnotator (Beast package module) to compute the final consensus tree. Postprocessing for the final presentation of the tree was performed using FigTree (Beast package module) and MEGA V10.1.

      Data availability

      The crystal structure presented in this paper was deposited in the Protein Data Bank (PDB entry 6V0A). The ICP-OES data, crystallographic statistics, Coomassie-stained gel and Western blotting, UV-Vis spectrum, EPR spectrum, data from size-exclusion chromatography, G. lovleyi and S. deleyianum NrfA protein alignment, G. lovleyi active-site difference density map, activity assay with and without EDTA, sequence alignment of NrfA proteins from select species, and the uncollapsed phylogenetic tree can be found in the supporting information. The alignment of all 445 sequences is uploaded separately as Dataset S1. The dynamic light scattering data, activity assay, collapsed phylogenetic tree, and all remaining data are contained within the paper.

      Acknowledgments

      We thank Prof. Cheryl Kerfeld and Dr. Bryan Ferlez (MSU-Plant Research Laboratory) for the use of their DLS instrument and for their technical expertise.

      Supplementary Material

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