title: Azotobacter vinelandii ferredoxin I: a sequence and structure comparison approach to alteration of [4Fe-4S]2+/+ reduction potential.
abstract: The reduction potential (E(0)') of the [4Fe-4S](2+/+) cluster of Azotobacter vinelandii ferredoxin I (AvFdI) and related ferredoxins is approximately 200 mV more negative than the corresponding clusters of Peptostreptococcus asaccharolyticus ferredoxin and related ferredoxins. Previous studies have shown that these differences in E(0)' do not result from the presence or absence of negatively charged surface residues or in differences in the types of hydrophobic residues found close to the [4Fe-4S](2+/+) clusters. Recently, a third, quite distinct class of ferredoxins (represented by the structurally characterized Chromatium vinosum ferredoxin) was shown to have a [4Fe-4S](2+/+) cluster with a very negative E(0)' similar to that of AvFdI. The observation that the sequences and structures surrounding the very negative E(0)' clusters in quite dissimilar proteins were almost identical inspired the construction of three additional mutations in the region of the [4Fe-4S](2+/+) cluster of AvFdI. The three mutations, V19E, P47S, and L44S, that incorporated residues found in the higher E(0)' P. asaccharolyticus ferredoxin all led to increases in E(0)' for a total of 130 mV with a 94-mV increase in the case of L44S. The results are interpreted in terms of x-ray structures of the FdI variants and show that the major determinant for the large increase in L44S is the introduction of an OH-S bond between the introduced Ser side chain and the Sgamma atom of Cys ligand 42 and an accompanying movement of water.
authors.affiliation: Department of Molecular Biology and Biochemistry and Irvine Research Unit in Macromolecular Structure, University of California, Irvine, California 92697, USA.
authors.forename: Kaisheng
authors.initials: K
authors.lastname: Chen
authors.forename: Yean-Sung
authors.initials: YS
authors.lastname: Jung
authors.forename: Christopher A
authors.initials: CA
authors.lastname: Bonagura
authors.forename: Gareth J
authors.initials: GJ
authors.lastname: Tilley
authors.forename: G Sridhar
authors.initials: GS
authors.lastname: Prasad
authors.forename: Vandana
authors.initials: V
authors.lastname: Sridhar
authors.forename: Fraser A
authors.initials: FA
authors.lastname: Armstrong
authors.forename: C David
authors.initials: CD
authors.lastname: Stout
authors.forename: Barbara K
authors.initials: BK
authors.lastname: Burgess
mesh_terms: D000595:Amino Acid Sequence; D016948:Azotobacter vinelandii / Q000737:chemistry*; D001665:Binding Sites; D018360:Crystallography, X-Ray; D004563:Electrochemistry; D004578:Electron Spin Resonance Spectroscopy; D005288:Ferredoxins / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007501:Iron / Q000378:metabolism*; D008024:Ligands; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D011485:Protein Binding; D011487:Protein Conformation; D017386:Sequence Homology, Amino Acid; D014466:Ultraviolet Rays
chemical_list: D005288:Ferredoxins; D008024:Ligands; D007501:Iron