Interaction of cytochrome c with the blue copper proteins, plastocyanin and azurin

Direct monitoring of the electron pool effect of cytochrome c3 by highly sensitive EQCM measurements

Cytochrome c(3) from Desulfovibrio vulgaris has four hemes per molecule, and a redox change at the hemes alters the conformation of the protein, leading to a redox-dependent change in the interaction of cytochrome c(3) with redox partners (an electron acceptor or an electron donor). The redox-dependent change in this interaction was directly monitored by the high-performance electrochemical quartz crystal microbalance (EQCM) technique that has been improved to give high sensitivity in solution. In this method, cytochrome c(3) molecules in solution associate electrostatically with a viologen-immobilized quartz crystal electrode as a monolayer, and redox of the associating cytochrome c(3) is controlled by the immobilized viologen. This technique makes it possible to measure the access of cytochrome c(3) to the electrode or repulsion from the electrode, and hence interconversion between an electrostatic complex and an electron transfer complex on the cytochrome c(3) and the viologen as a mass change accompanying a potential sweep is monitored. In addition, simultaneous measurement of a mass change and a potential step reveals that the cytochrome c(3) stores electrons when the four hemes are reduced (an electron pool effect), that is, the oxidized cytochrome c(3) facilitates acceptance of electrons from the immobilized viologen molecule, but the reduced cytochrome c(3) donates the accepted electrons to the viologen with difficulty.

Metalloprotein association, self-association, and dynamics governed by hydrophobic interactions: simultaneous occurrence of gated and true electron-transfer reactions between cytochrome f and cytochrome c(6) from Chlamydomonas reinhardtii

Noninvasive reconstitution of the heme in cytochrome c(6) with zinc(II) ions allowed us to study the photoinduced electron-transfer reaction (3)Zncyt c(6) + cyt f(III) --> Zncyt c(6)(+) + cyt f(II) between physiological partners cytochrome c(6) and cytochrome f, both from Chlamydomonas reinhardtii. The reaction kinetics was analyzed in terms of protein docking and electron transfer. In contrast to various protein pairs studied before, both the unimolecular and the bimolecular reactions of this oxidative quenching take place at all ionic strengths from 2.5 through 700 mM. The respective intracomplex rate constants are k(uni) (1.2 +/- 0.1) x 10(4) s(-1) for persistent and k(bi) (9 +/- 4) x 10(2) s(-1) for the transient protein complex. The former reaction seems to be true electron transfer, and the latter seems to be electron transfer gated by a structural rearrangement. Remarkably, these reactions occur simultaneously, and both rate constants are invariant with ionic strength. The association constant K(a) for zinc cytochrome c(6) and cytochrome f(III) remains (5 +/- 3) x 10(5) M(-1) in the ionic strength range from 700 to 10 mM and then rises slightly to (7 +/- 2) x 10(6) M(-1), as ionic strength is lowered to 2.5 mM. Evidently, docking of these proteins from C. reinhardtii is due to hydrophobic interaction, slightly augmented by weak electrostatic attraction. Kinetics, chromatography, and cross-linking consistently show that cytochrome f self-dimerizes at ionic strengths of 200 mM and higher. Cytochrome f(III) quenches triplet state (3)Zncyt c(6), but its dimer does not. Formation of this unreactive dimer is an important step in the mechanism of electron transfer. Not only association between the reacting proteins, but also their self-association, should be considered when analyzing reaction mechanisms.

Photoinduced electron transfer from carboxymethylated cytochrome c to plastocyanin

Photoinduced electron transfer from cytochrome c to plastocyanin was investigated using a novel method. Reduced carboxymethylated cytochrome c (CmCyt c), with carbon monoxide bound to the heme iron, and oxidized plastocyanin were mixed. At 1 mM CO the reduced state of CmCyt c is stabilized by about 350 meV. After flash photolysis of CO the apparent redox potential of CmCyt c drops resulting in electron transfer to plastocyanin. The electron transfer characteristics were investigated at approximately 30 different wavelengths in the range 390-460 nm. A global fit of the data showed that the electron transfer rate is 960+/-30 s-1 at pH 7.

Complex of plastocyanin and cytochrome c characterized by NMR chemical shift analysis

The complexes of horse ferrous and ferric cytochrome c with Cd-substituted pea plastocyanin have been characterized by nuclear magnetic resonance, in order to determine the binding sites and to study the effects of complex formation. Reproducible, small chemical shift changes (0.005-0.05 ppm) were observed for protons in both proteins upon formation of a 1:1 complex. The chemical shift changes depended on the ratio of free to bound protein, with a binding constant of 1.0 +/- 0.5 x 10(5) M(-1), indicating that they were caused by complex formation and that free and bound proteins were in fast exchange. Two-dimensional spectra of the complex of ferrocytochrome c and plastocyanin were screened systematically for chemical shift changes. For about 760 protons, or 70% of the assigned protons in the two proteins, the chemical shift in the complex could be established. In plastocyanin and cytochrome c 14% and 17% of the protons, respectively, showed a significant chemical shift change. These protons form two groups. The first consists of a limited number of surface-exposed side-chain protons. These map on the so-called east side of plastocyanin and the front side of cytochrome c. This group of chemical shift changes is interpreted as representing direct effects of binding, and the respective surfaces thus represent the binding sites. The second group includes backbone amide protons and a few aliphatic and aromatic protons in the hydrophobic core of each protein. The chemical shift changes of this group are interpreted as secondary, i.e., caused by very small structural changes which are transmitted deep into the core of the protein. Ferric cytochrome c caused the same chemical shift effects in plastocyanin as the ferrous form; no intermolecular paramagnetic effects were observed. The small size of the chemical shifts and the absence of intermolecular paramagnetic shifts and NOEs suggest that the complex consists of a dynamic ensemble of structures which are in fast exchange, rather than a single static complex. This study shows that small, reproducible chemical shifts can be used effectively to characterize protein complexes in detail.

Analysis of the 1H-NMR chemical shifts of Cu(I)-, Cu(II)- and Cd-substituted pea plastocyanin. Metal-dependent differences in the hydrogen-bond network around the copper site

To compare cadmium-substituted plastocyanin with copper plastocyanin, the 1H-NMR spectra of CuI-, CuII- and Cd-plastocyanin from pea have been analyzed. Full assignments of the spectra of CuI- and Cd-plastocyanin indicate chemical shift differences up to 1 ppm. The affected protons are located in the four loops that surround the Cu site. The largest differences were found for protons in the hydrogen bond network which stabilizes this part of the protein. This suggests that the chemical shift differences are caused by very small but extensive structural changes in the network upon replacement of CuI by Cd. For CuII-plastocyanin the resonances of 72% of the protons observed in the CuI form have been identified. Protons within approximately 0.9 nm of the CuII were not observed due to fast paramagnetic relaxation. The protons between 0.9-1.7 nm from the CuII showed chemical shift differences up to 0.4 ppm compared to both CuI- and Cd-plastocyanin. These differences can be predicted assuming that they represent pseudocontact shifts. When corrected for the pseudocontact shift contribution, the CuII-plastocyanin chemical shifts were nearly all identical within error to those of the Cd form, but not of the CuI-plastocyanin, indicating that the CuII-plastocyanin structure, in as far as it can be observed, resembles Cd-rather than CuI-plastocyanin. In a single stretch of residues (64-69) chemical shift differences remained between all three forms after correction. The fact that pseudocontact shifts were observed for protons which were not broadened may be attributable to the weaker distance dependence of the pseudocontact shift effect compared to paramagnetic relaxation. This results in two shells around the Cu atom, an inner paramagnetic shell (0-0.9 nm), in which protons are not observed due to broadening, and an outer paramagnetic shell (0.9-1.7 nm), in which protons can be observed and show pseudocontact shifts. It is concluded that Cd-plastocyanin is a suitable redox-inactive substitute for Cu-plastocyanin.

Beta-thiopropionyl cytochromes c modified at lysyl residues: preparation and characterization of the monosubstituted horse cytochromes c

beta-Thiopropionyl derivatives of horse cytochrome c singly modified at each of 18 different lysine epsilon-amino groups have been prepared using sulfosuccinimidyl-2-(biotinamido)ethyl-1,3-dithiopropionate and purified to homogeneity by high-pressure liquid chromatography. These derivatives were characterized by determination of: (i) the location of the modification; (ii) reduction potentials; (iii) visible and NMR spectra: and by (iv) measurement of electron transfer activity with cytochrome-c oxidase. No significant changes in structure were indicated, except for the ferric forms of the derivatives modified at lysines 72, 73, and 79 which are discussed separately. The electron transfer activity of the beta-thiopropionyl cytochromes c with bovine heart cytochrome-c oxidase was decreased to extents dependent on the position of the modification. Aminoethylation, a secondary modification which reverses the charge change, restored the electron transfer rate to that observed with the unmodified cytochrome c, irrespective of the location of the primary modification. These results afford a direct experimental demonstration that alterations in kinetics with physiological electron transfer partners resulting from modifications which cause a change of the charge of surface side chains are solely due to the electrostatic effects. Of the many chemically modified cytochromes c prepared to date, the singly substituted beta-thiopropionyl cytochromes c are likely to be particularly useful as the thiol allows covalent linkage of any sulfhydryl-reactive reagent to a well-defined location on the protein surface by a simple procedure, even when the secondary modifier is relatively unstable, a crucial advantage not otherwise readily achieved.

Analysis of the bimolecular reduction of ferricytochrome c by ferrocytochrome b5 through mutagenesis and molecular modelling

Site-directed mutagenesis has been used to produce variants of cytochrome c in which selected structural or functional properties of this protein are altered that have been implicated previously in contributing to the rate at which ferricytochrome c is reduced by ferrocytochrome b5. In total, 18 variants have been studied by kinetics and electrochemical methods to assess the contributions of thermodynamic driving force, surface charge and hydrophobic interactions, and redox-linked structural reorganization of the protein to the rate of electron transfer between these two proteins under conditions where the reaction is bimolecular. While some variants (those at position-38) appear to affect primarily the driving force of the reaction, others appear to influence the rearrangement barrier to electron transfer (those at positions-67 and -52) while the interface between electron donor and acceptor centers is the principal effect of substitutions for a conserved aromatic heme contact residue at the surface of the protein (position-82). Interpretation of these results has been facilitated through the use of energy minimization calculations to refine the hypothetical models previously suggested for the cytochrome c- cytochrome b5 precursor complex on the basis of Brownian dynamics simulations of the bimolecular encounter event.

Plastocyanin: Structure and function

The aim of this review is to analyze the current state of knowledge concerning the blue copper protein plastocyanin (PC) focusing on its interactions with its reaction partners cytochromef and P700. Plastocyanin is a 10 kD blue copper protein which is located in the lumen of the thylakoid where it functions as a mobileelectron carrier shuttling electrons from cytochromef to P700 in Photosystem I. PC is a typical β-barrel protein containing a single copper center which is ligated to two histidines, a methionine and a cysteine in a distorted tetrahedral geometry. PC has two potential binding sites for reaction partners. Site 1 consists of the H87 ligand to the copper and Site 2 consists of Y83 which is surrounded by two clusters of negative charges which are highly conserved in higher plant PCs.The interaction of PC with cytochromef has been studied extensively. It is electrostatic in nature with negative charges on PC interacting with positive charges on cytochromef. Evidence from cross-linking, chemical modification, kinetics and site-directed mutagenesis studies implicate Site 2 as the binding site for Cytf. The interaction is thought to occur in two stages: an initial diffusional approach guided by electrostatic interactions, followed by more precise docking to form a final electron transfer complex.Due to the multisubunit nature of the Photosystem I complex, the evidence is not as clear for the binding site for P700. However, a small positively-charged subunit (Subunit III) of Photosystem I has been implicated in PC binding. Also, both chemical modification and site-directed mutagenesis experiments have suggested that PC interacts with P700 via Site 1.

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)

Comparison of electrostatic interactions and of protein-protein orientations in electron-transfer reactions of plastocyanin with the triplet state of zinc cytochrome c and with zinc cytochrome c cation radical

Photoinduced reduction of cupriplastocyanin by the triplet state of zinc cytochrome c (the "forward" reaction) and the subsequent thermal oxidation of cuproplastocyanin by zinc cytochrome c cation radical (the "back" reaction) at ionic strengths from 40 mM to 3.00 M are studied by laser kinetic spectroscopy (so-called flash photolysis). Variation of the bimolecular rate constants over the entire range of ionic strength cannot be explained in terms of monopole-monopole interactions between the protein molecules, but it can be explained in terms of monopole-monopole, monopole-dipole, and dipole-dipole interactions. Analysis of the kinetic results in terms of these electrostatic interactions reveals the overall protein-protein orientation for electron transfer. In both the forward and back reactions the exposed heme edge in zinc cytochrome c apparently abuts the negatively-charged (acidic) patch on the plastocyanin surface, which is remote from the copper atom, and not the electroneutral (hydrophobic) patch, which is proximate to the copper atom. The acidic patch is large, and this analysis cannot rule out a relatively small difference in protein-protein orientations for the forward and back reactions. These two reactions are compared with the previously studied reduction of cupriplastocyanin by ferrocytochrome c. Although native cytochrome c and its zinc derivative have very similar structural and electrostatic properties, the reactive forms of the cytochrome c/plastocyanin and zinc cytochrome c/plastocyanin complexes may adopt somewhat different protein-protein orientations or may adopt similar orientations but differ in dynamic properties.

Photoinduced electron-transfer reaction in a ternary system involving zinc cytochrome c and plastocyanin. Interplay of monopolar and dipolar electrostatic interactions between metalloproteins

A carbodiimide promotes noninvasive cross-linking between amino groups surrounding the exposed heme edge in zinc cytochrome c and carboxylic groups in the acidic patch in plastocyanin. Eight derivatives of the covalent complex Zncyt/pc(I), which have similar structures but different overall charges because of different numbers and locations of N-acylurea groups, are separated by cation-exchange chromatography. Kinetics of electron transfer from the diprotein complex in the triplet excited state, 3Zncyt/pc(I), to free cupriplastocyanin at pH 7.0 and various ionic strengths is studied by laser flash spectroscopy. This reaction is purely bimolecular for all eight N-acylurea derivatives of the diprotein complex. The overall charges of the derivatives 1 and 2 at pH 7.0 are -2 and 0, respectively; both of them, however, have very large dipole moments of 410-480 D. The rate constants for their reactions with cupriplastocyanin, whose charge at pH 7.0 is -8 and whose dipole moment is 362 D, are determined over the range of ionic strengths from 2.5 mM to 3.00 M. The observed dependence of the rate constants on ionic strength cannot be explained in terms of net charges (monopole-monopole interactions) alone, but it can be fitted quantitatively with a theory that recognizes also monopole-dipole and dipole-dipole interactions [van Leeuwen, J. W. (1983) Biochim. Biophys. Acta 743, 408]. At ionic strengths up to ca. 10 mM monopole-monopole interactions predominate and Brønsted-Debye-Hückel theory applies.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural and functional features of Pseudomonas cytochrome c peroxidase

The secondary structure of Pseudomonas cytochrome c peroxidase (ferrocytochrome c: hydrogen-peroxide oxidoreductase, EC 1.11.1.5) has been predicted from the established amino acid sequence of the enzyme using a Chou-Fasman-type algorithm. The amount of alpha-helicity thus obtained is in agreement with previously obtained results based on circular dichroic measurements at far UV. The two heme c moieties of the enzyme have earlier been shown to have widely different characteristics, e.g., the redox potentials of the hemes differ with about 600 mV, and carry out different functions in the enzyme molecule. The structural comparisons made in this study enlighten the observed functional differences. The first heme in the polypeptide chain, heme 1, has in its environment a folding pattern generally encountered in cytochromes. In the region of the sixth ligand, however, profound differences are noted. The cytochromal methionine has been replaced by a lysine with a concomitant lowering of redox-potential thus making peroxidatic activity possible. Around heme 2, extra amino acid residues have been added to the peroxidase as compared with Rhodospirillum molischianum cytochrome c2 core structure in the 20's loop. After completion of the cytochromal fold around heme 2 an additional tail consisting of 25 residues is linked. This tail shows no stabilizing elements of secondary structure, but contains a strongly hydrophobic segment which suggests a possible membrane contact site of this extrinsic membrane protein. Heme 2 is concluded to have a cytochromal function in the molecule. To further elucidate the functional properties of the enzyme, a noncovalent two-fragment complex was produced by specific cleavage of the peroxidase by Pseudomonas elastase. The complex was studied with respect to its properties to the native enzyme. The two-fragment complex of Pseudomonas peroxidase retains the overall conformation of the native enzyme showing, however, no heme-heme interaction. Thus, a comparison of the properties of the native enzyme with those of the two-fragment complex permitted some conclusions to be drawn on the structure of the enzyme as well as the mechanism of heme-heme interaction. From the present results we conclude that the two distal heme surfaces in the peroxidase are oriented toward each other. This structural arrangement allows an inter-heme communication in the enzyme molecule and it also forms the structural basis for the enzyme mechanism. The structural comparisons also give insight into the evolution of an ancestral cytochrome c into an efficient peroxidase that has a versatile control mechanism in heme-heme interaction.

Involvement of the hydrophobic patch of azurin in the electron-transfer reactions with cytochrome C551 and nitrite reductase

The electron-transfer reactions of site-specific mutants of the blue copper protein azurin from Pseudomonas aeruginosa with its presumed physiological redox partners cytochrome c551 and nitrite reductase were investigated by temperature-jump and stopped-flow experiments. In the hydrophobic patch of azurin Met44 was replaced by Lys, and in the His35 patch His35 was replaced by Phe, Leu and Gln. Both patches were previously thought to be involved in electron transfer. 1H-NMR spectroscopy revealed only minor changes in the three-dimensional structure of the mutants compared to wild-type azurin. Observed changes in midpoint potentials could be attributed to electrostatic effects. The slow relaxation phase observed in temperature-jump experiments carried out on equilibrium mixtures of wild-type azurin and cytochrome c551 was definitively shown to be due to a conformational relaxation involving His35. Analysis of the kinetic data demonstrated the involvement of the hydrophobic but not the His35 patch of azurin in the electron transfer reactions with both cytochrome c551 and nitrite reductase.

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

The direct electrochemistry of the cytochrome c/cytochrome b5 and cytochrome c/plastocyanin complexes has been investigated at edge-plane graphite and modified gold electrode surfaces, which are selective for one of the two components of the complex. Electrochemical response of one protein at an otherwise electrostatically unfavorable electrode surface was achieved in the presence of the other protein, and the calculated heterogeneous electron-transfer rate constant and diffusion coefficient were found to be in good agreement with the values determined previously from the electrochemistry of the individual proteins [Armstrong, F. A., Hill, H. A. O., & Walton, N. J. (1988) Acc. Chem. Res. 21, 407 and references therein]. A dynamic model of the protein-protein-electrode ternary complex is proposed to explain the promotion effect, and this model is supported by a study comparing the electrochemical responses of covalent and electrostatic cytochrome c/plastocyanin complexes. It is also suggested that the behavior of protein-protein complexes at electrode surfaces could be related to that of the complexes associated with biological membranes.

Photoinduced electron transfer within complexes between plastocyanin and ruthenium bisbipyridine dicarboxybipyridine cytochrome c derivatives

A new technique has been developed to measure intracomplex electron transfer between cytochrome c and its redox partners. Cytochrome c derivatives labeled at single lysine amino groups with ruthenium bisbipyridine dicarboxybipyridine were prepared as previously described [Pan, L.P., Durham, B., Wolinska, J., & Millett, F. (1988) Biochemistry 27, 7180-7184]. Excitation of RuII with a short light pulse resulted in the formation of the excited-state RuII*, which rapidly transferred an electron to the ferric heme group to form FeII and RuIII. Aniline was included in the buffer to reduce RuIII to RuII, leaving the heme group in the ferrous state. This process was complete within the lifetime of the light pulse. When plastocyanin was present in the solution, electron transfer from the ferrous heme of cytochrome c to CuII in plastocyanin was observed. All of the ruthenium cytochrome c derivatives formed electrostatic complexes with plastocyanin at low ionic strength, allowing intracomplex electron-transfer rate constants to be measured. The rate constants for derivatives modified at the indicated lysines were as follows: Lys 13, 1920 s-1; Lys 8, 1480 s-1; Lys 7, 1340 s-1; Lys 86, 1020 s-1; Lys 25, 820 s-1; Lys 72, 800 s-1; Lys 27, 530 s-1. It is interesting that the derivative modified at lysine 13 at the top of the heme crevice had the largest rate constant, while lysine 27 at the right side of the heme crevice had the smallest.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrostatic interactions of 4-carboxy-2,6-dinitrophenyllysine-modified cytochromes c with physiological and non-physiological redox partners

An analysis of the effect of electrostatic properties of 4-carboxy-2,6-dinitrophenyllysine (CDNP-lysine) cytochromes c on their reactions with strongly and weakly binding redox partners is given. For strongly binding systems (cytochrome-c oxidase, cytochrome-c reductase, sulphite oxidase and yeast cytochrome-c peroxidase) the magnitude of the dipole moments of the CDNP cytochromes c determines their relative reactivities. For weakly binding redox agents, such as hexacyanoferrate(III), cobalt(III)tris(1,10-phenanthroline), azurin and plastocyanin, the electrostatic potential at the haem edge accounts for the greater part of the relative activities. Relative rate data were obtained from the literature. It is concluded that the dipole moment of native cytochromes c may account for an approx. 50-fold increase in the efficiency of its physiological activity towards membrane-bound enzymes. A correction on a formula to describe the contribution of a molecular dipole moment to the ionic strength dependence of a bimolecular rate constant (Koppenol, W. H. (1980) Biophys. J. 29, 493-508) leads to an equation nearly identical to that obtained by Van Leeuwen et al. (Van Leeuwen, J.W., Mofers, F.J.M. and Verrman, E.C.I. (1981) Biochim. Biophys. Acta 635, 434-439).

The effect of complex formation upon the reduction rates of cytochrome c and cytochrome c peroxidase compound II

The effect of complex formation between ferricytochrome c and cytochrome c peroxidase (Ferrocytochrome-c:hydrogen peroxide oxidoreductase, EC 1.11.1.5) on the reduction of cytochrome c by N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD), reduced N-methylphenazonium methosulfate (PMSH), and ascorbate has been determined at low ionic strength (pH 7) and 25 degrees C. Complex formation with the peroxidase enhances the rate of ferricytochrome c reduction by the neutral reductants TMPD and PMSH. Under all experimental conditions investigated, complex formation with cytochrome c peroxidase inhibits the ascorbate reduction of ferricytochrome c. This inhibition is due to the unfavorable electrostatic interactions between the ascorbate dianion and the negatively charged cytochrome c-cytochrome c peroxidase complex. Corrections for the electrostatic term by extrapolating the data to infinite ionic strength suggest that ascorbate can reduce cytochrome c peroxidase-bound cytochrome c faster than free cytochrome c. Reduction of cytochrome c peroxidase Compound II by dicyanobis(1,10-phenanthroline)iron(II) (Fe(phen)2(CN)2) is essentially unaffected by complex formation between the enzyme and ferricytochrome c at low ionic strength (pH 6) and 25 degrees C. However, reduction of Compound II by the negatively changed tetracyano-(1,10-phenanthroline)iron(II) (Fe(phen)(CN)4) is enhanced in the presence of ferricytochrome c. This enhancement is due to the more favorable electrostatic interactions between the reductant and cytochrome c-cytochrome c peroxidase Compound II complex then for Compound II itself. These studies indicate that complex formation between cytochrome c and cytochrome c peroxidase does not sterically block the electron-transfer pathways from these small nonphysiological reductants to the hemes in these two proteins.

Kinetics of electron transfer between two Hansenula anomala flavocytochrome b2 derivatives and two simple copper proteins (azurin and stellacyanin)

Two derivatives of Hansenula anomala flavocytochrome b2 have been prepared, one deprived of the flavin prosthetic group (deflavocytochrome b2), and the other consisting of the heme-b-carrying globule (b2 core). The redox potential of the heme in the two derivatives is -5 (+/- 5) mV and -10 (+/- 5) mV respectively, fairly similar to the value of -20 (+/- 5) mV reported for the holoenzyme, indicating a minor effect of the flavin and of the flavodehydrogenase domain on heme potential. The kinetics of azurin and stellacyanin reduction by both derivatives have been investigated. At pH 7.0, I = 0.2 M and 20 degrees C the second-order rate constants are: k = 8 X 10(5) M-1 S-1 for azurin reduction by deflavocytochrome b2; k = 1.6 X 10(6) M-1 S-1 for azurin reduction by b2 core; k = 1 X 10(7) M-1 S-1 for stellacyanin reduction by deflavocytochrome b2; k = 3 X 10(7) M-1 S-1 for stellacyanin reduction by b2 core. The change in pH markedly affects the kinetics in the case of azurin, but has no effect on stellacyanin reduction. The change in ionic strength has a significant effect when deflavocytochrome b2 is the reductant, indicating that the flavodehydrogenase domain plays a role in the stabilization of the transient kinetic complex by means of electrostatic interactions. The kinetic results are discussed in the framework of the Marcus theory.

Crystal structure analyses of reduced (CuI) poplar plastocyanin at six pH values

The structure of poplar plastocyanin in the reduced (CuI) state has been determined and refined, using counter data recorded from crystals at pH 3.8, 4.4, 5.1, 5.9, 7.0 and 7.8 (resolution 1.9 A, 1.9 A, 2.05 A, 1.7 A, 1.8 A and 2.15 A; the final residual R value was 0.15, 0.15, 0.16, 0.17, 0.16 and 0.15, respectively). The molecular and crystal structure of the protein is substantially the same in the reduced state as in the oxidized state. The refinements of the structures of the six forms of the reduced protein could therefore be commenced with a model derived from the known structure of CuII-plastocyanin. The refinements were made by reciprocal space least-squares calculations interspersed with inspections of electron-density difference maps. Precautions were taken to minimize any bias of the results of the refinements in the direction of the starting model. The most significant differences among the structures of the reduced protein at the six pH values, or between them and the structure of the oxidized protein, are concentrated at the Cu site. In the reduced protein at high pH (pH 7.8), the CuI atom is co-ordinated by the N delta(imidazole) atoms of His37 and His87, the S gamma(thiolate) atom of Cys84, and the S delta(thioether) atom of Met92, just as in CuII-plastocyanin. The distorted tetrahedral geometry and the unusually long Cu-S(Met92) bond are retained. The only effects of the change in oxidation state are a lengthening of the two Cu-N(His) bonds by about 0.1 A, and small changes in two bond angles involving the Cu-S(Cys) bond. The high-pH form of reduced plastocyanin accordingly meets all the requirements for efficient electron transfer. As the pH is lowered, the Cu atom and the four Cu-binding protein side-chains appear to undergo small but concerted movements in relation to the rest of the molecule. At low pH (pH 3.8), the CuI atom is trigonally co-ordinated by N delta(His37), S gamma(Cys84) and S delta(Met92). The fourth Cu-ligand bond is broken, the Cu atom making only a van der Waals' contact with the imidazole ring of His87. The trigonal geometry of the Cu atom strongly favours CuI, so that this form of the protein should be redox-inactive. This is known to be the case.(ABSTRACT TRUNCATED AT 400 WORDS)

Preferred sites on cytochrome c for electron transfer with two positively charged blue copper proteins, Anabaena variabilis plastocyanin and stellacyanin

Rate constants for the reactions of horse cytochrome c (E'0 of +260 mV) with the copper proteins Anabaena variabilis plastocyanin (E'0 of +360 mV) used as oxidant and stellacyanin (E'0 of +187 mV) used as reductant have been determined at 25 degrees C, pH 7.5 and 7.0, respectively, and an ionic strength of 0.10 M (NaCl). These rate constants were also measured with eight different singly substituted 4-carboxy-2,6-dinitrophenyl (CDNP) horse cytochrome c derivatives, modified at lysine-7, -13, -25, -27, -60, -72, -86, or -87 and with the trinitrophenyl (TNP) derivative modified at lysine-13. The influence of the modifications on the bimolecular rate constants for these reactions defines the region on the protein that is involved in the electron-exchange reactions and demonstrates that the preferred site is at or near the solvent-accessible edge of the heme prosthetic group on the "front" surface of the molecule. Both reactions are strongly influenced by the lysine-72 modification to the left of the exposed heme edge and, to this extent, behave similar to the earlier studied reaction with azurin. These effects span only an order of magnitude in rate constants and are thus many times smaller than those for the physiological protein redox partners of cytochrome c. While the preferred sites of reaction on the surface of cytochrome c for small inorganic complexes appear to be dependent only on the net charge of the reactants, with the copper proteins additional factors intervene. These influences are discussed in terms of hydrophobic patches and the distribution of charges on the surface of the four copper proteins so far examined.

NMR and kinetic characterization of the interaction between French bean plastocyanin and horse cytochrome c

French bean plastocyanin is shown by stopped-flow kinetics to oxidize horse cytochrome c with k (298 K, I = 0.10 M) = 5.1 X 10(6) M-1 X s-1. The activation parameters demonstrate a satisfactory isokinetic correlation with those previously reported for plastocyanin-cytochrome f reactions. NMR line broadening and shifts of the hyperfine shifted resonances of cytochrome c(III) reveal that strong 1:1 complexes are formed with plastocyanin. The negative patch of plastocyanin and the heme edge region of cytochrome c are shown to be the interacting sites by the hyperfine shift perturbations and competitive binding experiments with Gd3+, which associates selectively with the negative patch of plastocyanin. Complexation of plastocyanin and cytochrome c causes a small change in the heme electronic structure, but there is no NMR or optical evidence for significant conformational changes at either metal center. The rate of the reverse electron-transfer reaction within the plastocyanin-cytochrome c complex has been directly measured by NMR line broadening (krev (298 K) = 87 s-1). A rate for the forward intracomplex electron-transfer reaction (kf (298 K) = 4.8 X 10(3) s-1) has been calculated from krev and the optically measured equilibrium constant.

Preparation of a covalently linked adduct between plastocyanin and cytochrome f

Plastocyanin can be covalently cross-linked to the monomeric cytochrome f from turnip by incubation in the presence of a water-soluble carbodiimide. The adduct between the two proteins has a molecular weight of approximately 43,000 suggesting a 1:1 stoichiometry between the two proteins of the adduct. This stoichiometry has been verified by spectral characterization of the adduct. The efficiency of the cross-linking reaction is pH dependent with a higher degree of cross-linking being observed at pH 6.5 than at pH 7.0.