Direct electrochemistry of protein-protein complexes involving cytochrome c, cytochrome b5, and plastocyanin

Iron-based redox centres of reductase and oxygenase components of phenol hydroxylase from A. radioresistens: a redox chain working at highly positive redox potentials

This is the first report of the direct electrochemistry of the reductase (PHR) and oxygenase (PHO) components of phenol hydroxylase from Acinetobacter radioresistens S13 studied by cyclic and differential pulse voltammetry. The PHR contains one 2Fe2S cluster and one FAD that mediate the transfer of electrons from NAD(P)H to the non-heme diiron cluster of PHO. Cyclic and differential pulse voltammetry (CV and DPV) on glassy carbon showed two redox pairs with midpoint potentials at +131.5 ± 13 mV and -234 ± 3 mV versus normal hydrogen electrode (NHE). The first redox couple is attributed to the FeS centre, while the second one corresponds to free FAD released by the protein. DPV scans on native and guanidinium chloride treated PHR highlighted the presence of a split signal (ΔE ≈ 100 mV) attributed to heterogeneous properties of the 2Fe2S cluster interacting with the electrode, possibly due to the presence of two protein conformers and consistently with the large peak-to-peak separation and the peak broadening observed in CV. DPV experiments on gold electrodes performed on PHO confirm a consistently higher reduction potential at +396 mV vs. NHE. The positive redox potentials measured by direct electrochemistry for the FeS cluster in PHR and for the non-heme diiron cluster of PHO show that the entire phenol hydroxylase system works at higher potentials than those reported for structurally similar enzymes, for example methane monooxygenases.

Molecular mechanism of interaction and electron transfer between dihydric phenols and L-cysteine

The mechanism and electron transfer for pollutant dihydric phenol and biomolecule L-Cysteine (L-Cys) interaction in aqueous solution were studied by means of electrochemistry and UV-VIS spectrophotometry. Two forms of L-Cys, fixed on Au-electrode and free dissolved in the solution, were examined. The results showed that L-Cys of an ordered monolayer fixed on an Au electrode facilitated electron transfer and electrocatalytic redox of three isomers of dihydric phenol. However, free L-Cys does not show such facility. Furthermore, neither cleavage of the original chemical bond nor formation of a new chemical bond was observed in the molecules investigated, suggesting that L-Cys molecules may associate tightly with dihydric phenol molecules to form L-Cys . C(6)H(6)O(2)or (L-Cys) (2) . C(6)H(6)O(2) complex molecule via hydrogen-bonding. Different coordination numbers influence the electrochemical activity and behavior of associated complexes; thus, the function of biomolecules could be affected.

Characterization and calculation of a cytochrome c-cytochrome b5 complex using NMR data

To identify the binding site for bovine cytochrome b(5) (cyt b(5)) on horse cytochrome c (cyt c), cross-saturation transfer NMR experiments were performed with (2)H- and (15)N-enriched cyt c and unlabeled cyt b(5). In addition, chemical shift changes of the cyt c backbone amide and side chain methyl resonances were monitored as a function of cyt b(5) concentration. The chemical shift changes indicate that the complex is in fast exchange, and are consistent with a 1:1 stoichiometry. A K(a) of (4 +/- 3) x 10(5) M(-)(1) was obtained with a lower limit of 855 s(-)(1) for the dissociation rate of the complex. Mapping of the chemical shift variations and intensity changes upon cross-saturation NMR experiments in the complex reveals a single, contiguous interaction interface on cyt c. Using NMR data as constraints, a protein docking program was used to calculate two low-energy model complex clusters. Independent calculations of the effect of the cyt b(5) heme ring current-induced magnetic dipole on cyt c were used to discriminate between the different models. The interaction surface of horse cyt c in the current experimentally constrained model of the cyt c-cyt b(5) complex is similar but not identical to the interface predicted in yeast cyt c by Brownian dynamics and docking calculations. The occurrence of different amino acids at the protein-protein interface and the dissimilar assumptions employed in the calculations can largely account for the nonidentical interfaces.

Effects of pH on kinetics of the structural rearrangement that gates the electron-transfer reaction between zinc cytochrome c and plastocyanin: Analysis of protonation states in a diprotein complex

Electron transfer from zinc cytochrome c to copper(II)plastocyanin in the electrostatically- stabilized complex [Crnogorac MM, Shen C, Young S Hansson O, Kostic NM (1996) Biochemistry 35, 16465?74]. We study this rearrangement in four complexes Zncyt/pc(II), which zinc cytochrome c makes with the wild-type form and the single mutants Asp42Asn, Glu59Gln, and Glu60Gln of plastocyanin. The rate constant for the rearrangement, kF differs for the four forms of plastocyanin but is independent of pH from 5.4 to 9.0 in all four cases. That kF is affected by the single mutations but not by pH changes suggests that the residues Asp 42, Glu59, and Glu60 in the wild-type plastocyanin remain deprotonated (i.e., as anions) within the Zncyt/pc(II) complex throughout the pH range examined. This fact agrees with the notion that loss of salt bridges in the initial (redox-inactive) configuration of the complex is compensated by formation of new salt bridges in the rearranged (redox-active) configuration.

Effects of pH on protein association: modification of the proton-linkage model and experimental verification of the modified model in the case of cytochrome c and plastocyanin

Effects of pH on protein association are not well understood. To understand them better, we combine kinetic experiments, calculations of electrostatic properties, and a new theoretical treatment of pH effects. The familiar proton-linkage model, when used to analyze the dependence of the association constant K on pH, reveals little about the individual proteins. We modified this model to allow determination not only of the numbers of the H+ ions involved in the association but also of the pK(a) values, in both the separate and the associated proteins, of the side chains that are responsible for the dependence of K on pH. Some of these side chains have very similar pK(a) values, and we treat them as a group having a composite pK(a) value. Use of these composite pK(a) values greatly reduces the number of parameters and allows meaningful interpretation of the experimental results. We experimentally determined the variation of K in the interval 5.4 < or = pH < or = 9.0 for four diprotein complexes, those that the wild-type cytochrome c forms with the wild-type plastocyanin and its mutants Asp42Asn, Glu59Gln, and Glu60Gln. The excellent fittings of the experimental results to the modified model verified this model and revealed some unexpected and important properties of these prototypical redox metalloproteins. Protein association causes a decrease in the pK(a) values of the acidic side chains and an increase in the pK(a) values of the basic side chains. Upon association, three carboxylic side chains in wild-type plastocyanin each release a H+ ion. These side chains in free plastocyanin have an anomalously high composite pK(a) value, approximately 6.3. Upon association, five or six side chains in cytochrome c, likely those of lysine, each take up a H+ ion. Some of these side chains have anomalously low pK(a) values, less than 7.0. The unusual pK(a) values of the residues in the recognition patches of plastocyanin and cytochrome c may be significant for the biological functions of these proteins. Although each mutation in plastocyanin markedly, and differently, changed the dependence of K on pH, the model consistently gave excellent fittings. They showed decreased numbers of H+ ions released or taken up upon protein association and altered composite pK(a) values of the relevant side chains. Comparisons of the fitted composite pK(a) values with the theoretically calculated pK(a) values for plastocyanin indicated that Glu59 and Asp61 in the wild-type plastocyanin each release a H+ ion upon association with cytochrome c. Information of this kind cannot readily be obtained by spectroscopic methods. Our modification of the proton-linkage model is a general one, applicable also to ligands other than H+ ion and to processes other than association.

Electrostatic effects on electron-transfer kinetics in the cytochrome f-plastocyanin complex

In a complex of two electron-transfer proteins, their redox potentials can be shifted due to changes in the dielectric surroundings and the electrostatic potentials at each center caused by the charged residues of the partner. These effects are dependent on the geometry of the complex. Three different docking configurations (DCs) for intracomplex electron transfer between cytochrome f and plastocyanin were studied, defined by 1) close contact of the positively charged region of cytochrome f and the negatively charged regions of plastocyanin (DC1) and by (2, 3) close contact of the surface regions adjacent to the Fe and Cu redox centers (DC2 and DC3). The equilibrium energetics for electron transfer in DC1-DC3 are the same within approximately +/-0.1 kT. The lower reorganization energy for DC2 results in a slightly lower activation energy for this complex compared with DC1 and DC3. The long heme-copper distance (approximately 24 A) in the DC1 complex drastically decreases electronic coupling and makes this complex much less favorable for electron transfer than DC2 or DC3. DC1-like complexes can only serve as docking intermediates in the pathway toward formation of an electron-transfer-competent complex. Elimination of the four positive charges arising from the lysine residues in the positive patch of cytochrome f, as accomplished by mutagenesis, exerts a negligible effect (approximately 3 mV) on the redox potential difference between cyt f and PC.

Determination of rate and equilibrium constants for the reactions between electron transfer mediators and proteins by linear sweep voltammetry

Redox proteins undergo measurable charge transfer at electrodes only under special circumstances, while they readily take part in electron transfer reactions with mediators in solution. Advantage was taken of the latter fact to develop a new method to study the kinetics and equilibria of protein-mediator electron transfer reactions. It was shown that rate and equilibrium constants for the electron exchange between electron transfer mediator and the protein can be obtained from the analysis of the perturbation of the linear sweep voltammetry (LSV) response of the mediator due to the presence of the protein. The experiments were carried out under conditions where the protein does not interact with the electrode. Theoretical data obtained by digital simulation are presented to show the conditions under which rate and equilibrium constants are accessible by the LSV technique. The electron transfer reactions between ferri- and ferrocytochrome c and N,N,N',N'-tetramethylphenylenediamine and the corresponding radical cation in phosphate-buffered saline (0.04 M phosphate, pH 7.4, 0.1 M NaCl) buffer were selected to demonstrate the technique. These studies resulted in an equilibrium constant equal to 1.0 and forward and reverse rate constants equal to 1.6 x 10(4) M-1 s-1. The data available from this method include forward and reverse rate constants for electron transfer and the formal potential for the protein redox couple.

Voltammetry of Plastocyanin at a Graphite Electrode: Effects of Structure, Charge, and Electrolyte

Comparative voltammetric studies on Anabaena variabilis plastocyanin (positively charged at neutral pH) and spinach and poplar plastocyanins (negatively charged at neutral pH) have been undertaken at an edge-plane graphite electrode as a function of ionic strength, pH, and Mg(2+) concentration at 3 degrees C. The aim was to provide a more detailed understanding of the influence of the electrode-protein (solution) interfacial characteristics, as well as the variation of the formal potential with both the nature of the plastocyanin species and the pH. As might be expected, some of the interfacial properties associated with the positive charge on A. variabilis plastocyanin are the opposite of those observed with the negatively charged plastocyanins. For example, the linear diffusion component of the mass transport process for A. variabilis plastocyanin under the conditions of cyclic voltammetry is decreased and the radial diffusion component is increased by the addition of Mg(2+), whereas the reverse occurs with poplar and spinach plastocyanins. The voltammetrically determined reversible potentials for A. variabilis plastocyanin are considerably less positive than those for spinach and poplar plastocyanins, in agreement with values calculated from chemically based redox studies. Ionic strength effects, as determined by addition of NaClO(4) over the concentration range 0.005-0.20 M, are negligible for all three proteins. The addition of Mg(2+) causes a significant shift in the reversible potential toward more positive values for spinach and poplar plastocyanin but only a small positive shift for A. variabilis plastocyanin. The difference is attributed to a specific binding effect. The addition of Mg(2+) also dramatically alters the pH dependence of the reversible potential, indicating that the equilibrium between the protonated and unprotonated forms of reduced plastocyanin is modified by binding of Mg(2+) to the protein. It is concluded that the effects of biologically relevant redox-inactive cations such as Mg(2+) or Ca(2+) have to be considered carefully in studies of the redox chemistry of metalloproteins.

Experimental and theoretical analysis of the interaction between cytochrome c and cytochrome b5

Experimental and theoretical investigation of the interaction of cytochrome c and cytochrome b5 performed over nearly twenty years has produced considerable insight into the manner in which these proteins recognize and bind to each other. The results of these studies and the experimental and theoretical strategies that have been developed to achieve these results have significant implications for understanding the behavior of similar complexes formed by more complex and less-well characterized electron transfer proteins. The current review provides a comprehensive summary and critical evaluation of the literature on which the current status of our understanding of the interaction of cytochrome c and cytochrome b5 is based. The general issues related to the study of electron transfer complexes of this type are discussed and some new directions for future investigation of such systems are considered.

Recent progress in the electrochemistry of c-type cytochromes

C-type cytochromes are classified into two main groups: i) cytochromes which give fast electrochemical responses at the conventional electrodes in the absence of any promoter (eg multi-heme cytochromes c3); ii) cytochromes which need the presence of promoters or the use of modified electrodes to exhibit fast electrochemical responses (eg one-heme mitochondrial cytochrome c). In the latter case, careful design of electrode surface and composition of the solution are required for the attainment of rapid and reversible electron-exchange reactions. Some general considerations are given on the 'electrochemical model'. In particular, binding interactions between the electrode and the protein can take place in a similar manner to that occurring between physiological partner proteins. Electrochemistry when coupled to other physical techniques can give more complete insights in the relationship between the redox properties, structure and function of c-type cytochromes. In particular, in the case of polyheme cytochromes, promising results are expected from the study of site-directed mutagenesis-modified cytochrome c3.

Transmutation of a heme protein

Residue Asn57 of bovine liver cytochrome b5 has been replaced with a cysteine residue, and the resulting variant has been isolated from recombinant Escherichia coli as a mixture of four major species: A, BI, BII, and C. A combination of electronic spectroscopy, 1H NMR spectroscopy, resonance Raman spectroscopy, electrospray mass spectrometry, and direct electrochemistry has been used to characterize these four major cytochrome derivatives. The red form A (E(m) = -19 mV) is found to possess a heme group bound covalently through a thioether linkage involving Cys57 and the alpha carbon of the heme 4-vinyl group. Form BI has a covalently bound heme group coupled through a thioether linkage involving the beta carbon of the heme 4-vinyl group. Form BII is similar to BI except that the sulfur involved in the thioether linkage is oxidized to a sulfoxide. The green form C (E(m) = 175 mV) possesses a noncovalently bound prosthetic group with spectroscopic properties characteristic of a chlorin. A mechanism is proposed for the generation of these derivatives, and the implications of these observations for the biosynthesis of cytochrome c and naturally occurring chlorin prosthetic groups are discussed.

Importance of protein rearrangement in the electron-transfer reaction between the physiological partners cytochrome f and plastocyanin

Cytochrome f from turnip and plastocyanin from French bean were noninvasively cross-linked in the presence of the carbodiimide EDC so that the exposed heme edge in the former protein abuts the acidic patch remote from the copper site in the latter [Morand, L.Z., Frame, M.K., Colvert, K.K., Johnson, D.A., Krogmann, D.W., & Davis, D.J. (1989) Biochemistry 28, 8039]. The molecular mass, reduction potentials, and UV-visible and ESR spectra of the covalent complex were consistent with the composition cyt/pc and with a lack of noticeable structural perturbations of the protein molecules. Isoelectric focusing showed the presence of N-acylurea groups, byproducts of the cross-linking reaction [Zhou, J.S., Brothers, H.M. II, Neddersen, J.P., Peerey, L.M., Cotton, T.M., & Kostić, N.M. (1992) Bioconjugate Chem. 3, 382]. Laser flash spectroscopy, with riboflavin semiquinone as the reductant, showed that the electrontransfer reaction within the covalent complex cyt(II)/pc(II) is either undetectably slow or reversible. The question was resolved by monitoring, during redox titrations, the 1H NMR line widths of the heme methyl groups in free ferricytochrome f and in this protein cross-linked to plastocyanin.(ABSTRACT TRUNCATED AT 250 WORDS)

Transient kinetics of electron transfer from a variety of c-type cytochromes to plastocyanin

Plastocyanin (PC) and its physiological reaction partner cytochrome (cyt) f form a complex which is electrostatically stabilized by interactions between complementary localized charges. We have measured the kinetics of intracomplex electron transfer between several reduced cytochromes and PC using laser flash photolysis. With spinach cyt f and spinach PC, we obtain first-order rate constants, kforward = 2780 s-1 and kreverse = 1050 s-1, for the reversible reaction and a complex dissociation constant of about 23 microM at an ionic strength (I) of 5 mM. The observed rate constant increases by a factor of 2 between I = 5 and 40 mM and then decreases monotonically at higher ionic strengths. This indicates that the complex is not completely dissociated until I = 150 mM and that the proteins within the electrostatically most stable complex are not optimally oriented for electron transfer. Similar results were obtained with turnip cyt f and spinach PC, although in this case intracomplex electron transfer is about 4 times as fast. Horse cyt c also forms an electrostatically stabilized complex with PC, and yields a limiting rate constant for intracomplex electron transfer (1750 s-1) and a dissociation constant (10 microM) comparable to those for spinach cyt f. The ionic strength dependence shows that the complex is more readily dissociated (complete at I = 25 mM) than is that of cyt f and that rearrangement is not required for optimal electron transfer. Addition of polylysine results in 10-fold inhibition of the rate of electron transfer. Pseudomonas cyt c-551 is an acidic cytochrome which does not form a complex with PC.(ABSTRACT TRUNCATED AT 250 WORDS)

Direct electrochemistry of proteins. Investigations of yeast cytochrome c mutants and their complexes with cytochrome b5

Direct electrochemistry of site-specific mutants of yeast iso-1-cytochrome c (cyt c) and their complexes with bovine cytochrome b5 (cyt b5) has been investigated at edge-plane pyrolytic graphite (EPG) and bis(4-pyridyl)-disulphide-modified gold electrodes. Structure/function relationships have been investigated with the particular aim of clarifying the factors controlling the interactions of proteins at electrode/electrolyte interfaces and the determinants for direct electrochemistry in ternary protein/protein/electrode adducts, e.g. cyt c/cyt b5/EPG. Investigations of the cyt c mutants alone revealed a variety of electrochemical responses: all the mutants show similar voltammetric reversibility at modified gold electrodes, whereas at EPG electrodes the reversibility follows the order: Asn52Ile-Cys102Thr greater than Cys102Thr greater than Asn52Ala-Cys102Thr. Mid-point potentials follow the order: Arg13Ile (+60 +/- 5 mV vs. standard calomel electrode) greater than Cys102Thr (+40 +/- 5 mV) greater than Lys27Gln (+30 +/- 5 mV) approximately Lys72Asp (+30 +/- 5 mV) greater than Asn52Ala-Cys102Thr (+15 +/- 5 mV) greater than Asn52Ile-Cys102Thr (-10 +/- 5 mV). The structural basis for these differences is briefly discussed. When these mutants are bound to cyt b5, the differences in electrochemical response are greatly enhanced in the ternary cyt c/cyt b5/EPG adducts. A minimal analysis of these differences supports a model of multiple overlapping binding and recognition domains on cyt c which may be finely tuned to allow ternary complex formation so that a single-site variation could modify or abolish direct electrochemistry in the ternary adduct.

Direct electrochemistry of the enzyme, methylamine dehydrogenase, from bacterium W3A1

The electrochemical response of methylamine dehydrogenase from bacterium W3A1 at edge-plane-oriented pyrolytic graphite (epg) and modified gold electrodes has been investigated. Quasi-reversible electron transfer has been observed. Variations in concentration of different cations and anions gave rise to both promotion and inhibition of the direct response. A catalytic response of the enzyme in the presence of methylamine has been observed at both an epg electrode and a 2,2'-dithiodiglycolic-acid-modified gold electrode surface, and the effects of various cations and anions on the catalytic peak current have been investigated. The spectroelectrochemical results obtained at an optically transparent thin-layer electrode, modified with 2,2'-dithiodiglycolic acid, are also reported. In the presence of 1,1'-dimethylferrocene-3-(1-ethanol-2-amine) (14.8 microM), the results reveal a midpoint potential of -148 mV for methylamine dehydrogenase from bacterium W3A1. This is in very close agreement to the value obtained in the cyclic voltammetric investigations of -140 mV.