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J. Biol. Chem., Vol. 278, Issue 26, 23868-23873, June 27, 2003
The 1.5-Å Structure of Chryseobacterium meningosepticum Zinc
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| ABSTRACT |
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-lactamase, BlaB, from the
pathogenic bacterium, Chryseobacterium meningosepticum, in complex
with the inhibitor, D-captopril, has been solved at 1.5-Å
resolution. The enzyme has the typical 
/
metallo-
-lactamase fold and the characteristic two metal binding sites
of members of the subclass B1, in which two Zn2+ ions
were identified. D-Captopril, a diastereoisomer of the commercial
drug, captopril, acts as an inhibitor by displacing the catalytic hydroxyl ion
required for antibiotic hydrolysis and intercalating its sulfhydryl group
between the two Zn2+ ions. Interestingly,
D-captopril is located on one side of the active site cleft. The
x-ray structure of the complex of the closely related enzyme, IMP-1, with a
mercaptocarboxylate inhibitor, which also contains a sulfhydryl group bound to
the two Zn2+ ions, shows the ligand to be located on the
opposite side of the active site cleft. A molecule generated by fusion of
these two inhibitors would cover the entire cleft, suggesting an interesting
approach to the design of highly specific inhibitors. | INTRODUCTION |
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-lactamases
(MBLs)1 or class B
-lactamases (1). These
enzymes were initially described as a curiosity in innocuous strains of
Bacillus cereus (2)
but were subsequently identified in several pathogenic species responsible for
human infections in which the enzymes are either encoded by resident
chromosomal genes or by horizontally acquired genes carried on mobile genetic
elements
(37).
Class B
-lactamases have a broad spectrum of substrates, including
penicillin, cephalosporin, and carbapenem, some of which are not substrates
for active site serine
-lactamases
(3,
4). At present no inhibitors of
these enzymes are used clinically, making them a potential source of
antibiotic resistance.
The MBL family has been divided into three different subclasses, B1, B2,
and B3, on the basis of sequence similarities
(8). MBLs have two metal
binding sites (the His site, or first site, and the Cys site, or second site),
which bind Zn2+ ions required for enzyme activity. In
general, both the mono-Zn2+ (Zn2+
in the His site) and di-Zn2+ forms are active but have
different kinetic properties
(9,
10); the exception is CphA
from Aeromonas hydrophilia, which is inhibited by the presence of the
second zinc ion (11). In the
mono-Zn2+ enzyme, a water-mediated mechanism for the
hydrolysis of the antibiotic
-lactam ring has been suggested in which a
water molecule is activated by the presence of the Zn2+
ion in the His site, then carries out nucleophilic attack against the carbonyl
carbon of the
-lactam. Residues Asp-120 and Cys-221 from the second site
would help keep the active water molecule in position. Asp-120 would also act
as a general base, transferring the proton from the water molecule to the
nitrogen atom of the
-lactam ring, causing bond cleavage and opening of
the ring, leading to inactivation of the antibiotic
(12,
13). In the
bi-Zn2+ enzyme, a hydroxide ion is positioned close to
the two Zn2+ ions. The second Zn2+
would also help to anchor Asp-120
(14) and would orient the
-lactam ring by binding to the
-lactam nitrogen of the antibiotic
(15). Although both the mono-
and di-Zn2+ forms exist, it has been hypothesized that
only the mono-Zn2+ form is present under normal
physiological conditions
(16).
Because of the great interest in MBL inhibition, several classes of inhibitors have been described (17, 18); these include phenazines (19), ketone derivatives of L- and D-alanine and trifluoromethyl alcohol (20), thioesters (21, 22), biphenyl tetrazoles (23, 24), amino acid-derived hydroxamates (25), thiols (2630), and tricyclic natural products (31). However, despite many of these having good inhibitory properties, only a few thiols have a broad spectrum of inhibition for MBLs. In this context, the analysis of the three-dimensional structures of MBLs in complex with potential inhibitors becomes a powerful tool for rational drug design. At present, the structures of Bacteroides fragilis CcrA complexed with Mes, biphenyl tetrazole, or a tricyclic natural product (32) and of (23, 31) Pseudomonas aeruginosa IMP-1 complexed with mercaptocarboxylate (30) or with succinic acid derivatives (33) have been reported.
BlaB is a chromosome-encoded MBL produced by Chryseobacterium
meningosepticum. It is a member of subclass B1 has a broad substrate
profile
(3436)
and is produced constitutively
(37). C.
meningosepticum is an ubiquitous Gramnegative rod bacterium of clinical
relevance because it can cause neonatal meningitis, adult septicemia, and
nosocomial infections and is resistant to most
-lactams, including
carbapenems (38,
39).
D-Captopril (1-(D-3-mercapto-2-methylpropionyl)-D-proline) is a diastereoisomer of the commercial drug captopril (1-(D-3-mercapto-2-methylpropionyl)-L-proline), used clinically to control high blood pressure. It acts as an angiotensin-converting enzyme inhibitor and increases plasma level of the vasodilator, bradykinin (40). D-Captopril, which was synthesized to test its inhibitory effect on MBLs, is a good competitive inhibitor of BlaB (Ki = 70100 µM) at neutral pH.2 In this work, we describe the first high resolution (1.5 Å) three-dimensional structure of BlaB complexed with D-captopril.
| EXPERIMENTAL PROCEDURES |
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Expression, Purification, and Crystallization of the
D-Captopril ComplexThe overexpression, purification,
and characterization of the protein have been described
(34). For crystallization, the
hanging drop method was used. Crystals were obtained by mixing 2 µl of
protein (5 mg/ml in 10 mM sodium cacodylate, 100 µM
zinc acetate, 100 µM dithiothreitol, pH 6.5) and 1 µl of the
reservoir solution (28% polyethylene glycol 4000, 0.2 M sodium
acetate, and 0.1 M Tris-HCl, pH 8.4) at 8 °C; crystals appeared
after approximately 3 weeks. A crystal was soaked for
5 weeks in
previously equilibrated drops of the reservoir solution containing 2 mM
D-captopril, then the crystal was flash-cooled in liquid nitrogen using
precipitant containing 15% glycerol as cryoprotectant and stored in liquid
nitrogen for subsequent data collection at a synchrotron source.
Data Collection and ProcessingDiffraction data sets were
collected at the ID144 beamline of the European Synchrotron Radiation
Facility in Grenoble, France. A data set of 360 frames at
= 0.9792
Å was collected at 1.5-Å resolution with 0.5° oscillation
steps. Data integration performed using DENZO/SCALEPACK
(43) gave an initial ambiguity
in the space group determination, since according to the automated indexing
procedure, the crystal could be cubic or trigonal, with similar distortion
indexes. The data were re-indexed using DENZO/SCALE-PACK and scaled using
SCALA (44), trying different
space groups to solve this ambiguity. After this stage, the crystal was
considered to be cubic I23 or I213, with a = b =
c = 112.85 Å,
=
=
= 90°,
Rsym = 0.117 (1 molecule/asymmetric unit), or trigonal R3, with
a = b = 159.69 Å, c = 97.82 Å,
=
= 90°,
= 120°, Rsym = 0.119 (3 or 4
molecules/asymmetric unit) (Table
I).
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Structural Determination and RefinementThe structure of the
MBL, BcII, from B. cereus at 1.8-Å resolution
(45) with no ions or water
molecules was used as a model for molecular replacement using the program
AMoRe (46). Because the x-ray
data could be indexed in three space groups (I213, I23, and R3),
various tests were performed. In brief, data indexed in R3 were used
successfully in AMoRe with a BcII model performing a four molecule/asymmetric
unit search (c-factor = 36.3%, R-factor = 48.0%). With these solutions and
using a noncrystallographic symmetry-averaging map calculated using the CNS
program (47), a partial model
of BlaB was built using TURBO
(48). A full model of BlaB at
1.5 Å was made using ARP/WARP
(49). From the initial
(2Fo Fc) and
(Fo Fc)
maps, the inhibitor, D-captopril, was localized close to the
Zn2+ ions in the active site. Further cycles of energy
minimization, B-factor refinement, and water picking were performed using CNS
programs (47). Models for
D-captopril were not included until their conformations were well
defined by the unbiased |Fo
Fc|,
calc electron density maps.
D-Captopril molecules in each of the four monomers were modeled.
The model was refined without any non-crystallographic-symmetry restrains,
yielding Rworking = 18.95% and Rfree = 20.76%.
| RESULTS |
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fold, in which a compact core of two
-sheets is surrounded by
-helices
(Fig. 1). The crystal, which
shows a cubic pseudo-symmetry, belongs to the space group R3 and has four
monomers per asymmetric unit (A, B, C, and D).
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The four monomers are practically identical, with a few local differences
due to slightly different environments or to effects of crystal packing
(e.g. monomers A, B, and D are missing the last four C-terminal
residues, whereas monomer C is missing the last six). The root mean square
deviation between the conserved C
atoms of the four molecules is about
0.1 Å. The active site of BlaB shares a feature with other subclass B1
structures, namely that two Zn2+ ions (Zn1 and Zn2)
occupy the two metal binding sites, the His site (His-116, His-118, His-196)
and the Cys site (Asp-120, Cys-221, His-263)
(Fig. 2, a and
c). A clear electron density corresponding to a molecule
of D-captopril was found close to the active site. The inhibitor
intercalates its sulfhydryl group between the two Zn2+
ions (distance S-Zn1, 2.32 Å; S-Zn2, 2.30 Å in monomer A)
(Fig. 2). The sulfur atom is
also close to the NE2 of His-196 (3.34 Å)
(Table II). The inhibitor is
located in the positively charged groove leading to the active site
(Fig. 3a) and is
present in the four molecules/asymmetric unit with full occupancy.
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Other interactions enhance the binding of the inhibitor molecule. The carboxylic group of D-captopril is stabilized by hydrogen bonds between its O2 and the NZ of Lys-167 and two water molecules; the other carboxylic oxygen, O3, is bound to the amide main chain of Asp-119. The O1 carbonyl oxygen of the inhibitor also makes a hydrogen bond with another water molecule (Fig. 2b). In addition, the inhibitor is stabilized by several hydrophobic contacts between its carbons and His-118, Asp-120, and Tyr-233 (Fig. 2b and Table II).
| DISCUSSION |
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-lactamases. The secondary
structure topology of the four available x-ray structures (BcII, CcrA, BlaB,
and IMP-1) is very similar, the only difference being that the first short
-strand seen in BcII and CcrA is absent in BlaB and IMP-1. Pairwise
superposition of the C
atoms of BlaB and BcII (1BMC
[PDB]
), Ccra
(1ZNB
[PDB]
), or IMP-1 (1JJE
[PDB]
) gives a root mean difference density of 1.5, 1.4, and
1.9 Å, respectively.
In subclass B1, there is a loop between
2 and
3 (residues
6165), which is disordered in the structure of native BcII
(12). However, in the presence
of an inhibitor, the loop is stabilized
(30), transforming the active
site groove into a tunnel-shaped cavity. In the BlaB structure, the loop is
also well defined, probably due to the presence of D-captopril in
the active site (Fig.
2c). Interestingly, D-captopril has no direct
interaction with any residues in the loop
(Fig. 2, b and
c; Table
II). The flexibility in the loop may help to accommodate the
inhibitor, which itself may help to stabilize the loop. This stabilization
takes place indirectly by Trp-87, which makes a double hydrogen bond between
its NE1 and the OD1 of Asp-119 and the water molecule that hydrogen bonds to
the O1 carbonyl oxygen of D-captopril
(Table II). Trp-87 also makes
hydrophobic contacts with the active site residue, His-263. All these
interactions determine the position of Trp-87, which establishes hydrophobic
interactions between its ring and two residues, Phe-61 and Asn-59. The loop is
strongly stabilized by hydrogen bonds between the carbonyl oxygens and the
amide nitrogens of the backbone of this region (amino acids 5769). In
the apical part of the loop facing the solvent, a water molecule forms a
bridge between the ND2 of Asn-62 and the carbonyl oxygen of Phe-61, which also
makes a hydrogen bond with the amide backbone of Thr-64. In addition, the
amide backbone of Phe-61 makes hydrogen bonds with the carbonyl oxygen of the
main chain of Thr-64. A second water molecule makes hydrogen bonds with this
bridging water molecule and the hydroxyl group of Tyr-67. In BlaB, the loop
appears to be closer to the rest of the protein than in the other three
structures, with the hydrophobic residues, Phe-61, Tyr-67, and Trp-87,
pointing toward the inside of the protein and creating a hydrophobic
environment even though the loop does not interact directly with the
inhibitor. In addition, the loops in BlaB and BcII are in different positions,
e.g. the distance between equivalent residues in BlaB and BcII is
5.44 Å for Phe-61 and 5.77 Å for Asn-62. Interestingly, in
subclass B1 MBLs, bulky hydrophobic residues are often found at positions 61,
67, and 87. In all MBL structures, the Asp or Asn residue at residue 84 has an
angle outside the allowed range (
60 ± 1°) and
plays an important structural role, contributing to the architecture of the
active site. Of the residues that are highly conserved and/or potentially
involved in substrate binding in subclass B1
-lactamases, three
positions are particularly interesting in the BlaB structure, 224, 233, and
236 (Fig. 2c).
Lys-224 is very well defined in the electron density map (NZ, B factor of
13.5 Å2). The NZ makes three hydrogen bonds with the carbonyl
oxygen of the preceding amino acid (Ile-223) and two water molecules. Lys-224
is assumed to interact with the carboxyl group of the
-lactam antibiotic
(30). However, Lys-224 is not
found in subclass B3 MBLs, and no equivalent residue is observed in a similar
spatial position in the L1
(15) or FEZ-1 structures
(50).
Based on x-ray crystallographic structures, Asn-233 and Zn1 were thought to
form an oxyanion hole to polarize the carbonyl group of the
-lactam
antibiotic (50,
51). In BlaB, Asn-233 is
replaced by Tyr, and the hydroxyl of the tyrosine is 7.88 Å away from
Zn1 (Fig. 2c).
Interestingly, Glu-236, which is an aspartic acid residue in the BcII, CcrA,
and IMP-1 enzymes, makes three hydrogen-bonds with OH Tyr-233, NEZ His-118 (a
zinc ligand), and a water molecule, whereas Asp-236 in the other three x-ray
structures only hydrogen-bonds to NEZ His-118 and a water molecule.
Consequently, no oxyanion hole is observed in the present structure. However,
the observed position of Tyr-233 could be the result of complex formation,
since C3, C5, and C6 of
D-captopril make hydrophobic contacts with the tyrosine side chain.
Nevertheless, rotation around
1 is sterically forbidden, and
only a large conformational change of the 227237 loop
(Fig. 2c) upon
substrate binding could bring OH Tyr-233 or other side chain residues into
positions where they could form part of the oxyanion hole.
Binding of the InhibitorThe main interaction between BlaB and D-captopril occurs through intercalation of the sulfur atom of the inhibitor between the two Zn2+ ions, leading to the removal of the activated water (OH group) (Fig. 2, a and b; Table II). A similar situation has been seen in the three-dimensional structure of IMP-1 in complex with a mercaptocarboxylate inhibitor (2-mercaptomethyl-4-phenyl-butyrylimino)-(5-tetrazol-1-ylmethyl-thiophen-2-yl)-acetic acid) (30).
Structural superposition of the structures of the two protein-inhibitor complexes shows that the inhibitors intercalate their sulfur groups almost identically between the two Zn2+ ions of the active site. A striking observation is that, although they are located in different parts of the active site, together the two inhibitors cover the entire active site cavity (Fig. 3a). The fusion of these two inhibitor molecules could be a good starting point for generating a new family of more effective inhibitors (Fig. 3b). Based on the structural superposition using this new molecule, many interactions would be retained, including those between the S atom and the two Zn2+ ions, the carboxylate groups and the main-chain amide nitrogens (N119 and N233), the carboxylate groups and the NZ of two lysines (Lys-167 and Lys-224), and many hydrophobic contacts.
In the crystal structure of IMP-1 complexed with a succinic acid derivative (33), one oxygen of the carboxylic group of the inhibitor is located between the two Zn2+ ions, displacing the hydroxyl. Curiously, in the three-dimensional structure of the subclass B3 MBL FEZ-1 complexed with D-captopril (50), the inhibitor is not located in the active site, because a sulfate ion from the crystallization solution intercalates one of its oxygen atoms between the two Zn2+ ions. In the complex of B. fragilis CcrA with a biphenyl tetrazole (23), the tetrazole ring of the inhibitor acts as an extra ligand for the second Zn2+, changing its coordination and provoking a distortion of the active site and the removal of the water molecules from the active site.
Taking together our results and all currently known three-dimensional
structures of MBLs complexed with inhibitors
(23,
30,
32,
33), we can conclude that
there is a high variability in the modes of binding of the inhibitors
depending on the
-lactamase sequence and the size and chemical
properties of the inhibitor. However, one feature is constant, which is the
intercalation of a charged group between the Zn2+ ions
and the removal of the bridging hydroxide directly involved in water-mediated
hydrolysis of the substrate. Moreover, the new BlaB structure presented here
highlights the structural diversity in the MBL family. The evolution of broad
substrate profile enzymes, such as BlaB, poses a serious threat to
antibacterial therapy, and a detailed analysis of inhibitor-enzyme complexes
is vital for the design of new inhibitors.
| FOOTNOTES |
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* This work was supported by European Union Human and Mobility Grant
HPRN-CT-2002-00264 and Belgian Government Grant PAI P5/33. The costs of
publication of this article were defrayed in part by the payment of page
charges. This article must therefore be hereby marked
"advertisement" in accordance with 18 U.S.C. Section 1734
solely to indicate this fact. ![]()

To whom correspondence should be addressed. Tel.: 33-4-38-78-56-09; Fax:
33-4-38-78-54-94; E-mail:
otto{at}ibs.fr.
1 The abbreviations used are: MBL, metallo-
-lactamase; Mes,
4-morpholineethanesulfonic acid. ![]()
2 H. W. Adolph, personal communication. ![]()
| ACKNOWLEDGMENTS |
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| REFERENCES |
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