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Volume 272, Number 25, Issue of June 20, 1997 pp. 15620-15627
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.

Y13C Azotobacter vinelandii Ferredoxin I
A DESIGNED [Fe-S] LIGAND MOTIF CONTAINS A CYSTEINE PERSULFIDE

(Received for publication, January 10, 1997, and in revised form, March 3, 1997)

Mary A. Kemper , C. David Stout ^§ , Sarah E. J. Lloyd ^§ , G. Sridhar Prasad ^§ , Sarah Fawcett ^¶ , Fraser A. Armstrong ^¶ , Binghui Shen and Barbara K. Burgess

From the  Department of Molecular Biology and Biochemistry, University of California, Irvine, California 92697-3900, ^§ Department of Molecular Biology, The Scripps Research Institute, La Jolla, California 92037, and ^¶ Department of Chemistry, Oxford University, Oxford, United Kingdom

Ferredoxins that contain [4Fe-4S]^2+/+ clusters often obtain three of their four cysteine ligands from a highly conserved CysXXCysXXCys sequence motif. Little is known about the in vivo assembly of these clusters and the role that this sequence motif plays in that process. In this study, we have used structure as a guide in attempts to direct the formation of a [4Fe-4S]^2+/+ in the [3Fe-4S]^+/0 location of native (7Fe) Azotobacter vinelandii ferredoxin I (AvFdI) by providing the correct three-dimensional orientation of cysteine ligands without introducing a CysXXCysXXCys motif. Tyr¹³ of AvFdI occupies the position of the fourth ligating cysteine in the homologous and structurally characterized 8Fe ferredoxin from Peptococcus aerogenes and a Y13C variant of AvFdI could be easily modeled as an 8Fe protein. However, characterization of purified Y13C FdI by UV-visible spectra, circular dichroism, electron paramagnetic resonance spectroscopies, and by x-ray crystallography revealed that the protein failed to use the introduced cysteine as a ligand and retained its [3Fe-4S]^+/0 cluster. Further, electrochemical characterization showed that the redox potential and pH behavior of the cluster were unaffected by the substitution of Tyr by Cys. Although Y13C FdI is functional in vivo it does differ significantly from native FdI in that it is extremely unstable in the reduced state possibly due to increased solvent exposure of the [3Fe-4S]⁰ cluster. Surprisingly, the x-ray structure showed that the introduced cysteine was modified to become a persulfide. This modification may have occurred in vivo via the action of NifS, which is known to be expressed under the growth conditions used. It is interesting to note that neither of the two free cysteines present in FdI was modified. Thus, if NifS is involved in modifying the introduced cysteine there must be specificity to the reaction.

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