X-ray structural, functional and computational studies of the O2-sensitive E. coli hydrogenase-1 C19G variant reveal an unusual [4Fe-4S] cluster

The crystal structure of the Escherichia coli O2-sensitive C19G [NiFe]-hydrogenase-1 variant shows that the mutation results in a novel FeS cluster, proximal to the Ni-Fe active site. While the proximal cluster of the native O2-tolerant enzyme can transfer two electrons to that site, EPR spectroscopy shows that the modified cluster can transfer only one electron, this shortfall coinciding with O2 sensitivity. Computational studies on electron transfer help to explain how the structural and redox properties of the novel FeS cluster modulate the observed phenotype.

Generating single metalloprotein crystals in well-defined redox states: electrochemical control combined with infrared imaging of a NiFe hydrogenase crystal

We describe an approach to generating and verifying well-defined redox states in metalloprotein single crystals by combining electrochemical control with synchrotron infrared microspectroscopic imaging. For NiFe hydrogenase 1 from Escherichia coli we demonstrate fully reversible and uniform electrochemical reduction from the oxidised inactive to the fully reduced state, and temporally resolve steps during this reduction.

Influence of electrochemical properties in determining the sensitivity of [4Fe-4S] clusters in proteins to oxidative damage

Interconversion between [4Fe-4S] cubane and [3Fe-4S] cuboidal states represents one of the simplest structural changes an iron-sulphur cluster can undertake. This reaction is implicated in oxidative damage and in modulation of the activity and regulation of certain enzymes, and it is therefore important to understand the factors governing cluster stability and the processes that activate cluster conversion. In the present study, protein film voltammetry has been used to induce and monitor the oxidative conversion of [4Fe-4S] into [3Fe-4S] clusters in different variants of Azotobacter vinelandii ferredoxin I (AvFdI; the 8Fe form of the native protein), and DeltaThr(14)/DeltaAsp(15), Thr(14)-->Cys (T14C) and C42D mutants. The electrochemical results have been correlated with the differing oxygen sensitivities of [4Fe-4S] clusters, and comparisons have been drawn with other ferredoxins (Desulfovibrio africanus FdIII, Clostridium pasteurianum Fd, Thauera aromatica Fd and Pyrococcus furiosus Fd). In contrast with high-potential iron-sulphur proteins (HiPIPs) for which the oxidized species [4Fe-4S](3+) is inert to degradation and can be isolated, the hypervalent state in these ferredoxins (most obviously the 3+ level) is very labile, and the reduction potential at which this is formed is a key factor in determining the cluster's resistance to oxidative damage.

A distal histidine mutant (H52Q) of yeast cytochrome c peroxidase catalyzes the oxidation of H(2)O(2) instead of its reduction

A H52Q variant of yeast cytochrome c peroxidase (CcP), in which the distal histidine is replaced by glutamine, catalyzes oxidation of H(2)O(2) instead of reduction. This redirection of catalytic action is detected by protein film voltammetry. In the presence of H(2)O(2), wild-type CcP, adsorbed on a graphite electrode, shows a strong catalytic reduction wave commencing at about 0.8V (pH 5.4); by contrast, H52Q does not exhibit this activity but instead shows a catalytic oxidation current at potentials in the region of 0.9 V. The oxidation current is partly suppressed in the presence of tetranitromethane (a superoxide scavenger) and is not observed for other mutants studied, including H52A. The only significant structural change in the H52Q variant is that the Q-52 side chain occupies the space vacated by the H-52 imidazole; specifically, the N-epsilon atom that is believed to transfer a proton and induce O--O cleavage is replaced, to within 0.75 A, by the carbamide-O. Thus, while the weakly basic amide functionality is unable to serve in the reorganization of bound H(2)O(2), it is able to facilitate its oxidation, most obviously by serving as a H-bond acceptor to assist formation of a labile superoxide intermediate.

Enzyme electrokinetics: energetics of succinate oxidation by fumarate reductase and succinate dehydrogenase

Protein film voltammetry is used to probe the energetics of electron transfer and substrate binding at the active site of a respiratory flavoenzyme--the membrane-extrinsic catalytic domain of Escherichia coli fumarate reductase (FrdAB). The activity as a function of the electrochemical driving force is revealed in catalytic voltammograms, the shapes of which are interpreted using a Michaelis-Menten model that incorporates the potential dimension. Voltammetric experiments carried out at room temperature under turnover conditions reveal the reduction potentials of the FAD, the stability of the semiquinone, relevant protonation states, and pH-dependent succinate--enzyme binding constants for all three redox states of the FAD. Fast-scan experiments in the presence of substrate confirm the value of the two-electron reduction potential of the FAD and show that product release is not rate limiting. The sequence of binding and protonation events over the whole catalytic cycle is deduced. Importantly, comparisons are made with the electrocatalytic properties of SDH, the membrane-extrinsic catalytic domain of mitochondrial complex II.

Determination of an optimal potential window for catalysis by E. coli dimethyl sulfoxide reductase and hypothesis on the role of Mo(V) in the reaction pathway

Protein film voltammetry (PFV) of Escherichia coli dimethyl sulfoxide (DMSO) reductase (DmsABC) adsorbed at a graphite electrode reveals that the catalytic activity of this complex Mo-pterin/Fe-S enzyme is optimized within a narrow window of electrode potential. The upper and lower limits of this window are determined from the potential dependences of catalytic activity in reducing and oxidizing directions; i.e., for reduction of DMSO (or trimethylamine-N-oxide) and oxidation of trimethylphosphine (PMe(3)). At either limit, the catalytic activity drops despite the increase in driving force: as the potential is lowered below -200 mV (pH 7.0-8.9), the rate of reduction of DMSO decreases abruptly, while for PMe(3), an oxidative current is observed that vanishes as the potential is raised above +20 mV (pH 9.0). Analysis of the waveshapes reveals that both activity thresholds result from one-electron redox reactions that arise, most likely, from groups within the enzyme; if so, they represent "switches" that reflect the catalytic mechanism and may be of physiological relevance. The potential window of activity coincides approximately with the appearance of the Mo(V) EPR signal observed in potentiometric titrations, suggesting that crucial stages of catalysis are facilitated while the active site is in the intermediate Mo(V) oxidation state.

Fast voltammetric studies of the kinetics and energetics of coupled electron-transfer reactions in proteins

A wealth of information on the reactions of redox-active sites in proteins can be obtained by voltammetric studies in which the protein sample is arranged as a layer on an electrode surface. By carrying out cyclic voltammetry over a wide range of scan rates and exploiting the ability to poise or pulse the electrode potential between cycles, data are obtained that are conveniently (albeit simplistically) analysed in terms of plots of peak potentials against scan rate. A simple reversible electron-transfer process gives rise to a 'trumpet'-shaped plot because the oxidation and reduction peaks separate increasingly at high scan rate; the electrochemical kinetics are then determined by fitting to Butler-Volmer or Marcus models. Much more interesting though are the ways in which this 'trumpet plot' is altered, often dramatically, when electron transfer is coupled to biologically important processes such as proton transfer, ligand exchange, or a change in conformation. It is then possible to derive particularly detailed information on the kinetics, energetics and mechanism of reactions that may not revealed clearly or even at all by other methods. In order to interpret the voltammetry of coupled systems, it is important to be able to define 'ideal behaviour' for systems that are expected to show simple and uncoupled electron transfer. Accordingly, this paper describes results we have obtained for several proteins that are expected to show such behaviour, and compares these results with theoretical predictions.

Structure of C42D Azotobacter vinelandii FdI. A Cys-X-X-Asp-X-X-Cys motif ligates an air-stable [4Fe-4S]2+/+ cluster

All naturally occurring ferredoxins that have Cys-X-X-Asp-X-X-Cys motifs contain [4Fe-4S](2+/+) clusters that can be easily and reversibly converted to [3Fe-4S](+/0) clusters. In contrast, ferredoxins with unmodified Cys-X-X-Cys-X-X-Cys motifs assemble [4Fe-4S](2+/+) clusters that cannot be easily interconverted with [3Fe-4S](+/0) clusters. In this study we changed the central cysteine of the Cys(39)-X-X-Cys(42)-X-X-Cys(45) of Azotobacter vinelandii FdI, which coordinates its [4Fe-4S](2+/+) cluster, into an aspartate. UV-visible, EPR, and CD spectroscopies, metal analysis, and x-ray crystallography show that, like native FdI, aerobically purified C42D FdI is a seven-iron protein retaining its [4Fe-4S](2+/+) cluster with monodentate aspartate ligation to one iron. Unlike known clusters of this type the reduced [4Fe-4S](+) cluster of C42D FdI exhibits only an S = 1/2 EPR with no higher spin signals detected. The cluster shows only a minor change in reduction potential relative to the native protein. All attempts to convert the cluster to a 3Fe cluster using conventional methods of oxygen or ferricyanide oxidation or thiol exchange were not successful. The cluster conversion was ultimately accomplished using a new electrochemical method. Hydrophobic and electrostatic interaction and the lack of Gly residues adjacent to the Asp ligand explain the remarkable stability of this cluster.

Investigations of the oxidative disassembly of Fe-S clusters in Clostridium pasteurianum 8Fe ferredoxin using pulsed-protein-film voltammetry

Rapid responses of biological [4Fe-4S] clusters to conditions of oxidative stress have been studied by protein-film voltammetry by using precise pulses of electrode potential to trigger reactions. Investigations with Clostridium pasteurianum 8Fe ferredoxin exploit the fact that [3Fe-4S] clusters display a characteristic pattern of voltammetric signals, so that their appearance and disappearance after an oxidative pulse can be tracked unambiguously under electrochemical control. Adsorbed to monolayer coverage at a graphite electrode, the protein initially shows a strong signal (B') at -0.36 V vs standard hydrogen electrode due to two [4Fe-4S](2+/+) clusters at similar potentials. Short square pulses (0.1-5 s) to potentials in the range 0.5-0.9 V cause extensive loss of B', and new signals appear (A'and C') that arise from [3Fe-4S] species (+/0 and 0/2- couples). The A' and B' intensities quantify transformations which are induced by the pulse and which occur subsequently when more reducing conditions are restored. Optimal [3Fe-4S] formation (in excess over [4Fe-4S]) is achieved with a 3-s pulse to 0.7 V, following which there is rapid partial recovery to yield a 1:1 3Fe:4Fe ratio, consistent with 7Fe protein. Thus, a 6Fe protein is formed, but one of the clusters is rapidly repaired. The [3Fe-4S]:[4Fe-4S] ratio follows a bell-shaped curve spanning the same potential range that defines complete loss of signals, while double-pulse experiments show that [3Fe-4S](+) resists further oxidative damage. Oxidative disassembly involves successive one-electron oxidations of [4Fe-4S] (i.e., 2+ --> 3+ --> 4+), with [3Fe-4S](+) being a relatively stable byproduct, that is, not an intermediate. Disassembly of [3Fe-4S] in the 7Fe protein continues after reducing conditions are restored, with lifetimes depending on oxidation level; thus 1+ (most stable) > 0 > 2-. In the presence of Fe(2+), the 0 level is stabilized by conversion back to [4Fe-4S](2+/+). By pulsing in the presence of Zn(2+), the [3Fe-4S] clusters that are formed are trapped rapidly as their Zn adducts.

Atomically defined mechanism for proton transfer to a buried redox centre in a protein

The basis of the chemiosmotic theory is that energy from light or respiration is used to generate a trans-membrane proton gradient. This is largely achieved by membrane-spanning enzymes known as 'proton pumps. There is intense interest in experiments which reveal, at the molecular level, how protons are drawn through proteins. Here we report the mechanism, at atomic resolution, for a single long-range electron-coupled proton transfer. In Azotobacter vinelandii ferredoxin I, reduction of a buried iron-sulphur cluster draws in a solvent proton, whereas re-oxidation is 'gated' by proton release to the solvent. Studies of this 'proton-transferring module' by fast-scan protein film voltammetry, high-resolution crystallography, site-directed mutagenesis and molecular dynamics, reveal that proton transfer is exquisitely sensitive to the position and pK of a single amino acid. The proton is delivered through the protein matrix by rapid penetrative excursions of the side-chain carboxylate of a surface residue (Asp 15), whose pK shifts in response to the electrostatic charge on the iron-sulphur cluster. Our analysis defines the structural, dynamic and energetic requirements for proton courier groups in redox-driven proton-pumping enzymes.

Unusual spectroscopic and electrochemical properties of the 2[4Fe-4S] ferredoxin of Thauera aromatica

A reduced ferredoxin serves as the natural electron donor for key enzymes of the anaerobic aromatic metabolism in the denitrifying bacterium Thauera aromatica. It contains two [4Fe-4S] clusters and belongs to the Chromatium vinosum type of ferredoxins (CvFd) which differ from the "clostridial" type by a six-amino acid insertion between two successive cysteines and a C-terminal alpha-helical amino acid extension. The electrochemical and electron paramagnetic resonance (EPR) spectroscopic properties of both [4Fe-4S] clusters from T. aromatica ferredoxin have been investigated using cyclic voltammetry and multifrequency EPR. Results obtained from cyclic voltammetry revealed the presence of two redox transitions at -431 and -587 mV versus SHE. X-band EPR spectra recorded at potentials where only one cluster was reduced (greater than -500 mV) indicated the presence of a spin mixture of S = (3)/(2) and (5)/(2) spin states of one reduced [4Fe-4S] cluster. No typical S = (1)/(2) EPR signals were observed. At lower potentials (less than -500 mV), the more negative [4Fe-4S] cluster displayed Q-, X-, and S-band EPR spectra at 20 K which were typical of a single S = (1)/(2) low-spin [4Fe-4S] cluster with a g(av) of 1.94. However, when the temperature was decreased stepwise to 4 K, a magnetic interaction between the two clusters gradually became observable as a temperature-dependent splitting of both the S = (1)/(2) and S = (5)/(2) EPR signals. At potentials where both clusters were reduced, additional low-field EPR signals were observed which can only be assigned to spin states with spins of >(5)/(2). The results that were obtained establish that the common typical amino acid sequence features of CvFd-type ferredoxins determine the unusual electrochemical properties of the [4Fe-4S] clusters. The observation of different spin states in T. aromatica ferredoxin is novel among CvFd-type ferredoxins.

Ferredoxin III of Desulfovibrio africanus: sequencing of the native gene and characterization of a histidine-tagged form

Desulfovibrio africanus ferredoxin III (Da FdIII) contains one [4Fe-4S](2+/1+) cluster and one [3Fe-4S](1+/0) cluster, bound by seven Cys residues, in which the [3Fe-4S] cluster is co-ordinated by the unusual sequence, Cys(11)-Xaa-Xaa-Asp(14)-Xaa-Xaa-Cys(17)-Xaa(n)-Cys(51)-Glu. The [3Fe-4S] core of this ferredoxin is so far unique in showing rapid bi-directional [3Fe-4S]<-->[4Fe-4S] cluster interconversion with a wide range of metal ions. In order to obtain protein for mutagenesis studies Da FdIII has been cloned, sequenced, and expressed as a hexa-histidine tagged (ht) polypeptide in Escherichia coli strain BL21(DE3) pLysS. Expression of ht Da FdIII, whether translated from a synthetic gene (pJB10) or from the native nucleotide sequence (pJB11), occurred at similar levels (approx. 6 mg.l(-1)), but without incorporation of metal clusters. The nucleotide sequence confirms the protein sequence reported previously [Bovier-Lapierre, Bruschi, Bonicel and Hatchikian (1987) Biochim. Biophys. Acta 913, 20-26]. Cluster incorporation was achieved using FeCl(3) together with cysteine sulphur transferase, NifS, plus cysteine to generate low levels of sulphide ions. Absorption and EPR spectroscopy show that both [3Fe-4S] and [4Fe-4S] clusters are correctly inserted. Thin-film electrochemistry provides evidence that the [3Fe-4S] cluster undergoes reversible cluster transformation in the presence of Fe(II) and Zn(II) ions with properties identical to the native protein. Nevertheless the protein has lower stability than native Da FdIII during chromatography. The one-dimensional 600 MHz NMR spectrum of the apoprotein indicates an unstructured protein with random coil chemical shifts whereas spectra of the reconstituted ht protein show secondary structural elements and 18 peaks shifted downfield of 9.6 p.p.m. The spectra are unique but have similarities with the shift patterns seen with 7Fe Desulfurolobus ambivalens Fd. The ht does not affect iron-sulphur cluster incorporation, but NMR evidence suggests that excess Fe binds to the tag. This may account for the lower stability of the ht compared with the native protein.

Alteration of the reduction potential of the [4Fe-4S](2+/+) cluster of Azotobacter vinelandii ferredoxin I

The [4Fe-4S](2+/+) cluster of Azotobacter vinelandii ferredoxin I (FdI) has an unusually low reduction potential (E(0')) relative to other structurally similar ferredoxins. Previous attempts to raise that E(0') by modification of surface charged residues were unsuccessful. In this study mutants were designed to alter the E(0') by substitution of polar residues for nonpolar residues near the cluster and by modification of backbone amides. Three FdI variants, P21G, I40N, and I40Q, were purified and characterized, and electrochemical E(0') measurements show that all had altered E(0') relative to native FdI. For P21G FdI and I40Q FdI, the E(0') increased by +42 and +53 mV, respectively validating the importance of dipole orientation in control of E(0'). Protein Dipole Langevin Dipole calculations based on models for those variants accurately predicted the direction of the change in E(0') while overestimating the magnitude. For I40N FdI, initial calculations based on the model predicted a +168 mV change in E(0') while a -33 mV change was observed. The x-ray structure of that variant, which was determined to 2.8 A, revealed a number of changes in backbone and side chain dipole orientation and in solvent accessibility, that were not predicted by the model and that were likely to influence E(0'). Subsequent Protein Dipole Langevin Dipole calculations (using the actual I40N x-ray structures) did quite accurately predict the observed change in E(0').

Voltammetric studies of bidirectional catalytic electron transport in Escherichia coli succinate dehydrogenase: comparison with the enzyme from beef heart mitochondria

The succinate dehydrogenases (SDH: soluble, membrane-extrinsic subunits of succinate:quinone oxidoreductases) from Escherichia coli and beef heart mitochondria each adsorb at a pyrolytic graphite 'edge' electrode and catalyse the interconversion of succinate and fumarate according to the electrochemical potential that is applied. E. coli and beef heart mitochondrial SDH share only ca. 50% homology, yet the steady-state catalytic activities, when measured over a continuous potential range, display very similar catalytic operating potentials and energetic biases (the relative ability to catalyse succinate oxidation vs. fumarate reduction). Importantly, E. coli SDH also exhibits the interesting 'tunnel-diode' behaviour previously reported for the mitochondrial enzyme. Thus as the potential is lowered below ca. -60 mV (pH 7, 38 degrees C) the rate of catalytic fumarate reduction decreases abruptly despite an increase in driving force. Since the homology relates primarily to residues associated with active site regions, the marked similarity in the voltammetry reaffirms our previous conclusions that the tunnel-diode behaviour is a characteristic property of the enzyme active site. Thus, succinate dehydrogenase is an excellent fumarate reductase, but its activity in this direction is limited to a very specific range of potential.

Catalytic electron transport in Chromatium vinosum [NiFe]-hydrogenase: application of voltammetry in detecting redox-active centers and establishing that hydrogen oxidation is very fast even at potentials close to the reversible H+/H2 value

The nickel-iron hydrogenase from Chromatium vinosum adsorbs at a pyrolytic graphite edge-plane (PGE) electrode and catalyzes rapid interconversion of H(+)((aq)) and H(2) at potentials expected for the half-cell reaction 2H(+) right arrow over left arrow H(2), i.e., without the need for overpotentials. The voltammetry mirrors characteristics determined by conventional methods, while affording the capabilities for exquisite control and measurement of potential-dependent activities and substrate-product mass transport. Oxidation of H(2) is extremely rapid; at 10% partial pressure H(2), mass transport control persists even at the highest electrode rotation rates. The turnover number for H(2) oxidation lies in the range of 1500-9000 s(-)(1) at 30 degrees C (pH 5-8), which is significantly higher than that observed using methylene blue as the electron acceptor. By contrast, proton reduction is slower and controlled by processes occurring in the enzyme. Carbon monoxide, which binds reversibly to the NiFe site in the active form, inhibits electrocatalysis and allows improved definition of signals that can be attributed to the reversible (non-turnover) oxidation and reduction of redox centers. One signal, at -30 mV vs SHE (pH 7.0, 30 degrees C), is assigned to the [3Fe-4S](+/0) cluster on the basis of potentiometric measurements. The second, at -301 mV and having a 1. 5-2.5-fold greater amplitude, is tentatively assigned to the two [4Fe-4S](2+/+) clusters with similar reduction potentials. No other redox couples are observed, suggesting that these two sets of centers are the only ones in CO-inhibited hydrogenase capable of undergoing simple rapid cycling of their redox states. With the buried NiFe active site very unlikely to undergo direct electron exchange with the electrode, at least one and more likely each of the three iron-sulfur clusters must serve as relay sites. The fact that H(2) oxidation is rapid even at potentials nearly 300 mV more negative than the reduction potential of the [3Fe-4S](+/0) cluster shows that its singularly high equilibrium reduction potential does not compromise catalytic efficiency.

Thermodynamic influences on the fidelity of iron-sulphur cluster formation in proteins

In an organism, two thermodynamic factors are important in ensuring that homometallic [4Fe-4S] cubane clusters are formed in preference to clusters containing heterometals such as Zn or Cu. These are the electronic resonance stabilisation, which boosts the binding of Fe(II) within an Fe-S cluster relative to its normally low position in the Irving-Williams order, and attenuation of the cytoplasmic concentrations of competing metals such as Zn or Cu by specific ligands.

Redox properties of flavocytochrome c3 from Shewanella frigidimarina NCIMB400

The thermodynamic and catalytic properties of flavocytochrome c3 from Shewanella frigidimarina have been studied using a combination of protein film voltammetry and solution methods. As measured by solution kinetics, maximum catalytic efficiencies for fumarate reduction (kcat/Km = 2.1 x 10(7) M-1 s-1 at pH 7.2) and succinate oxidation (kcat/Km = 933 M-1 s-1 at pH 8.5) confirm that flavocytochrome c3 is a unidirectional fumarate reductase. Very similar catalytic properties are observed for the enzyme adsorbed to monolayer coverage at a pyrolytic graphite "edge" electrode, thus confirming the validity of the electrochemical method for providing complementary information. In the absence of fumarate, the adsorbed enzyme displays a complex envelope of reversible redox signals which can be deconvoluted to yield the contributions from each active site. Importantly, the envelope is dominated by the two-electron signal due to FAD [E degrees ' = -152 mV vs the standard hydrogen electrode (SHE) at pH 7.0 and 24 degrees C] which enables quantitative examination of this center, the visible spectrum of which is otherwise masked by the intense absorption bands due to the hemes. The FAD behaves as a cooperative two-electron center with a pH-dependent reduction potential that is modulated (pKox at 6.5) by ionization of a nearby residue. In conjunction with the kinetic pKa values determined for the forward and reverse reactions (7.4 and 8.6, respectively), a mechanism for fumarate reduction, incorporating His365 and an anionic form of reduced FAD, is proposed. The reduction potentials of the four heme groups, estimated by analysis of the underlying envelope, are -102, -146, -196, and -238 mV versus the SHE at pH 7.0 and 24 degrees C and are comparable to those determined by redox potentiometry.

A T14C variant of Azotobacter vinelandii ferredoxin I undergoes facile [3Fe-4S]0 to [4Fe-4S]2+ conversion in vitro but not in vivo

[4Fe-4S]2+/+ clusters that are ligated by Cys-X-X-Cys-X-X-Cys sequence motifs share the general feature of being hard to convert to [3Fe-4S]+/0 clusters, whereas those that contain a Cys-X-X-Asp-X-X-Cys motif undergo facile and reversible cluster interconversion. Little is known about the factors that control the in vivo assembly and conversion of these clusters. In this study we have designed and constructed a 3Fe to 4Fe cluster conversion variant of Azotobacter vinelandii ferredoxin I (FdI) in which the sequence that ligates the [3Fe-4S] cluster in native FdI was altered by converting a nearby residue, Thr-14, to Cys. Spectroscopic and electrochemical characterization shows that when purified in the presence of dithionite, T14C FdI is an O2-sensitive 8Fe protein. Both the new and the indigenous clusters have reduction potentials that are significantly shifted compared with those in native FdI, strongly suggesting a significantly altered environment around the clusters. Interestingly, whole cell EPR have revealed that T14C FdI exists as a 7Fe protein in vivo. This 7Fe form of T14C FdI is extremely similar to native FdI in its spectroscopic, electrochemical, and structural features. However, unlike native FdI which does not undergo facile cluster conversion, the 7Fe form T14C FdI quickly converts to the 8Fe form with a high efficiency under reducing conditions.

Fast-scan cyclic voltammetry of protein films on pyrolytic graphite edge electrodes: characteristics of electron exchange

The rapid electron-exchange characteristics of metalloproteins adsorbed at a pyrolytic graphite "edge" electrode have been studied by analog dc cyclic voltammetry at scan rates up to 3000 V s-1. The voltammetry of four proteins, azurin (a "blue" copper protein) and three 7Fe ferredoxins, reveals oxidation and reduction peaks that display only modest increases in width and peak separation as the scan rate is raised. This is indicative of a substantially homogeneous population of noninteracting centers which undergo rapid electron exchange with the electrode. Both the Butler--Volmer and Marcus models have been tested. The electrochemical kinetics, as reflected by k0 (the rate at zero overpotential), are too fast to allow the determination of reorganization energies by this method. Nonetheless, the rapid and energetically coherent nature of the electron transfer enables the cyclic oxidation and reduction of protein redox centers to be examined on a time scale sufficiently short to recognize coupled processes occurring in the millisecond time domain, which are characteristic of the protein under investigation. Two of the ferredoxins display increasingly asymmetric voltammetry as the scan rate is increased, which is attributed to the coupling of electron transfer to conformational (or orientational) changes. For azurin, the use of higher electrolyte concentrations enables studies to be made at scan rates up to 3000 V s-1, from which a standard electron-transfer rate constant in the region of 5000 s-1 is obtained. At these high scan rates, azurin still shows very symmetrical voltammograms but with peak shapes displaying a more gradual decrease in current, at increasing overpotential, than is predicted using realistic values of the reorganization energy. The ability to measure even faster rate constants and access coupled reactions occurring in shorter time domains is likely to be limited by complex processes occurring on the graphite surface.

Voltammetric studies of the reactions of iron-sulphur clusters ([3Fe-4S] or [M3Fe-4S]) formed in Pyrococcus furiosus ferredoxin

Reactions of the [3Fe-4S] cluster and various metallated [M3Fe-4S] adducts co-ordinated in the ferredoxin from the hyperthermophile Pyrococcus furiosus have been studied by protein-film voltammetry, bulk-solution voltammetry, solution kinetics and magnetic CD (MCD). The [3Fe-4S] cluster exhibits two couples, [3Fe-4S]+/0 and [3Fe-4S]0/2-. Film voltammetry is possible over a wide pH range (2-8), revealing that the [3Fe-4S]+/0 couple shows a complex pH dependence with pKred1=2.8, pKox=4.9 and pKred2=6.7. From MCD, pKred1 corresponds with protonation of [3Fe-4S]0 to give a spectroscopically distinct species, as reported for ferredoxins from Azotobacter and Sulfolobus. The status of the disulphide/disulphydryl entity makes no significant difference to the data (given for the -S-S- form). Formation of the hyper-reduced [3Fe-4S]2- state is observed, requiring 3H+ for the overall 3e- reduction of [3Fe-4S]+, the change therefore being electroneutral. By comparison with the ferredoxin from Desulfovibrio africanus, uptake of Fe(II) and other M(II) by [3Fe-4S]0 to give [M3Fe-4S] clusters is slow (t1/2>10 min at room temperature, slower still if the protein is adsorbed on the electrode), whereas reaction with Tl(I) to produce [Tl3Fe-4S] is very rapid (t1/2<<1 s), suggesting that co-ordination of Tl does not require reorganization of the protein structure. Rates of formation of [3Fe-4S] from [M3Fe-4S] adducts increase sharply at high potentials, showing that metal release involves a labile 'super-oxidized' [M3Fe-4S]3+ state.

Delta T 14/Delta D 15 Azotobacter vinelandii ferredoxin I: creation of a new CysXXCysXXCys motif that ligates a [4Fe-4S] cluster

In clostridial-type ferredoxins, each of the two [4Fe-4S]2+/+ clusters receives three of its four ligands from a CysXXCysXXCys motif. Azotobacter vinelandii ferredoxin I (AvFdI) is a seven-iron ferredoxin that contains one [4Fe-4S]2+/+ cluster and one [3Fe-4S]+/0 cluster. During the evolution of the 7Fe azotobacter-type ferredoxins from the 8Fe clostridial-type ferredoxins, one of the two motifs present changed to a CysXXCysXXXXCys motif, resulting in the inability to form a 4Fe cluster and the appearance of a 3Fe cluster in that position. In a previous study, we were unsuccessful in using structure as a guide in designing a 4Fe cluster in the 3Fe cluster position of AvFdI. In this study, we have reversed part of the evolutionary process by deleting two residues between the second and third cysteines. UV/Vis, CD, and EPR spectroscopies and direct electrochemical studies of the purified protein reveal that this DeltaT14/DeltaD15 FdI variant is an 8Fe protein containing two [4Fe-4S]2+/+ clusters with reduction potentials of -466 and -612 mV versus SHE. Whole-cell EPR shows that the protein is present as an 8Fe protein in vivo. These data strongly suggest that it is the sequence motif rather than the exact sequence or the structure that is critical for the assembly of a 4Fe cluster in that region of the protein. The new oxygen-sensitive 4Fe cluster was converted in partial yield to a 3Fe cluster. In known ferredoxins and enzymes that contain reversibly interconvertible [4Fe-4S]2+/+ and [3Fe-4S]+/0 clusters, the 3Fe form always has a reduction potential ca. 200 mV more positive than the 4Fe cluster in the same position. In contrast, for DeltaT14/DeltaD15 FdI, the 3Fe and 4Fe clusters in the same location have extremely similar reduction potentials.

Discovery of a novel ferredoxin from Azotobacter vinelandii containing two [4Fe-4S] clusters with widely differing and very negative reduction potentials

Ferredoxins that contain 2[4Fe-4S]2+/+ clusters can be divided into two classes. The "clostridial-type" ferredoxins have two Cys-Xaa-Xaa-Cys-Xaa-Xaa-Cys-Xaa-Xaa-Xaa-Cys-Pro motifs. The "chromatium-type" ferredoxins have one motif of that type and one more unusual Cys-Xaa-Xaa-Cys-Xaa7-9-Cys-Xaa-Xaa-Xaa-Cys-Pro motif. Here we report the purification of a novel ferredoxin (FdIII) from Azotobacter vinelandii which brings to 12 the number of small [Fe-S] proteins that have now been reported from this organism. NH2-terminal sequencing of the first 56 amino acid residues shows that FdIII is a chromatium-type ferredoxin with 77% identity and 88% similarity to Chromatium vinosum ferredoxin. Studies of the purified protein by matrix-assisted laser desorption ionization-time of flight mass spectroscopy, iron analysis, absorption, circular dichroism, and electron paramagnetic resonance spectroscopies show that FdIII contains 2[4Fe-4S]2+/+ clusters in a 9,220-Da polypeptide. All 2[4Fe-4S]2+/+ ferredoxins that have been studied to date, including C. vinosum ferredoxin, are reported to have extremely similar or identical reduction potentials for the two clusters. In contrast, electrochemical characterization of FdIII clearly establishes that the two [4Fe-4S]2+/+ clusters have very different and highly negative reduction potentials of -486 mV and -644 mV versus the standard hydrogen electrode.

Y13C Azotobacter vinelandii ferredoxin I. A designed [Fe-S] ligand motif contains a cysteine persulfide

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.

[3Fe-4S] <--> [4Fe-4S] cluster interconversion in Desulfovibrio africanus ferredoxin III: properties of an Asp14 --> Cys mutant

The 8Fe ferredoxin III from Desulfovibrio africanus is a monomeric protein which contains two [4Fe-4S]2+/1+ clusters, one of which is labile and can readily and reversibly lose one Fe under oxidative conditions to yield a [3Fe-4S]1+/0 cluster. This 4Fe cluster has an S = 3/2 ground sping state insteaed of S = 1/2 in the reduced +1 state [George, Armstrong, Hatchikian and Thomson (1989) Biochem. J. 264, 275-284]. The co-ordination to this cluster is unusual in that an aspartate (Asp14, D14, is found where a cysteine residue normally occurs. Using a mutant protein obtained from the overexpression in Escherichia coli of a synthetic gene in which Asp14, the putative ligand to the removable Fe, has been changed to Cys, we have studied the cluster interconversion properties of the labile cluster. Analysis by EPR and magnetic-circular-dichroism spectroscopies showed that the Asp14 --> Cys (D14C) mutant contains two [4Fe-4S]2+/1+ clusters, both with S = 1/2 in the reduced state. Also, unlike in native 8Fe D. africanus ferredoxin III, the 4Fe <--> 3Fe cluster interconversion reaction was found to be sluggish and did not go to completion. It is inferred that the reversibility of the reaction in the native protein is due to the presence of the aspartate residue at position 14 and that this residue might protect the [3Fe-4S] cluster from further degradation.

Identification of the iron-sulfur clusters in a ferredoxin from the archaeon Sulfolobus acidocaldarius. Evidence for a reduced [3Fe-4S] cluster with pH-dependent electronic properties

A ferredoxin isolated from the archaeon Sulfolobus acidocaldarius strain DSM 639 has been shown to contain one [3Fe-4S]1 + 10 cluster with a reduction potential of -275 mV and one [4Fe-4S]2+/1+ cluster with a reduction potential of -529 mV at pH 6.4, in the temperature range 0-50 degrees C. The monomer molecular mass was confirmed to be 10907.5 +/- 1.0 Da by electrospray mass spectrometry, as calculated from the published amino acid sequence [Minami, Y. Wakabayashi. S., Wada, K., Matsubara, H., Kerscher, L. & Oesterhelt, D. (1985) J. Biochem. (Tokyo) 97, 745-751], while the holoprotein molecular mass was found to be 11,550 +/- 1.0 Da. The reduced [3Fe-4S]0 cluster was also shown by direct electrochemistry and magnetic circular dichroic spectroscopy to undergo a one-proton uptake reaction as first observed for Azotobacter chroococcum ferredoxin I [George, S. J., Richards, A. J. M., Thomson, A. J. & Yates, M. G. (1984) Biochem. J. 224, 247-251]. The pKa of the protonation step has been determined by a novel thin film electrochemical method to be 5.8. This is significantly different from the pKa of 7.7 determined for A. vinelandii ferredoxin I [Shen, B., Martin, L. L., Butt, J. N., Armstrong, F. A., Stout, C. D., Jensen, J. M., Stephens, P. J., LaMar, G. N., Gorst, C. M. & Burgess, B. K. (1993) J. Biol. Chem. 268, 25928-25939] and indicates that the polypeptide chain around the [3Fe-4S] cluster controls this reaction. Although this appears to be only the second reported case of protonation at or near the reduced [3Fe-4S]0 cluster, its observation in S. acidocaldarius ferredoxin raises the question of the generality of this chemistry for 3Fe clusters. The similarity of the pKa to the estimated intracellular pH of S. acidocaldarius strongly suggests a physiological role for this process.

Formation and properties of a stable 'high-potential' copper-iron-sulphur cluster in a ferredoxin

A ferredoxin isolated from Desulfovibrio africanus contains a [3Fe-4S] cluster that reversibly binds a copper atom, yielding a stable product with a greatly increased reduction potential. The reaction is readily detected in protein molecules adsorbed as a film on an electrode surface. Electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD) spectra of oxidized and reduced bulk solution products support their assignment as [Cu3Fe-4S]2+ (S = 1/2) and [Cu3Fe-4S]1+ (S = 2) respectively, with copper bound formally as Cu(I). Cyanide causes selective loss of copper and regeneration of the [3Fe-4S] reactant. The results demonstrate the chemical feasibility of CuFeS clusters and suggest that they could exist naturally in biological systems.

Azotobacter vinelandii ferredoxin I. Alteration of individual surface charges and the [4FE-4S]2+/+ cluster reduction potential

The structures of Azotobacter vinelandii ferredoxin I (AvFdI) and Peptococcus aerogenes ferredoxin (PaFd), near their analogous [4e-4S]2+/+ clusters, are highly conserved (Backes, G., Mino, Y., Loehr, T.M., Meyer, T.E., Cusanovich, M.A., Sweeney, W.V., Adman, E.T., and Sanders-Loehr, J. (1991) J. Am. Chem. Soc. 11, 2055-2064). Despite these similarities, the reduction potential (E0') of the AvFdI [4Fe-4S]2+/+ cluster is more than 200 mV more negative than that of PaFd. We have tested the contribution that individual amino acid residues make to the control of E0' by converting residues in AvFdI into the corresponding residue in PaFd. Four mutations involved substitutions of negatively charged surface residues with neutral residues and two involved substitution of buried hydrophobic residues. All AvFdI variants were characterized by x-ray crystallography, absorption, CD, EPR, and 1H NMR spectroscopies and by electrochemical methods. For the F25I mutation, significant structural changes occurred that affected the EPR and 1H NMR spectroscopic properties of AvFdI and had a minor influence on E0'. For all other mutations there were no changes in reduction potential. Thus we conclude, that variations in charged surface residues do not account for the observed differences in E0' between the analogous [4Fe-4S]2+/+ cluster of PaFd and AvFdI. These differences are therefore most likely to be due to differences in solvent accessibility.

Azotobacter vinelandii ferredoxin I. Aspartate 15 facilitates proton transfer to the reduced [3Fe-4S] cluster

The [3Fe-4S]+/0 cluster of Azotobacter vinelandii ferredoxin I (AvFdI) has an unusually low and strongly pH-dependent reduction potential (E'0). The reduced cluster exists in two forms, depending upon pH, that exhibit substantially different magnetic circular dichroism (MCD) spectra. Recent studies have established that the MCD changes observed on decreasing the pH from 8.3 (alkaline form) to 6.0 (acid form) cannot be explained either by a change in spin state of the cluster (Stephens, P.J., Jensen, G.M., Devlin, F.J., Morgan, T.V., Stout, C. D., Martin, A.E., and Burgess, B.K. (1991) Biochemistry 30, 3200-3209) or by a major structural change (e.g. ligand exchange) (Stout, C.D. (1993) J. Biol. Chem. 268, 25920-25927). Here, we have examined the influence of aspartate 15 on the pH dependence of the spectroscopic and electrochemical properties of AvFdI by construction of a D15N mutant. Aspartate 15, which is salt-bridged to lysine 84 at the protein surface, is the closest ionizable residue to the [3Fe-4S] cluster. The results show that replacement of aspartate by asparagine results in an approximately 20-mV increase in E'0 for the [3Fe-4S]+/0 cluster at high pH concomitant with an approximately 0.8-pH unit decrease in the pK of the reduced form. The major pH dependence of E'0 is preserved as is the effect observed by MCD. These data eliminate the possibility that the MCD change is due to the presence of Asp-15 and support the conclusion that it originates in direct protonation of the [3Fe-4S]0 cluster, probably on a sulfide ion. Voltammetric studies show that interconversion between [3Fe-4S]+ and [3Fe-4S]0 at acidic pH involves rapid electron transfer followed by proton transfer (for reduction) and then proton transfer followed by electron transfer (for oxidation). Ionized aspartate 15 facilitates proton transfer. Thus, protonation and deprotonation are much slower for D15N relative to the native protein at pH > 5.5. Proton transfer reactions necessary for further reduction of the [3Fe-4S]0 cluster to the [3Fe-4S]- and [3Fe-4S]2- states are also retarded in D15N. The results suggest that the carboxylate-ammonium salt bridge afforded by Asp-15-Lys-84 conducts protons between the cluster and solvent H2O molecules. Overproduction of D15N FdI, but not native FdI, in A. vinelandii has a negative effect on the growth rate of the organism, suggesting that the rate of protonation or deprotonation of the [3Fe-4S]0 cluster may be important in vivo.

Electrocatalytic reduction of hydrogen peroxide at a stationary pyrolytic graphite electrode surface in the presence of cytochrome c peroxidase: a description based on a microelectrode array model for adsorbed enzyme molecules

Electrochemical reduction of H2O2 at pyrolytic graphite disc electrodes of radius 2.5 mm occurs at readily accessible potentials (600 mV versus the standard hydrogen electrode) in the presence of yeast cytochrome c peroxidase. Introduction of the enzyme into the electrolyte solution initiates large changes in the ellipsometric angles measured for the electrode-solution interface, consistent with time-dependent enzyme adsorption. This process may be correlated with changes in electrochemical activity. Over the same time course, linear-sweep voltammograms are characterized by a transition from a sigmoidal to a peak-type waveform. It is proposed that the time-dependent behaviour may be rationalized by use of a microscopic model for substrate mass transport, in which the two-electron reduction of peroxide occurs at electrocatalytic sites consisting of adsorbed enzyme molecules. A voltammetric theory based on treating the adsorbed redox enzymes as an expanding array of microelectrodes is in excellent agreement with experiment.

Classification of fumarate reductases and succinate dehydrogenases based upon their contrasting behaviour in the reduced benzylviologen/fumarate assay

Reduction of fumarate by soluble beef heart succinate dehydrogenase has been shown previously by voltammetry to become increasingly retarded as the potential is lowered below a threshold potential of -80 mV at pH 7.5. The behaviour resembles that of a tunnel diode, an electronic device exhibiting the property of negative resistance. The enzyme thus acts to oppose fumarate reduction under conditions of high thermodynamic driving force. We now provide independent evidence for this phenomenon from spectrophotometric kinetic assays. With reduced benzylviologen as electron donor, we have studied the reduction of fumarate catalysed by various enzymes classified either as succinate dehydrogenases or fumarate reductases. For succinate dehydrogenases, the rate increases as the concentration of reduced dye (driving force) decreases during the reaction. In contrast, authentic fumarate reductases of anaerobic cells (and 'succinate dehydrogenase' from Bacillus subtilis) neither exhibit the electrochemical effect nor deviate from simple kinetic behaviour in the cuvette assay. The 'tunnel-diode' effect may thus represent an evolutionary adaptation to aerobic metabolism.

Reversible electrochemistry of fumarate reductase immobilized on an electrode surface. Direct voltammetric observations of redox centers and their participation in rapid catalytic electron transport

Fumarate reductase (Escherichia coli) can be immobilized in an extremely electroactive state at an electrode, with retention of native catalytic properties. The membrane-extrinsic FrdAB component adsorbs to monolayer coverage at edge-oriented pyrolytic graphite and catalyzes reduction of fumarate or oxidation of succinate, depending upon the electrode potential. In the absence of substrates, reversible redox transformations of centers in the enzyme are observed by cyclic voltammetry. The major component of the voltammograms is a pair of narrow reduction and oxidation signals corresponding to a pH-sensitive couple with formal reduction potential E degree' = -48 mV vs SHE at pH 7.0 (25 degrees C). This is assigned to two-electron reduction and oxidation of the active-site FAD. A redox couple with E degree' = -311 mV at pH 7 is assigned to center 2 ([4Fe-4S]2+/1+). Voltammograms for fumarate reduction at 25 degrees C, measured with a rotating-disk electrode, show high catalytic activity without the low-potential switch-off that is observed for the related enzyme succinate dehydrogenase. The catalytic electrochemistry is interpreted in terms of a basic model incorporating mass transport of substrate, interfacial electron transfer, and intrinsic kinetic properties of the enzyme, each of these becoming a rate-determining factor under certain conditions. Electrochemical reversibility is approached under conditions of low turnover rate, for example, as the supply of substrate to the active site is limited. In this situation, electrocatalytic half-wave potentials, E1/2, are similar for oxidation of bulk succinate and reduction of bulk fumarate and coincide closely with the E degree' value assigned to the FAD. At 25 degrees C and pH 7, the apparent KM for fumarate reduction is 0.16 mM, and kcat is 840 s-1. Accordingly the second-order rate constant, kcat/KM, is 5.3 x 10(6) M-1 s-1. Under the same conditions, oxidation of succinate is much slower. As the supply of fumarate to the enzyme is raised to increase turnover, the electrochemical reaction eventually becomes limited by the rate of electron transfer from the electrode. Under these conditions a second catalytic wave becomes evident, the E1/2 value of which corresponds to the reduction potential of the redox couple suggested to be center 2. This small boost to the catalytic current indicates that the low-potential [4Fe-4S] cluster can function as a second center for relaying electrons to the FAD.

Iron-sulphur clusters with labile metal ions

A study has been carried out of the redox-linked metal ion uptake processes of the iron-sulphur cluster [3Fe-4S] in the bacterial ferredoxin, Fd III from Desulphovibrio africanus using a combination of electron paramagnetic resonance (EPR) and low-temperature magnetic circular dichroism (MCD) spectroscopy and direct, unmediated electrochemistry of the Fd in a film deposited at a pyrolytic graphite electrode. Reduction of the three-iron cluster is required before a divalent metal ion becomes bound as in the reaction sequence [formula: see text] The redox potentials of these processes and the metal binding constants have been determined. The affinities of the [3Fe-4S]0 cluster for divalent ions lie in the sequence Cd greater than Zn much greater than Fe. In addition, specific binding of a monovalent ion, Thallium(I), is detected for [3Fe-4S]1+ as well as for [3Fe-4S]0. The results provide a clear and quantitative demonstration of the capability of the open triangular tri-mu 2-sulphido face of a [3Fe-4S] cluster to bind a variety of metal ions if the protein environment permits. In each case the entering metal ion is coordinated by at least one additional ligand which may be from solvent (H2O or OH-) or from a protein side chain (e.g., carboxylate from aspartic acid). Hence the [3Fe-4S] core can be a redox-linked sensor of divalent metal ions, Fe(II) or Zn(II), that may trigger conformational change.

Diode-like behaviour of a mitochondrial electron-transport enzyme

In mitochondria, electrons derived from the oxidation of succinate by the tricarboxylic acid cycle enzyme succinate-ubiquinone oxido-reductase are transferred directly to the quinone pool. Here we provide evidence that the soluble form of this enzyme (succinate dehydrogenase) behaves as a diode that essentially allows electron flow in one direction only. The gating effect is observed when electrons are exchanged rapidly and directly between fully active succinate dehydrogenase and a graphite electrode. Turnover is therefore measured under conditions of continuously variable electrochemical potential. The otherwise rapid and efficient reduction of fumarate (the reverse reaction) is severely retarded as the driving force (overpotential) is increased. Such behaviour can arise if a rate-limiting chemical step like substrate binding or product release depends on the oxidation state of a redox group on the enzyme. The observation provides, for a biological electron-transport system, a simple demonstration of directionality that is enforced by kinetics as opposed to that which is assumed from thermodynamics.

Site-directed mutagenesis of Azotobacter vinelandii ferredoxin I. Changes in [4Fe-4S] cluster reduction potential and reactivity

We have used site-directed mutagenesis to obtain two variants of Azotobacter vinelandii ferredoxin I (AvFdI), whose x-ray structures are now available. In the C20A protein, a ligand to the [4Fe-4S] cluster was removed whereas in the C24A mutant a free cysteine next to that cluster was removed. Like native FdI, both mutants contain one [4Fe-4S] cluster and one [3Fe-4S] cluster. The structure of C24A is very similar to that of native FdI, while the structure of C20A is rearranged in the region of the [4Fe-4S] cluster to allow it to use the free Cys-24 as a replacement ligand. Here we compare the properties of the native, C20A, and C24A proteins. Although all three proteins are O2 stable in vitro, the C20A protein is much less stable toward proteolysis than the other two in vivo. Spectroscopic results show that all three proteins exhibit the same general redox behavior during O2-oxidation and dithionite reduction. Electrochemical data show that the [3Fe-4S] clusters in all three proteins have the same pH-dependent reduction potentials (-425 mV versus SHE, pH 7.8), whereas the [4Fe-4S] cluster potentials vary over a approximately 150 mV range from -600 mV (C24A) to -647 mV (native) to -746 mV (C20A). Despite this variation in potential both the C20A and C24A proteins appear to be functional in vivo. Native FdI reacts with three equivalents of Fe(CN)3-(6) to form a paramagnetic species previously proposed to be a cysteinyl-disulfide radical. Neither the C20A nor the C24A variant undergoes this reaction, strongly suggesting that it involves the free Cys-24.

Electrochemical and spectroscopic characterization of the conversion of the 7Fe into the 8Fe form of ferredoxin III from Desulfovibrio africanus. Identification of a [4Fe-4S] cluster with one non-cysteine ligand

Desulfovibrio africanus ferredoxin III is a protein (Mr 6585) containing one [3Fe-4S]1+,0 and one [4Fe-4S]2+,1+ core cluster when aerobically isolated. The amino acid sequence contains only seven cysteine residues, the minimum required to ligand these two clusters. Cyclic voltammery by means of direct electrochemistry at a pyrolytic-graphite-'edge' electrode promoted by neomycin shows that, when reduced, the [3Fe-4S]0 centre reacts rapidly with Fe(II) ion to form a [4Fe-4S]2+ cluster. The latter, which can be reduced at a redox potential similar to that of the other [4Fe-4S] cluster, must include non-thiolate ligation. We propose that the carboxylate side chain of aspartic acid-14 is the most likely candidate, since this amino acid occupies the position of a cysteine residue in the sequence typical of an 8Fe ferredoxin. The magnetic properties at liquid-He temperature of this novel cluster, studied by low-temperature magnetic-c.d. and e.p.r. spectroscopy, are diamagnetic in the oxidized state and S = 3/2 in the one-electron-reduced state. This cluster provides a plausible model for the ligation states of the [4Fe-4S]1+ core in the S = 3/2 cluster of the iron protein of nitrogenase and in Bacillus subtilis glutamine:phosphoribosyl pyrophosphate amidotransferase.

Electrochemical and spectroscopic characterization of the 7Fe form of ferredoxin III from Desulfovibrio africanus

Desulfovibrio africanus ferredoxin III is a monomeric protein (Mr 6585) containing seven cysteine residues and 7-8 iron atoms and 6-8 atoms of acid-labile sulphur. It is shown that reversible unmediated electrochemistry of the two iron-sulphur clusters can be obtained by using a pyrolytic-graphite-'edge' carbon electrode in the presence of an appropriate aminoglycoside, neomycin or tobramycin, as promoter. Cyclic voltammetry reveals two well-defined reversible waves with E0' = -140 +/- 10 mV and -410 +/- 5 mV (standard hydrogen electrode) at 2 degrees C. Bulk reduction confirms that each of these corresponds to a one-electron process. Low-temperature e.p.r. and magnetic-c.d. spectroscopy identify the higher-potential redox couple with a cluster of core [3Fe-4S]1+.0 and the lower with a [4Fe-4S]2+.1+ centre. The low-temperature magnetic-c.d. spectra and magnetization properties of the three-iron cluster show that it is essentially identical with that in Desulfovibrio gigas ferredoxin II. We assign cysteine-11, -17 and -51 as ligands of the [3Fe-4S] core and cysteine-21, -41, -44 and -47 to the [4Fe-4S] centre.

Inhibition of ferredoxin: NADP+ reductase activity by the hexacyanochromate (III) ion

The small inorganic complex Cr(CN)6(3-) is a clean inhibitor of the ferredoxin: NADP+ reductase-catalysed oxidation of reduced spinach ferredoxin by NADP+. Independent spectrophotometric measurements show that millimolar additions of Cr(CN)6(3-) to mixtures of ferredoxin and ferredoxin NADP+ reductase give a marked attenuation of the difference spectrum characteristic of ferredoxin-ferredoxin: NADP+ reductase complex formation. Since there is no evidence, from NMR studies, for significant binding of Cr(CN)6(3-) to ferredoxin, these results indicate that Cr(CN)6(3-) binds to ferredoxin: NADP+ reductase at a site which is crucial to its interaction with the electron-transfer protein. The effective kinetic binding constant for Cr(CN)6(3-), measured at low ferredoxin concentration, is 445 M-1 (ie Kdiss congruent to 2 mM) at 25 degrees, pH7.5, I = 0.10 M. With assumption of a simple electrostatic interaction, an enzyme domain with an effective charge of 3+/4+ is proposed.

Reactions of electron-transfer proteins at electrodes

Studies of electron-transfer reactions of redox proteins have, in recent years, attracted widespread interest and attention. Progress has been evident from both physical and biological standpoints, with the increasing availability of three-dimensional structural data for many small electron-transfer proteins prompting a variety of systematic investigations (Isied, 1985). Most recently, attention has been directed towards questions concerning the elementary transfer of electrons between spatially remote redox sites, and the nature of protein–protein interactions which, for intermolecular processes, stabilize specific precursor complexes which may be optimally juxtaposed for electron-transfer. These and other issues, including the necessary reversibility of protein interfacial interactions and the dynamic properties of proteins as carriers of electrons in biological electron-transport systems, are now being addressed in the rapidly emerging field of direct (unmediated) protein electrochemistry. It is our intention in this article to discuss developments made in this area and highlight points which we believe to have the most bearing on our current understanding of diffusion-dominated, protein-mediated electron transport at electrode surfaces. First we shall outline some basic considerations which are best considered with reference to homogeneous systems.

Mechanism of formation, spectrum and reactivity of half-reduced eight-iron Clostridium pasteurianum ferredoxin in pulse-radiolysis studies and the non-co-operativity of the four-iron clusters

Reduction of fully oxidized Clostridium pasteurianum 8-Feox.,ox. ferredoxin by using pulse-radiolysis techniques yields the half-reduced species 8-Feox.,red. ferredoxin. The subsequent oxidation of 8-Feox.,red. ferredoxin with Co(NH3)5Cl2+ was studied. From a comparison with stopped-flow studies on the 2:1 Co(NH3)5Cl2+ oxidation of 8-Fered.,red. ferredoxin to the 8-Feox.,ox. form it is concluded that there is no redox co-operativity between the two 4-Fe centres in these reactions.

Chronic toxicity of methylmercury in the adult cat. Interim report

Doses of 3, 8.4, 20, 46, 74 or 176 mug Hg/kg/day were fed to groups of 8--10 adult cats, either as methylmercuric chloride or as methylmercury-contaminated fish, 7 days/week for up to 2 years. Food consumption, body weight change, blood mercury levels, haematology, urine analysis, serum blood urea nitrogen (BUN) levels and neurological status were assessed regularly in all animals. Clinical signs of methylmercury toxicity -- consisting of ataxia, loss of balance and motor incorrdination -- occured in groups receiving 176 mug Hg/kg/day after 14 weeks of treatment. Pathological findings were confined to the nervous system and consisted of loss of nerve cells with replacement by reactive and fibrillary gloisis. Terminal blood and brain mercury levels were approx. 10 ppm. There were no differences in the time required to develop clinical signs of methylmercury toxicity, tissue mercury levels or pathology between the groups of cats receiving methylmercury as methylmercuric chloride or as methylmercury-contaminated fish, at either dose level. Blood mercury levels in the remaining doses groups appeared to plateau after 40 weeks of treatment. Groups receiving 46 mug Hg/kg/day began to show some neurological impairment after 60 weeks of treatment which did not progress in subsequent weeks. No treatment-related effects were present in groups receiving 20, 8.4 or 3 mug Hg/kg/day after 2 years.