pH dependence of the oxidation-reduction potential of cytochrome c2

Martin David Kamen (1913-2002): discoverer of carbon 14, and of new cytochromes in photosynthetic bacteria

Martin Kamen was a giant of twentieth century science. Trained as a physical chemist, he was the co-discoverer of radioactive Carbon 14, which has transformed many areas of science as a tracer and as a way to date artifacts. He later switched to the study of metabolism and biochemistry and made important contributions to the understanding of nitrogen fixation and photosynthesis. Finally, he studied cytochromes, primarily from anoxygenic photosynthetic bacteria.

Fluorescence relaxation in intact cells of photosynthetic bacteria: donor and acceptor side limitations of reopening of the reaction center

The dark relaxation of the yield of variable BChl fluorescence in the 10(-5)-10 s time range is measured after laser diode (808 nm) excitation of variable duration in intact cells of photosynthetic bacteria Rba. sphaeroides, Rsp. rubrum, and Rvx. gelatinosus under various treatments of redox agents, inhibitors, and temperature. The kinetics of the relaxation is complex and much wider extended than a monoexponential function. The longer is the excitation, the slower is the relaxation which is determined by the redox states, sizes, and accessibility of the pools of cytochrome [Formula: see text] and quinone for donor and acceptor side-limited bacterial strains, respectively. The kinetics of fluorescence decay reflects the opening kinetics of the closed RC. The relaxation is controlled preferentially by the rate of re-reduction of the oxidized dimer by mobile cytochrome [Formula: see text] in Rba. sphaeroides and Rsp. rubrum and by the rate constant of the [Formula: see text] interquinone electron transfer, (350 μs)(-1) and/or the quinol/quinone exchange at the acceptor side in Rvx. gelatinosus. The commonly used acceptor side inhibitors (e.g., terbutryn) demonstrate kinetically limited block of re-oxidation of the primary quinone. The observations are interpreted in frame of a minimum kinetic and energetic model of electron transfer reactions in bacterial RC of intact cells.

Quinone and non-quinone redox couples in Complex III

The Q cycle mechanism proposed by Peter Mitchell in the 1970's explicitly considered the modification of ubiquinone two-electron redox properties upon binding to Complex III to match the thermodynamics of the other single-electron redox cofactors in the complex, and guide electron transfer to support the generation of a proton electro-chemical gradient across native membranes. A better understanding of the engineering of Complex III is coming from a now moderately well defined thermodynamic description of the redox components as a function of pH, including the Qi/heme b(H) cluster. The redox properties of the most obscure component, Qo, is finally beginning to be resolved.

Plasmon waveguide resonance spectroscopic evidence for differential binding of oxidized and reduced Rhodobacter capsulatus cytochrome c2 to the cytochrome bc1 complex mediated by the conformation of the Rieske iron-sulfur protein

The dissociation constants for the binding of Rhodobacter capsulatus cytochrome c2 and its K93P mutant to the cytochrome bc1 complex embedded in a phospholipid bilayer were measured by plasmon waveguide resonance spectroscopy in the presence and absence of the inhibitor stigmatellin. The reduced form of cytochrome c2 strongly binds to reduced cytochrome bc1 (Kd = 0.02 microM) but binds much more weakly to the oxidized form (Kd = 3.1 microM). In contrast, oxidized cytochrome c2 binds to oxidized cytochrome bc1 in a biphasic fashion with Kd values of 0.11 and 0.58 microM. Such a biphasic interaction is consistent with binding to two separate sites or conformations of oxidized cytochrome c2 and/or cytochrome bc1. However, in the presence of stigmatellin, we find that oxidized cytochrome c2 binds to oxidized cytochrome bc1 in a monophasic fashion with high affinity (Kd = 0.06 microM) and reduced cytochrome c2 binds less strongly (Kd = 0.11 microM) but approximately 30-fold more tightly than in the absence of stigmatellin. Structural studies with cytochrome bc1, with and without the inhibitor stigmatellin, have led to the proposal that the Rieske protein is mobile, moving between the cytochrome b and cytochrome c1 components during turnover. In one conformation, the Rieske protein binds near the heme of cytochrome c1, while the cytochrome c2 binding site is also near the cytochrome c1 heme but on the opposite side from the Rieske site, where cytochrome c2 cannot directly interact with Rieske. However, the inhibitor, stigmatellin, freezes the Rieske protein iron-sulfur cluster in a conformation proximal to cytochrome b and distal to cytochrome c1. We conclude from this that the dual conformation of the Rieske protein is primarily responsible for biphasic binding of oxidized cytochrome c2 to cytochrome c1. This optimizes turnover by maximizing binding of the substrate, oxidized cytochrome c2, when the iron-sulfur cluster is proximal to cytochrome b and minimizing binding of the product, reduced cytochrome c2, when it is proximal to cytochrome c1.

Exposing the complex III Qo semiquinone radical

Complex III Qo site semiquinone has been assigned pivotal roles in productive energy-conversion and destructive superoxide generation. After a 30-year search, a genetic heme bH knockout arrests this transient semiquinone EPR radical, revealing the natural engineering balance pitting energy-conserving, short-circuit minimizing, split electron transfer and catalytic speed against damaging oxygen reduction.

Pseudomonas aeruginosa cytochrome C(551): probing the role of the hydrophobic patch in electron transfer

Cytochrome c(551) from Pseudomonas aeruginosa is a monomeric redox protein of 82 amino-acid residues, involved in dissimilative denitrification as the physiological electron donor of cd(1) nitrite reductase. The distribution of charged residues on the surface of c(551) is very anisotropic: one side is richer in acidic residues whereas the other shows a ring of positive side chains, mainly lysines, located at the border of an hydrophobic patch which surrounds the heme crevice. In order to map in cytochrome c(551) the surface involved in electron transfer, we have introduced specific mutations in three residues belonging to the hydrophobic patch, namely Val23-->Asp, Pro58-->Ala and Ile59-->Glu. The effect of these mutations was analyzed studying both the self-exchange rate and the electron-transfer activity towards P. aeruginosa cd(1) nitrite reductase, the physiological partner and P. aeruginosa azurin, a copper protein often used as a model redox partner in vitro. Our results show that introduction of a negative charge in the hydrophobic patch severely hampers both homonuclear and heteronuclear electron transfer.

Extinction coefficients and midpoint potentials of cytochrome c(6) from the cyanobacteria Arthrospira maxima, Microcystis aeruginosa, and Synechocystis 6803

Cytochrome c(6) is a soluble heme protein that serves as a photosynthetic electron transport component in cyanobacteria and algae, carrying electrons from the cytochrome bf complex to photosystem I. The rapid accumulation of cytochrome c(6) sequence data from a wide range of species, combined with significant advances in determining high resolution three-dimensional structures, provides a powerful database for investigating the relationship between structure and function. The fact that the gene encoding cytochrome c(6) can be readily modified in a number of species adds to the usefulness of cytochrome c(6) as a tool for comparative analysis. Efforts to relate cytochrome c(6) sequence information to structure, and structural information to function depend on knowledge of the physical and thermodynamic properties of the cytochrome from different species. To this end we have determined the optical extinction coefficient, the oxidation/reduction midpoint potential, and the pH dependence of the midpoint potential of cytochrome c(6) isolated from three cyanobacteria, Arthrospira maxima, Microcystis aeruginosa, and Synechocystis 6803.

Effects of temperature and deltaGo on electron transfer from cytochrome c2 to the photosynthetic reaction center of the purple bacterium Rhodobacter sphaeroides

The kinetics of electron transfer from cytochrome c2 to the primary donor (P) of the reaction center from the photosynthetic purple bacterium Rhodobacter sphaeroides have been investigated by time-resolved absorption spectroscopy. Rereduction of P+ induced by a laser pulse has been measured at temperatures from 300 K to 220 K in a series of specifically mutated reaction centers characterized by altered midpoint redox potentials of P+/P varying from 410 mV to 765 mV (as compared to 505 mV for wild type). Rate constants for first-order electron donation within preformed reaction center-cytochrome c2 complexes and for the bimolecular oxidation of free cytochrome c2 have been obtained by multiexponential deconvolution of the kinetics. At all temperatures the rate of the fastest intracomplex electron transfer increases by more than two orders of magnitude as the driving force -deltaGo is varied over a range of 350 meV. The temperature and deltaGo dependences of the rate constant fit the Marcus equation well. Global analysis yields a reorganization energy lambda = 0.96 +/- 0.07 eV and a set of electronic matrix elements, specific for each mutant, ranging from 1.2 10(-4) eV to 2.5 10(-4) eV. Analysis in terms of the Jortner equation indicates that the best fit is obtained in the classical limit and restricts the range of coupled vibrational modes to frequencies lower than approximately 200 cm(-1). An additional slower kinetic component of P+ reduction, attributed to electron transfer from cyt c2 docked in a nonoptimal configuration of the complex, displays a Marcus type dependence of the rate constant upon deltaGo, characterized by a similar value of lambda (0.8 +/- 0.1 eV) and by an average electronic matrix element smaller by more than one order of magnitude. In all of the mutants, as the temperature is decreased below 260 K, both intracomplex reactions are abruptly inhibited, their rate being negligible at 220 K. The free energy dependence of the second-order rate constant for oxidation of cyt c2 in solution suggests that the collisional reaction is partially diffusion controlled, reaching the diffusion limit at exothermicities between 150 and 250 meV over the temperature range investigated.

Expression and characterization of Pseudomonas aeruginosa cytochrome c-551 and two site-directed mutants: role of tryptophan 56 in the modulation of redox properties

The gene coding for Pseudomonas aeruginosa cytochrome c-551 was expressed in Pseudomonas putida under aerobic conditions, using two different expression vectors; the more efficient proved to be pNM185, induced by m-toluate. Mature holo-(cytochrome c-551) was produced in high yield by this expression system, and was purified to homogeneity. Comparison of the recombinant wild-type protein with that purified from Ps. aeruginosa showed no differences in structural and functional properties. Trp56, an internal residue in cytochrome c-551, is located at hydrogen-bonding distance from haem propionate-17, together with Arg47. Ionization of propionate-17 was related to the observed pH-dependence of redox potential. The role of Trp56 in determining the redox properties of Ps. aeruginosa cytochrome c-551 was assessed by site-directed mutagenesis, by substitution with Tyr (W56Y) and Phe (W56F). The W56Y mutant is similar to the wild-type cytochrome. On the other hand, the W56F mutant, although similar to the wild-type protein in spectral properties and electron donation to azurin, is characterized by a weakening of the Fe-Met61 bond, as shown in the oxidized protein by the loss of the 695 nm band approx. 2 pH units below the wild-type. Moreover, in W56F, the midpoint potential and its pH-dependence are both different from the wild-type. These results are consistent with the hypothesis that hydrogen-bonding to haem propionate-17 is important in modulation of the redox properties of Ps. aeruginosa cytochrome c-551.

Regulation of the redox order of four hemes by pH in cytochrome c3 from D. vulgaris Miyazaki F

The assignment of 1H-NMR signals of the heme methyl and propionate groups of cytochrome c3 of D. vulgaris Miyazaki F was performed. The heme assignment was revised for hemes 2 and 3 (sequential heme numbering). Namely, heme 4 is mainly reduced at first with hemes 1, 2 and 3 following it in this order. The p2H titration of heme methyl signals in four macroscopic oxidation states was performed in the p2H range of 5.2 to 9.0. While the heme methyl resonances in the fully oxidized state showed just small changes with p2H, most resonances in the intermediate oxidation states revealed clear p2H dependence. In particular, the methyl resonances of heme 1 shifted significantly in the acidic region. Then, the chemical shifts of beta-CH2 (next to the carboxyl group) of all propionate groups in the fully oxidized state were observed at various p2H in the range of 4.5 to 9.0. Only the propionate group at C-13 (IUPAC-IUB nomenclature) of heme 1 showed a clear change in this p2H range, its titration curve being similar to those of the methyl resonances of heme 1 in the intermediate oxidation states. pKa of the propionate group was 5.95 +/- 0.05. Analysis of the microscopic formal redox potentials was carried out for the observations at p2H 5.2, 7.1 and 9.0. The redox potentials of heme 1 showed the most remarkable p2H dependence, resulting in the change of the order of the redox potentials of four hemes. A significant change was also found in the interacting potential between hemes 1 and 2. In the light of the p2H-titration experiments, the propionate at C-13 of heme 1 was identified as the most plausible ionizable group responsible for the p2H dependence of microscopic redox potentials of heme 1 in the acidic region.

Cyclic voltammetry and 1H-NMR of Rhodopseudomonas palustris cytochrome c2 pH-dependent conformational states

The pH-induced protein conformational transitions and changes in the ligation state of the heme iron in cytochrome c2 from Rhodopseudomonas palustris were monitored by electrochemical and spectroscopic measurements. In the pH range 1.5-11, the E degree values (and/or the peak potentials) determined by cyclic voltammetry, the electronic spectra and the hyperfine-shifted 1H-NMR resonances of the protein are sensitive to a number of acid/base equilibria. In particular, four equilibria have been determined for the oxidized protein with pKa values of 2.5, 5.5, 6.6 and 9.0. The lowest pKa most probably involves disruption of both axial heme iron bonds and protein unfolding. The subsequent pKa is associated with a low-pH oxidation of the protein by dioxygen, which is accompanied by a conformational change. The equilibrium with an apparent pKa of 6.6 modulates the E degree values without determining any detectable spectral change and most likely involves the acid/base equilibrium of an histidine residue in close vicinity of the heme (possibly His53). Finally, the alkaline ionization is due to the replacement of the methionine axially bound to the heme iron with a stronger (most probably N-donor) ligand. The reduced alkaline form is unstable and spontaneously converts to the neutral reduced form with a kinetic constant of 0.98 s-1 at pH 9.2.

Mutations of iso-1-cytochrome c at positions 13 and 90. Separate effects on physical and functional properties

Residues at positions 13 (lysine or arginine) and 90 (glutamate or aspartate) of eukaryotic cytochromes c have been conserved during evolution; Cys102, however, is found only in yeast cytochrome c. The positively charged residue at position 13 and the negatively charged residue at position 90 are close together in those cytochromes c for which three-dimensional structures are available. We have replaced the amino acids at these two positions by cysteine in Saccharomyces cerevisiae iso-1-cytochrome c; in an earlier study, Cys102 was replaced by threonine without negatively influencing the physical or enzymic properties of the protein. The mutated proteins [R13C, C102T]cytochrome c (iso-1-cytochrome c containing Arg13-->Cys and Cys102-->Thr mutations), [D90C, C102T]cytochrome c (iso-1-cytochrome c containing Asp90-->Cys and Cys102-->Thr mutations) and [R13C, D90C, C102T]cytochrome c (iso-1-cytochrome c containing Arg13-->Cys, Asp90-->Cys, and Cys102-->Thr mutations) are functional in vivo. Free sulfhydryl titration shows that the doubly mutated forms each contain one sulfhydryl group while the triple mutant contains two sulfhydryl groups. The stability of mutant [R13C, C102T]cytochrome c resembles that of [C102T] cytochrome c, whereas the stability of [D90C, C102T]cytochrome c resembles the stability of [R13C, D90C, C102T]cytochrome c. The activity of cytochrome-c oxidase using cytochrome c was monitored polarographically. Compared to the wild-type or [C102T]cytochrome c, which shows two kinetic phases with cytochrome-c oxidase, [D90C, C102T]cytochrome c has much the same profile; [R13C, C102T]cytochrome c and [R13C, D90C, C102T]cytochrome c exhibit one kinetic phase with decreased activity. Electron-transfer activity of the mutant cytochromes c is inhibited by Hg2+. The inhibition is highest for the triple mutant, less for [R13C, C102T]cytochrome c, even less for [D90C, C102T]cytochrome c and insignificant for the wild type. It would appear as though the stability of the triple mutant follows the changes that result from the Asp90-->Cys mutation while the activity changes follow those of the Arg13-->Cys mutation.

Cytochrome c2 mutants of Rhodobacter capsulatus

Although structurally related to other members of the class I c-type cytochromes, the cytochromes c2 have little amino acid sequence homology to the eukaryotic cytochromes c. Moreover, the cytochromes c2 exhibit distinct properties such as redox potential and an isoelectric point. In an effort to understand the differences between the cytochromes c2 and the other class I c-type cytochromes, we have developed a genetic system to study Rhodobacter capsulatus cytochrome c2 by site-directed mutagenesis. We describe here overproduction of R. capsulatus wild-type cytochrome c2 in cytochrome c2-minus strains of R. capsulatus and Rhodobacter sphaeroides. We demonstrate that R. capsulatus wild-type cytochrome c2 can transcomplement for photosynthetic growth in R. sphaeroides. Further, we describe the generation, expression, and in vivo functionality properties of nine R. capsulatus site-directed mutants. We show that mutants K12D, K14E, K32E, K14E/K32E, P35A, W67Y, and Y75F are overproduced and functional in vivo. In contrast, mutants Y75C and Y75S are expressed at low levels and exhibit poor functionality in vivo. These findings establish an effective system for the production of R. capsulatus site-directed mutants and demonstrate that interspecies complementation can be used to detect defective cytochrome c2 mutants.

The effects of surface charges on the redox potential of cytochrome c2 from the purple phototrophic bacterium Rhodobacter capsulatus

Four site-directed mutants of Rhodobacter capsulatus cytochrome c2, which substitute lysines at three positions with aspartate or glutamate, have been prepared. Mutations included the single charge substitutions K12D, K14E, and K32E and a double charge substitution K14E/K32E. Characterization of the ionic strength dependence of the wild-type and mutant redox potentials in the "nonbinding" buffer Tris-cacodylate suggests that (i) at zero ionic strength introduction of negatively charged groups stabilizes the oxidized state by 11-14 mV per charge and (ii) at high ionic strengths where the charged groups are masked, the effects of single charge substitutions are overcome; however, the redox potential of the double charge substitution is still affected. These results indicate that at physiological ionic strengths charge distribution only affects redox potential when the heme environment has been perturbed by a structural perturbation and that the determinants of redox potential in c-type cytochromes is primarily due to the local heme environment.

Isolation and characterization of cytochrome c550 from the methylamine-oxidizing electron-transport chain of Thiobacillus versutus

The isolation and purification of cytochrome c550 from the methylamine-oxidizing electron-transport chain in Thiobacillus versutus is reported. The cytochrome is a single-heme-containing type I cytochrome c with a relative molecular mass of 16 +/- 1 kDa, an isoelectric point of 4.6 +/- 0.1, a midpoint potential of 272 +/- 3 mV at pH less than 4 and 255 +/- 5 mV at pH = 7.0, and an axial coordination of the Fe by a methionine and a histidine. The midpoint potential decreases with increasing pH due to the deprotonation of a group tentatively identified as a propionate (pKa = 6.5 +/- 0.1 and 6.7 +/- 0.1 in the oxidized and reduced protein, respectively) and a change in the Fe coordination at pH greater than 10. The electron-self-exchange rate appears to depend strongly on the ionic strength of the solution and is relatively insensitive to changes in pH. At 313 K and pH 5.2 the electron-exchange rate amounts to 0.7 x 10(2) M-1 s-1 and 5.3 x 10(2) M-1 s-1 at I = 40 mM and I = 200 mM, respectively. Amino acid composition and molar absorption coefficients at various wavelengths are reported. Resonances of heme protons and the epsilon H3 group of the ligand methionine of the Fe have been identified in the 1H-NMR spectrum of the reduced as well as the oxidized cytochrome.

Assignments of 15N and 1H NMR resonances and a neutral pH ionization in Rhodospirillum rubrum cytochrome c2

The phi NH proton and 15N resonances of the ligand histidine of Rhodospirillum rubrum fericytochrome c2 are found at 14.7 and 184 ppm, respectively, contradicting the proposal that this proton is absent in the R. rubrum ferricytochrome. Substitution of the deuterium atom for this proton causes small upfield shifts of the phi nitrogen in both oxidation states, indicating that the phi NH-peptide carboxyl hydrogen bond is not substantially weakened by the substitution. The proton and 15N resonances of the indolic NH group of the invariant tryptophan-62 and numerous proton resonances of the heme and extraheme ligands in the spectrum of the ferricytochrome are also assigned. An ionization in the ferrocytochrome occurring at neutral pH is assigned to the single nonligand histidine. This attribution is supported by the direct measurement of the ionization by NOE difference spectroscopy and by comparative structural arguments involving closely related cytochromes and chemically modified cytochromes.

Characterization of pH-dependent conformational heterogeneity in Rhodospirillum rubrum cytochrome c2 using 15N and 1H NMR

The 15N-enriched ferricytochrome c2 from Rhodospirillum rubrum has been studied by 15N and 1H NMR spectroscopy as a function of pH. The 15N resonances of the heme and ligand tau nitrogen are broadened beyond detection because of paramagnetic relaxation. The 15N resonance of the ligand histidine phi nitrogen was unambiguously identified at 184 ppm (pH 5.6). The 15N resonances of the single nonligand histidine are observed only at low pH, as in the ferrocytochrome because of the severe broadening caused by tautomerization. The dependence of the 15N and 1H spectra of the ferricytochrome on pH indicated that the ligand histidine tau NH does not dissociate in the neutral pH range and is involved in a hydrogen bond, similar to that in the reduced state. Because neither deprotonated nor non-hydrogen-bonded forms of the ligand histidine are observed in the spectra of either oxidation state, the participation of such forms in producing heterogeneous populations having different electronic g tensors is ruled out. Transitions having pKa's of 6.2, 8.6, and 9.2 are observed in the ferricytochrome. The localized conformational change around the omega loops is observed in the neutral pH range, as in the ferrocytochrome. Structural heterogeneity leads to multiple resonances of the heme ring methyl at position 8. The exchange rate between the conformations is temperature dependent. The transition with a pKa of 6.2 is assigned to the His-42 imidazole group. The displacement of the ligand methionine, which occurs with a pKa of 9.2, causes gross conformational change near the heme center.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of specific lysine residues in the reaction of Rhodobacter sphaeroides cytochrome c2 with the cytochrome bc1 complex

The reaction of Rhodobacter sphaeroides cytochrome c2 with the Rb. sphaeroides cytochrome bc1 complex was studied by using singly labeled cytochrome c2 derivatives. Cytochrome c2 was treated with chlorodinitrobenzoic acid to modify lysine amino groups to negatively charged carboxydinitrophenyllysines and separated into eight different fractions by ion-exchange chromatography on a Whatman SE 53 (sulfoxyethyl)cellulose column. Peptide mapping studies indicated that six of these fractions were modified at single lysine amino groups. Each of the derivatives had the same Vmax value as native cytochrome c2 in the steady-state reaction with the Rb. sphaeroides cytochrome bc1 complex. However, the Km values of the cytochrome c2 derivatives modified at lysines 10, 55, 95, 97, 99, and 106 were found to be larger than that of native cytochrome c2 by factors of 6, 2, 3, 32, 13, and 8, respectively. These results indicate that lysines located in the sequence 97-106 on the left side of the heme crevice have the greatest involvement in binding the cytochrome bc1 complex. The involvement of lysine 97 is especially significant because it is located in an extra loop comprising residues 89-98 that is not present in eukaryotic cytochrome c.

pH-induced changes in Rhodospirillum rubrum cytochrome c2 and subsequent renaturation: an 15N NMR study

The 15N-enriched ferrocytochrome c2 from Rhodospirillum rubrum was studied by 15N NMR at different solvent pH values. The mobility and chemical shift of the N-terminal glutamic acid (335.4 ppm at pH 5.1) were found to depend on pH. It was least mobile between pH 8 and 9.0, which is explained in terms of pH-dependent conformational changes and formation of salt linkages and/or hydrogen bonds. The resonances of the lysine side chains are centered around 341.7 ppm at low pH and move upfield with pH by about 8.4 ppm with pKa values of 10.8. The exchange rates of the epsilon NH protons are lowest near their pKa values. The protein is very stable in the pH range between 4.9 and 10.0 but unfolds abruptly at pH 10.5-11. Denaturation was verified by the measurement of several parameters by NMR. The renaturation of the protein demonstrates that the folding begins with reformation of heme coordination and establishment of a hydrophobic core, followed by positioning of side chains and peptide backbones linking the nucleation centers. The repositioning processes had time scales of minutes to hours in contrast to the reported values of seconds in some studies.

15N and 1H NMR studies of Rhodospirillum rubrum cytochrome c2

15N-Enriched cytochrome c2 was purified from Rhodospirillum rubrum that had been grown on 15NH4Cl, and the diamagnetic iron(II) form of the cytochrome was studied by 15N and 1H NMR spectroscopy. 15N resonances of the four pyrrole nitrogens, the ligand histidine nitrogens, the highly conserved tryptophan indole nitrogen, and some proline nitrogens are assigned. The resonances of the single nonligand histidine are observed only at low pH because of severe broadening produced by proton tautomerization. The resonances of exchangeable protons bonded to the nitrogens of the ligand histidine, the tryptophan, and some amide groups are also assigned. The exchange rates of the nitrogen-bound protons vary greatly: most have half-lives of less than minutes, the indolic NH of Trp-62 exchanges with a half-time of weeks, and the ligand histidine NH proton exchanges with a half-time of months. The latter observation is indicative of extreme exclusion of solvent from the area surrounding the ligand histidine and lends credence to theories implicating the degree of hydrophobicity in this region as an important factor in adjusting the midpoint potential. The dependence of the 15N and 1H NMR spectra of ferrocytochrome c2 on pH indicates neither the Trp-62 nor the ligand His side chains become deprotonated to any appreciable extent below pH 9.5. The His-18 NH remains hydrogen bonded, presumably to the Pro-19 carboxyl group, throughout the pH titrations. Because neither deprotonated nor non-hydrogen-bonded forms of His-18 are observed in spectra of the ferrocytochrome, the participation of such forms in producing a heterogeneous population having different g tensor values seems unlikely.(ABSTRACT TRUNCATED AT 250 WORDS)

On the energetics of conformational changes and pH dependent redox behaviour of electron transfer proteins

Calculations of the electrostatic interaction energies for four metalloproteins that carry out electron transfer are reported. Each protein has a pH dependent redox potential from which the measured electrostatic interaction energy is obtained. The calculations were made using the X-ray structure coordinates and a semimacroscopic model of the interactions. For cytochrome c-551 and HIPIP the calculated and observed interaction energies were found to be approximately the same, in agreement with the fact that significant conformational changes do not accompany the ionisations. For cytochrome c2 and azurin, however, major differences were found between the calculated and observed values. These are accounted for primarily by the occurrence of significant conformational changes accompanying the ionisations. The reorganisation energies for these conformational changes are approximately 7.0 and approximately 11.1 kJ.mol-1, respectively.

Alkylamine derivatives of cytochrome c. Comparison with other lysine-modified analogues illuminates structure/function relations in the protein

For investigations of the functional roles of the lysine residues of cytochrome c, analogues in which these residues are modified without charge loss are highly desirable. The 19 lysine residues of the horse heart protein have been modified by reductive alkylation. Two analogues were prepared, using formaldehyde and acetone as the dialkylating and monoalkylating reagent respectively. Modification was shown to be clean and quantitative. Characterisation of the alkylamine derivatives by physicochemical measurements and biological activity determinations was carried out. The potential of these analogues in structure/function studies of cytochrome c is discussed. It is illustrated by their use, in conjunction with other lysine-modified derivatives, to investigate the extent to which surface charge determines redox potential, and to study the physicochemical changes that accompany rising pH. In the latter case the observed phenomena are not as closely correlated as previously thought, suggesting that there is a complex set of rearrangements of structure underlying the functional changes. The data confirm that modification of the lysine residues influences these changes. These residues participate in numerous surface intramolecular links, so the lack of correlation may be explained if each of the changing parameters were under the influence of a different set of residues. However, neither a lysine residue, nor a histidine residue directly displaces methionine from the sixth coordination position of the haem iron at alkaline pH.

Soluble cytochrome composition of the purple phototrophic bacterium, Rhodopseudomonas sphaeroides ATCC 17023

A detailed study of the soluble cytochrome composition of Rhodopseudomonas sphaeroides (ATCC 17023) indicates that there are five c-type cytochromes and one b-type cytochrome present. The molecular weights, heme contents, amino acid compositions, isoelectric points, and oxidation-reduction potentials were determined and the proteins were compared with those from other bacterial sources. Cytochromes c2 and c' have previously been well characterized. Cytochrome c-551.5 is a diheme protein which has a very low redox potential, similar to certain purple bacterial and algal cytochromes. Cytochrome c-554 is an oligomer, which is spectrally similar to the low-spin isozyme of cytochrome c' found in other purple bacteria (e.g., Rhodopseudomonas palustris cytochrome c-556). An unusual high-spin c-type heme protein has also been isolated. It is spectrally distinguishable from cytochrome c' and binds a variety of heme ligands including oxygen. A large molecular-weight cytochrome b-558 is also present which appears related to a similar protein from Rhodospirillum rubrum, and the bacterioferritin from Escherichia coli. None of the soluble proteins appear to be related to the abundant membrane-bound c-type cytochrome in Rps. sphaeroides which has a larger subunit molecular weight similar to mitochondrial cytochrome c1 and chloroplast cytochrome f.

A spectroscopic investigation of the structure and redox properties of Escherichia coli cytochrome b-562

The six-coordinate monohaem ferricytochrome b-562 from Escherichia coli exhibits two haem-linked pH-dependent transitions detected by NMR and optical spectroscopy. Only one of these transitions, that of the Fe(III)-coordinated His-102, is detected by EPR and MCD; the ionisation of a haem propionate is not. Both ionisations are redox-state-dependent and the midpoint redox potential of the protein is markedly pH-dependent. Over the pH range 5.0 to 8.5 the potential drops from 260 mV to 110 mV and at least five single proton ionisations are responsible for this. In addition to the two spectroscopically identified ferricytochrome ionisations, there are at least three unidentified ionisations, two of which occur in the ferrous protein. From a consideration of the X-ray structure, together with NMR data, it seems probable that at least one of these ionisations involves an amino acid carboxylate. The X-ray structure also suggests that the relatively low pKa of His-102 is a result of its proximity to Arg-98. However, an appreciable interaction between these groups requires that the solution conformation differs slightly from the X-ray structure. The fast rate of electron self-exchange, over 4 X 10(6) M-1 X s-1 at 315 K and pH* 7, may be a reflection of the fact that, as shown by the X-ray structure, a large amount of the haem and axial histidine ligand are exposed at the molecular surface with an asymmetric distribution of charged groups surrounding them.

NMR studies of electron transfer mechanisms in a protein with interacting redox centres: Desulfovibrio gigas cytochrome c3

The proton NMR spectra of the tetrahaem cytochrome c3 from Desulfovibrio gigas were examined while varying the pH and the redox potential. The analysis of the NMR reoxidation pattern was based on a model for the electron distribution between the four haems that takes into account haem-haem redox interactions. The intramolecular electron exchange is fast on the NMR time scale (larger than 10(5) s-1). The NMR data concerning the pH dependence of the chemical shift of haem methyl resonances in different oxidation steps and resonance intensities are not compatible with a non-interacting model and can be explained assuming a redox interaction between the haems. A complete analysis at pH* = 7.2 and 9.6, shows that the haem-haem interacting potentials cover a range from -50 mV to +60 mV. The midpoint redox potentials of some of the haems, as well as some of their interacting potentials, are pH-dependent. The physiological relevance of the modulation of the haem midpoint redox potentials by both the pH and the redox potential of the solution is discussed.

The effect of iron-hexacyanide binding on the determination of redox potentials of cytochromes and copper proteins

The midpoint redox potentials of Pseudomonas aeruginosa cytochrome c-551 and Rhodopseudomonas viridis cytochrome c2 were measured as a function of pH in the presence of Euglena cytochrome c-558 and the results compared with those obtained in the presence of ferro-ferricyanide. The pattern of pH dependence observed for the two bacterial cytochromes was the same whether it was measured by equilibrium with another redox protein or with the inorganic redox couple. Thus, the pH dependence of redox potential is not a consequence of pH-dependent ligand binding. The midpoint potential of Ps. aeruginosa azurin was measured as a function of pH using both ferro-ferricyanide mixtures and redox equilibrium with horse cytochrome c or Rhodopseudomonas capsulata cytochrome c2. In this case also the pattern of pH dependence obtained did not vary with the redox system used and it closely resembled that of Ps. aeruginosa cytochrome c-551. This is consistent with the observation that the equilibrium between cytochrome c-551 and azurin is relatively independent of pH. An equation was derived which described ph-dependent ligand binding and which can produce theoretical curves to fit the experimental pH dependence of redox potential for both cytochrome and azurin. However, the pronounced effect on such curves produced by varying the ligand association constants, and the insensitivity of the experimental data to changes in ionic strength, suggest that ligand binding effects do not account for the pH dependence of redox potential.

Control of redox properties of cytochrome c by special electrostatic interactions

An assessment is made of the proposal: electrostatic interactions between the ferric ion of oxidised cytochrome c and its haem propionate sidechains assists in determining the value of the redox potential and plays an important role in the redox state conformation change. Differences between the properties of homologous cytochromes are proposed to be due to differences associated with the charge on their haem propionates.

NMR redox studies of Desulfovibrio vulgaris Cytochrome c3. Electron transfer mechanisms

The 300-MHz proton NMR spectra of the tetrahaem cytochrome c3 from Desulfovibrio vulgaris were examined while varying the pH and the redox potential. The analysis of the complete NMR reoxidation pattern was done taking into account all the 16 redox states that can be present in the redox titration of a tetra-redox-center molecule. A network of saturation transfer experiments performed at different oxidation stages, between the fully reduced and the fully oxidized states, allowed the observation of different resonances for some of the haem methyl groups. In the present experimental conditions, some of the haems show a fast intramolecular electron exchange rate, but the intermolecular electron exchange is always slow. In intermediate reoxidation stages, large shifts of the resonances of some haem methyl groups were observed upon changing the pH. These shifts are discussed in terms of a pH dependence of the haem midpoint redox potentials. The physiological relevance of this pH dependence is discussed.

Resolved difference spectra of redox centers involved in photosynthetic electron flow in Rhodopseudomonas capsulata and Rhodopseudomonas sphaeroides

1. In Rhodopseudomonas sphaeroides the Qx absorption band of the reaction center bacteriochlorophyll dimer which bleaches on photo-oxidation is both blue-shifted and has an increased extinction coefficient on solubilisation of the chromatophore membrane with lauryldimethylamine-N-oxide. These effects may be attributable in part to the particle flattening effect. 2. The difference spectrum of photo-oxidisable c type cytochrome in the chromatophore was found to have a slightly variable peak position in the alpha-band (lambda max at 551--551.25 nm); this position was always red-shifted in comparison to that of isolated cytochrome c2 (lambda max at 549.5 +/- 0.5 nm). The shift in wavelength maximum was not due to association with the reaction center protein. A possible heterogeneity in the c-type cytochromes of chromatophores is discussed. 3. Flash-induced difference spectra attributed to cytochrome b were resolved at several different redox potentials and in the presence and absence of antimycin. Under most conditions, one major component, cytochrome b50 appeared to be involved. However, in some circumstances, reduction of a component with the spectral characteristics of cytochrome b-90 was observed. 4. Difference spectra attributed to (BChl)2, (Formula: see text), c type cytochrome and cytochrome b50 were resolved in the Soret region for Rhodopseudomonas capsulata. 5. A computer-linked kinetic spectrophotometer for obtaining automatically the difference spectra of components functioning in photosynthetic electron transfer chains is described. The system incorporates a novel method for automatically adjusting and holding the photomultiplier supply voltage.

Proton nuclear-magnetic-resonance and resonance Raman studies of thermophilic cytochrome c-552 from Thermus thermophilus HB8

The pH and temperature dependences of the 270-MHz proton nuclear magnetic resonance and resonance Raman spectra of Thermus thermophilus cytochrome c-552 were studied. Observation of the NMR methyl signal of the iron-bound methionine indicates that a methionine residue is the sixth ligand of heme iron in both ferric and ferrous states, although the environment of this methionine is not similar to that in mitochondrial cytochrome c. The NMR methyl signal of the coordinated methionine in the ferrous state was observed even at 87 degrees C, indicating the retention of the methionine ligand at the sixth coordination position. None of resonance Raman lines in ferrous cytochrome c-552 at higher temperatures showed a prominant temperature-dependent frequency shift, which implies that the heme iron was still bound with strong ligands and retained the low-spin state. In either redox state overall thermal denaturation did not occur even at 87 degrees C, although the ferric form existed in thermal spin mixture of the low-spin and high-spin species at higher temperatures. The hyperfine-shifted NMR resonances of the ferric form indicated rapid exchange of the sixth ligand at alkaline pH in the process of a single-step alkaline isomerization.

pH dependence of the redox potential of Pseudomonas aeruginosa cytochrome c-551

The redox potential of Ps. aeruginosa cytochrome c-551 varies with pH between pH 5 and 8. The pH dependence can be analysed in terms of a pKa of 6.2 in the oxidised form and a pKa of 7.3 in the reduced form. The same pKa values are also observed in NMR spectra of the two oxidation states and the pKa of 7.3 is observed in titration of the visible absorption spectrum of the ferrocytochrome. From the NMR studies these pKa values have been assigned to the ionisation of one of the haem propionic acid groups. pH dependence of redox potential is of variable occurrence among cytochromes and the possible significance and basis of this variation is discussed.

Electron acceptors of bacterial photosynthetic reaction centers. II. H+ binding coupled to secondary electron transfer in the quinone acceptor complex

The photoreduction of ubiquinone in the electron acceptor complex (QIQII) of photosynthetic reaction centers from Rhodopseudomonas sphaeroides, R26, was studied in a series of short, saturating flashes. The specific involvement of H+ in the reduction was revealed by the pH dependence of the electron transfer events and by net H+ binding during the formation of ubiquinol, which requires two turnovers of the photochemical act. On the first flash QII receives an electron via QI to form a stable ubisemiquinone anion (QII-); the second flash generates QI-. At low pH the two semiquinones rapidly disproportionate with the uptake of 2 H+, to produce QIIH2. This yields out-of-phase binary oscillations for the formation of anionic semiquinone and for H+ uptake. Above pH 6 there is a progressive increase in H+ binding on the first flash and an equivalent decrease in binding on the second flash until, at about pH 9.5, the extent of H+ binding is the same on all flashes. The semiquinone oscillations, however, are undiminished up to pH 9. It is suggested that a non-chromophoric, acid-base group undergoes a pK shift in response to the appearance of the anionic semiquinone and that this group is the site of protonation on the first flash. The acid-base group, which may be in the reaction center protein, appears to be subsequently involved in the protonation events leading to fully reduced ubiquinol. The other proton in the two electron reduction of ubiquinone is always taken up on the second flash and is bound directly to QII-. At pH values above 8.0, it is rate limiting for the disproportionation and the kinetics, which are diffusion controlled, are properly responsive to the prevailing pH. Below pH 8, however, a further step in the reaction mechanism was shown to be rate limiting for both H+ binding electron transfer following the second flash.

Equilibrium and kinetic measurements of the redox potentials of cytochromes c2 in vitro and in vivo

The equilibrium oxidation-reduction mipoint potential (Em) of isolated Rhodopseudomonas sphaeroides cytochrome c2 exhibits a pH-dependent behavior which can be ascribed to a pK on the oxidized form at pH 8.0 (Pettigrew et al. (1975) Biochim. Biophys. Acta 430, 197-208). However, as with mammalian cytochrome c (Brandt, K.G. Parks, P.C., Czerlinski, G.H. and Hess, G.P. (1966) J. Biol. Chem. 241, 4180-4185) this pK can more properly be attributed to the combination of a pK beyond pH 11, and a slow conformational change of the ferricytochrome. This has been demonstrated by resolving the Em of cytochrome c2 before and after the conformational change. The Em of the unaltered form is essentially pH independent between pH 7 and 11.5, and the lower equilibrium Em is due solely to the conformational change. In vivo the conformational change is prevented by the binding of the cytochrome c2 to the photochemical reaction center, and the cytochrome exhibits an essentially pH-independent Em from pH 5 to 11. The alkaline transition thus has little physiological significance, and it is unlikely that the redox reactions of cytochrome c2 in vivo involve protons.

Redox potentials of the photosynthetic bacterial cytochromes c2 and the structural bases for variability

The cytochromes c2 of the Rhodospirillaceae show a much greater variation in redox potential and its pH dependence than the mitochondrial cytochromes c that have been studied. It is proposed that the range of redox potential for cytochromes c2 functioning as the immediate electron donor to photo-oxidised bacteriochlorophyll may be 345-395 mV at pH 5. Closely related cytochromes c2 with different redox potentials show patterns of amino acid substitution which are consistent with changes in hydrophobicity near the haem being at least a partial determinant of redox potential. More distantly related cytochromes are difficult to compare because of the large number of amino acid substitutions and the probability that there are subtle changes in overall peptide chain folding. The redox potential versus pH curves can be analysed in terms of either one ionisation in the oxidised form or two in the oxidised form and one in the reduced. The pK in the oxidised form at higher pH values can be correlated with the pK for the disappearance or shift of the near infrared absorption band located near 695 nm. The structural bases of these ionisations are not known but the possible involvement of the haem propionate residues is discussed.

Redox potentials in hydro-organic media at normal and subzero temperatures. Ferro-ferricyanide and cytochrome c as models

Redox potentials of ferro-ferricyanide and cytochrome c were measured in water/ethylene glycol and water/dimethylsulfoxide (volume ratio from 100/0 to 50/50) between 25 and -25 degrees C. For both systems, the midpoint potential decreases in the presence of organic solvents and increases by cooling. The magnitude of these variations is larger in dimethylsulfoxide than in ethylene glycol; moreover in the same solvent mixture it is larger with ferro-ferricyanide than with cytochrome c, so that the difference between the redox potentials of these two systems can be strongly affected and even reversed. While in pure water (cacodylate buffer pH 7.0, NaCl 0.1 M) they are respectively +388 and +265 mV, in 50% dimethylsulfoxide at 25 degrees C they decrease to +112 and +208 mV. Reduction of cytochrome c by ferro-ferricyanide, in this mixture, is then expected and was indeed observed. On the other hand, as (deltaE/deltaT)T, (E being the redox potential) is higher for ferro-ferricyanide than for cytochrome c, the oxidative power of the former for the latter is expected to increase as temperature decreases. This effect was observed in 50% ethylene glycol at -16 degrees C. Organic solvents and large temperature variations appear then as powerful perturbants of redox reactions. Their effects should be taken into account in studies of redox reactions carried out in cooled hydro-organic media.

The pH dependence of the oxidation-reduction midpoint potential of cytochromes c2 in vivo

A recent report by Pettigrew et al. [Biochim, Biophys. Acta 430, (1976), 197-208] has examined the pH dependence of the oxidation-reduction midpoint potential of cytochromes c2 in vitro. In media of low ionic strength, these workers identified several pKs on the oxidized forms of the cytochromes, and in some cases there were also pKs on the reduced species. In this work we examine the pH dependence of the midpoint potentials of the cytochromes in situ, attached to the chromatophore membrane. Under these conditions no pK values are detected, and we conclude that in vivo there is no net change in the protonation of cytochrome c2 during oxidation or reduction.