Azotobacter vinelandii Ferredoxin I

Abstract

The reduction potential (E 0 ) of the [4Fe-4S]2؉/؉ cluster of Azotobacter vinelandii ferredoxin I (AvFdI) and related ferredoxins is ϳ200 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 Most iron-sulfur ([Fe-S]) proteins are intimately involved in essential electron transfer reactions, including membranebound electron transport systems; their functions, however, are not restricted to electron transfer and include direct catalysis of hydration/dehydration reactions, O 2 and iron sensing, regulation of gene expression, iron storage, metal cluster assembly, and the generation and stabilization of radical intermediates (for reviews, see Refs. 1-6). To meet these diverse functions, individual proteins have increased the variety of [Fe-S] cluster structures by adding or subtracting iron and sulfide atoms to vary the cluster type, or by introducing noncysteine ligands. Even after adopting a specific cluster type (e.g. [4Fe-4S] clusters with four cysteine ligands), proteins can control reactivity further by stabilizing a particular redox couple (e.g. 3ϩ/2ϩ, 2ϩ/ϩ, or ϩ/0) or by modulation of the reduction potential (E 0 Ј) of a particular redox couple (7-19). Thus, high potential iron proteins have 3ϩ/2ϩ E 0 Ј ranging from 90 to 450 mV (7)(8)(9)(10)(11)(12)(13)(14)(15)(16)20), while ferredoxins that contain structurally indistinguishable 2ϩ/ϩ clusters have E 0 Ј ranging from Ϫ280 to Ϫ715 mV in different native proteins (10, 21).Both experimental and theoretical research has been directed toward understanding how the polypeptide surrounding the cluster controls the reduction potential. Factors that have been proposed as being important include (a) solvent exposure of the cluster, (b) specific hydrogen bonding networks especially NH-S bonds, (c) the proximity and orientation of protein backbone and side chain dipoles, and/or (d) the proximity of charged residues to the cluster (7,(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35). This study concerns protein control of the reduction potential of a 2ϩ/ϩ cluster that is ligated via a typical CXXCXXC motif with one remote Cys ligand.Previous studies in this area have focused on comparing the environments of the clusters in structurally characterized proteins. 2ϩ/ϩ cluster of Azotobacter vinelandii ferredoxin I (AvFdI) 1 has a low E 0 Ј of about Ϫ620 mV (pH 7.0) (36), whereas the analogous clusters in Peptostreptococcus asaccharolyticus ferredoxin (PaFd) and Clostridium acidiurici ferredoxin have E 0 Ј of about Ϫ430 mV (not pH-dependent) (37). Comparison of the structures of these proteins showed that the peptide folding around the analogous clusters is highly conserved with respect to the location of the four Cys ligands, the Cys dihedral angles, and the eight amide groups hydr...

Redox Reactions of the Iron-Sulfur Cluster in a Ribosomal RNA Methyltransferase, RumA

“…Of particular interest in many contemporary studies of FeS proteins is understanding the factors that influence the potentials, and thus functions, of these clusters (17,18). Proteins with four Cys-iron bonds to the [4Fe-4S] cluster fall into two general redox groups, ones with an accessible 2ϩ/1ϩ couple, exemplified by some ferredoxins (19) and ones, generally called HiPiPs, in which the 2ϩ form can be oxidized to a stable 3ϩ form. The potential of the 2ϩ/1ϩ couple is variable among known [4Fe-4S] clusters.…”

Theoretical and experimental investigation of surface-confined two-center metalloproteins by large-amplitude Fourier transformed ac voltammetry

“…The small ferredoxin known as AvFdI (FdI) is a 7Fe electrontransfer protein containing two FeS clusters -[3Fe-4S] and [4Fe-4S], that function independently with respect to their electron transfer reactions [10][11][12][13][14]. Three FdI variants (WT, F2H and F25H) that exhibit variations in the E o 0 values for the [3Fe-4S] and [4Fe-4S] clusters were investigated by dc cyclic and FT ac voltammetry.…”

“…The crystal structure of FdI, shown in Fig. 1, depicts the wild-type protein and two variants having single point mutations that directly affect the potentials of one or both FeS clusters [10][11][12][13][14]. The essential pH dependence of those proteins are also presented [13].…”

“…In the organism, FdI functions as an electron carrier with the [3Fe-4S] cluster operating at a potential close to that of the H + /H 2 couple (À0.6 V versus Ag/AgCl at pH 7) whereas the [4Fe-4S] cluster has a potential that is approximately 0.2 V more negative [10][11][12][13][14]. Extensive electrochemical studies of this wild-type protein are available [10][11][12][13][14]. Large changes in [Fe-S] reduction potentials ($100 mV to $200 mV) are observed following the replacement, by site directed mutagenesis, of a phenylalanine (F) residue close to the [4Fe-4S] or both [Fe-S] clusters by the ionizable residue histidine (H).…”

Unzipping double-stranded DNA with a force: Numerical results

“…The calculation of the probability function of the LN model is then translated into solving a Schrödinger type equation. For each generated sequence, the Schrödinger equation is solved and thus yields an exact result with no arbitrary approximations [12].…”