Second order kinetics of the reduction of cytochrome c2 by the ubiquinone cytochrome b-c2 oxidoreductase of Rhodopseudomonas sphaeroides

Engineering the assembly of heme cofactors in man-made proteins

Timely ligation of one or more chemical cofactors at preselected locations in proteins is a critical preamble for catalysis in many natural enzymes, including the oxidoreductases and allied transport and signaling proteins. Likewise, ligation strategies must be directly addressed when designing oxidoreductase and molecular transport functions in man-made, first-principle protein constructs intended to operate in vitro or in vivo. As one of the most common catalytic cofactors in biology, we have chosen heme B, along with its chemical analogues, to determine the kinetics and barriers to cofactor incorporation and bishistidine ligation in a range of 4-α-helix proteins. We compare five elementary synthetic designs (maquettes) and the natural cytochrome b562 that differ in oligomeric forms, apo- and holo-tertiary structural stability; qualities that we show can either assist or hinder assembly. The cofactor itself also imposes an assembly barrier if amphiphilicity ranges toward too hydrophobic or hydrophilic. With progressive removal of identified barriers, we achieve maquette assembly rates as fast as native cytochrome b562, paving the way to in vivo assembly of man-made hemoprotein maquettes and integration of artificial proteins into enzymatic pathways.

Cyclic electron transfer in Heliobacillus mobilis involving a menaquinol-oxidizing cytochrome bc complex and an RCI-type reaction center

Flash-induced absorption changes arising from b-type hemes were studied on whole cells of Heliobacillus mobilis under physiological and redox-controlled conditions. The sensitivity of the monitored redox changes to inhibitors of cytochrome bc complexes and the redox potential dependence of reduction and oxidation reactions of cytochrome b-hemes demonstrate that the respective b-hemes are part of a cytochrome bc complex. Both the half-time and the extent of flash-induced reduction of cytochrome b titrated with apparent potentials of about -60 and -50 mV (both n = 2), respectively, i.e., close to the Em,7 value of the menaquinone (MK) pool, indicating a collisional interaction between menaquinol and the Qo site of the cytochrome bc complex. At strongly reducing ambient potentials (< -150 mV), a net flash-induced oxidation of b-hemes was observed in agreement with the Em,7 values of the individual hemes of -90 mV (b(h)) and -190 mV (b(l)) determined in equilibrium redox titrations on membrane fragments. From the extent of photooxidized b- and c-type hemes as well as P798+, a stoichiometry of 0.6-0.75 cytochrome bc complexes per photosynthetic reaction center was estimated. The kinetic behavior and also the energy profiles for Q-cycle turnover of the heliobacterial complex are compared to those of cytochrome bc1 complexes from purple bacteria and of cytochrome b6f complexes from chloroplasts.

Requirement of histidine 217 for ubiquinone reductase activity (Qi site) in the cytochrome bc1 complex

Folding models suggest that the highly conserved histidine 217 of the cytochrome b subunit from the cytochrome bc1 complex is close to the quinone reductase (Qi) site. This histidine (bH217) in the cytochrome b polypeptide of the photosynthetic bacterium Rhodobacter capsulatus has been replaced with three other residues, aspartate (D), arginine (R), and leucine (L). bH217D and bH217R are able to grow photoheterotrophically and contain active cytochrome bc1 complexes (60% of wild-type activity), whereas the bH217L mutant is photosynthetically incompetent and contains a cytochrome bc1 complex that has only 10% of the wild-type activity. Single-turnover flash-activated electron transfer experiments show that cytochrome bH is reduced via the Qo site with near native rates in the mutant strains but that electron transfer between cytochrome bH and quinone bound at the Qi site is greatly slowed. These results are consistent with redox midpoint potential (Em) measurements of the cytochrome b subunit hemes and the Qi site quinone. The Em values of cyt bL and bH are approximately the same in the mutants and wild type, although the mutant strains have a larger relative concentration of what may be the high-potential form of cytochrome bH, called cytochrome b150. However, the redox properties of the semiquinone at the Qi site are altered significantly. The Qi site semiquinone stability constant of bH217R is 10 times higher than in the wild type, while in the other two strains (bH217D and bH217L) the stability constant is much lower than in the wild type. Thus H217 appears to have major effects on the redox properties of the quinone bound at the Qi site. These data are incorporated into a suggestion that H217 forms part of the binding pocket of the Qi site in a manner reminiscent of the interaction between quinone bound at the Qb site and H190 of the L subunit of the bacterial photosynthetic reaction center.

Abundance, subunit composition, redox properties, and catalytic activity of the cytochrome bc1 complex from alkaliphilic and halophilic, photosynthetic members of the family Ectothiorhodospiraceae

Ubiquinol-cytochrome c oxidoreductase (cytochrome bc1) complexes were demonstrated to be present in the membranes of the alkaliphilic and halophilic purple sulfur bacteria Ectothiorhodospira halophila, Ectothiorhodospira mobilis, and Ectothiorhodospira shaposhnikovii by protoheme extraction, immunoblotting, and electron paramagnetic resonance spectroscopy. The gy values of the Rieske [2Fe-2S] clusters observed in membranes of E. mobilis and E. halophila were 1.895 and 1.910, respectively. In E. mobilis membranes, the cytochrome bc1 complex was present in a stoichiometry of approximately 0.2 per reaction center. This complex was isolated and characterized. It contained four prosthetic groups: low-potential cytochrome b (cytochrome bL; Em = -142 mV), high-potential cytochrome b (cytochrome bH; Em = 116 mV), cytochrome c1 (Em = 341 mV), and a Rieske iron-sulfur cluster. The absorbance spectrum of cytochrome bL displayed an asymmetric alpha-band with a maximum at 564 nm and a shoulder at 559 nm. The alpha bands of cytochrome bH and cytochrome c1 peaked at 559.5 and 553 nm, respectively. These prosthetic groups were associated with three different polypeptides: cytochrome b, cytochrome c1, and the Rieske iron-sulfur protein, with apparent molecular masses of 43, 30, and 21 kDa, respectively. No evidence for the presence of a fourth subunit was obtained. Maximal ubiquinol-cytochrome c oxidoreductase activity of the purified complex was observed at pH 8; the turnover rate was 57 mol of cytochrome c reduced.(mol of cytochrome c1)-1.s-1. The complex showed a strikingly low sensitivity towards typical inhibitors of cytochrome bc1 complexes.

Kinetics of photosynthetic electron transfer in artificial vesicles reconstituted with purified complexes from Rhodobacter capsulatus. I. The interaction of cytochrome c2 with the reaction center

1. The kinetics of the interaction of cytochrome c2 and photosynthetic reaction centers purified from Rhodobacter capsulatus were studied in proteoliposomes reconstituted with a mixture of phospholipids simulating the native membrane (i.e. containing 25% L-alpha-phosphatidylglycerol). 2. At low ionic strength, the kinetics of cytochrome-c2 oxidation induced by a single turnover flash was very different, depending on the concentration of cytochrome c2: at concentrations lower than 1 microM, the process was strictly bimolecular (second-order rate constant, k = 1.7 x 10(9) M-1 s-1), while at higher concentrations a fast oxidation process (half-time lower than 20 microseconds) became increasingly dominant and encompassed the total process at a cytochrome c2 concentration around 10 microM. From the concentration dependence of the amplitude of this fast phase an association constant for a reaction-center--cytochrome-c2 complex of about 10(5) M-1 was evaluated. From the fraction of photo-oxidized reaction centers promptly re-reduced in the presence of saturating concentrations of externally added cytochrome c2, it was found that in approximately 60% of the centers the cytochrome-c2 site was exposed to the external compartment. 3. Both the second-order oxidation reaction and the formation of the reaction-center--cytochrome-c2 complex were very sensitive to ionic strength. In the presence of 180 mM KCl, the value of the second-order rate constant was decreased to 7.0 x 10(7) M-1 s-1 and no fast oxidation of cytochrome c2 could be observed at 10 microM cytochrome c2. 4. The kinetics of exchange of oxidized cytochrome c2 bound to the reaction center with the reduced form of the same carrier, following a single turnover flash, was studied in double-flash experiments, varying the dark time between photoactivations over the range 30 microseconds to 5ms. The experimental results were analyzed according to aminimal kinetic model relating the amounts of oxidized cytochrome c2 and reaction centers observable after the second flash to the dark time between flashes. This model included the rate constants for the electron transfer between the primary and secondary ubiquinone acceptors of the complex (k1) and for the exchange of cytochrome c2 (k2). Fitting to the experimental results indicated a value of k1 equal to 2.4 x 10(3) s-1 and a lower limit for k2 of approximately 2 x 10(4) s-1 (corresponding to a second-order rate constant of approximately 3 x 10(9) M-1 s-1).

How rapid are the internal reactions of the ubiquinol:cytochrome c 2 oxidoreductase?

The temperature dependence of the partial reactions leading to turn-over of the UQH2:cyt c 2 oxidoreductase of Rhodobacter sphaeroides have been studied. The redox properties of the cytochrome components show a weak temperature dependence over the range 280-330 K, with coefficients of about 1 m V per degree; our results suggest that the other components show similar dependencies, so that no significant change in the gradient of standard free-energy between components occurs over this temperature range. The rates of the reactions of the high potential chain (the Rieske iron sulfur center, cytochromes c 1 and c 2, reaction center primary donor) show a weak temperature dependence, indicating an activation energy < 8 kJ per mole for electron transfer in this chain. The oxidation of ubiquinol at the Qz-site of the complex showed a strong temperature dependence, with an activation energy of about 32 kJ mole(-1). The electron transfer from cytochrome b-566 to cytochrome b-561 was not rate determining at any temperature, and did not contribute to the energy barrier. The activation energy of 32 kJ mole(-1) for quinol oxidation was the same for all states of the quinone pool (fully oxidized, partially reduced, or fully reduced before the flash). We suggest that the activation barrier is in the reaction by which ubiquinol at the catalytic site is oxidized to semiquinone. The most economical scheme for this reaction would have the semiquinone intermediate at the energy level indicated by the activation barrier. We discuss the plausibility of this simple model, and the values for rate constants, stability constant, the redox potentials of the intermediate couples, and the binding constant for the semiquinone, which are pertinent to the mechanism of the ubiquinol oxidizing site.

The nature and magnitude of the charge-separation reactions of ubiquinol cytochrome c2 oxidoreductase

The transdielectric charge separation reaction catalyzed by the ubiquinol-cytochrome c2 oxidoreductase is achieved in two fractional steps. We present a detailed analysis which addresses the nature of the charge transferred, the redox groups directly involved in charge separation and the contributions of each to the full charge separation catalyzed by the enzyme. Accounting for light saturation effects, reaction centers unconnected to cytochrome c2 and the fraction of total cytochrome bc1 turning over per flash permits detailed quantitation of: (1) the red carotenoid bandshift associated with electron transfer between ubiquinol at site Qz and the high- (2Fe2S center, cytochrome c1) and low-potential (cytochrome bL, cytochrome bH) components of cytochrome bc1; (2) the blue bandshift accompanying reduction of cytochrome bH by ubiquinol via site Qc (the reverse of the physiological reaction); and (3) the effect of delta psi on the Qc-cytochrome bH redox equilibrium. Studies were performed at pH values above and below the redox-linked pK values of the redox centers known to be involved in each reaction at equilibrium. The conclusions of this study may be summarized as follows: (1) there is no transdielectric charge separation apparent in the redox reactions between Qz and cytochrome bL, 2Fe2S and cytochrome c1 (in agreement with Glaser, E. and Crofts, A.R. (1984) Biochim. Biophys. Acta 766, 223-235), i.e., charge separation accompanies electron transfer between cytochrome bL and cytochrome bH; (2) the redox reactions between cytochrome bL and cytochrome bH and between cytochrome bH and Qc constitute the full electrogenic span; (3) electron transfer between cytochrome bL and cytochrome bH contributes approx. 60% of this span; (4) electron transfer between cytochrome bH and Qc contributes 45-55% as calculated from the blue bandshift or the delta psi-dependent equilibrium shift; (5) there is no discernable pH dependence of the Qz-cytochrome bH or Qc-cytochrome bH charge-separation reactions; (6) cytochrome bL, Qz, 2Fe2S, and cytochrome c1 are on the periplasmic side out of the low dielectric part of the membrane while cytochrome bH is buried in the low dielectric medium; (7) electron transfer is the predominant if not the sole contributor to charge separation; (8) Qz and Qc are on opposite sides of the membrane dielectric profile.

Oxidation of cytochrome c2 and of cytochrome c by reaction centers of Rhodospirillum rubrum and Rhodobacter sphaeroides. The effect of ionic strength and of lysine modification on oxidation rates

The oxidation of cytochrome c2 by the photooxidized reaction center bacteriochlorophyll, P+-870, in chromatophores of Rhodospirillum rubrum can be described using second-order kinetics at all ionic strengths. In a system consisting of isolated R. rubrum reaction centers and purified R. rubrum cytochrome c2, the oxidation of cytochrome c2 also follows second-order kinetics. In both cases, the reaction rates at low ionic strength are weakly dependent on the ionic strength. The data suggest that the cytochrome remains mobile at very low ionic strength, since the observed kinetics can be easily explained assuming no significant tight binding of cytochrome c2 to the reaction center. In a system consisting of equine cytochrome c and reaction centers of either R. rubrum or Rhodobacter sphaeroides, the cytochrome c oxidation rate depends more strongly on the ionic strength. The high reaction rates at low ionic strength suggest that a significant portion of the cytochrome is bound. Using equine cytochrome c derivatives modified at specific lysine residues, it was shown that both R. rubrum and Rb. sphaeroides reaction centers react with equine cytochrome c through its exposed heme edge.

Photooxidation of mitochondrial cytochrome c by isolated bacterial reaction centers: Evidence for tight-binding and diffusional pathways

The binding of horse heart mitochondrial cytochrome c to isolated reaction centers from Rhodopseudomonas sphaeroides is described. The kinetics of photooxidation of cytochrome c following a short actinic flash is compared to the expected binding state of the cytochrome at various concentrations and at different ionic strengths. At low ionic strength a very tight binding site (KD≦10(-8) M) is apparent which is nonfunctional with respect to electron donation to the bound reaction center. This tightly bound cytochrome can react with another reaction center in a diffusion limited, second order process. A weaker binding site (KD≃0.3 · 10(-6) M) is also boserved which is associated with rapid, first order electron transfer from cytochrome to reaction center. Both binding processes are weakened in the presence of salt and there is no detectable binding in 100 mM NaCl. Under such conditions cytochrome oxidation is entirely a diffusional, second order process. However, analysis of the flash intensity dependence of the extent of cytochrome oxidation, by the method of van Grondelle (van Grondelle, R. (1978) Ph.D. Thesis, State University, Leiden) indicated that the cytochrome was not freely mobile even in 100 mM NaCl, at least in the sense that reduced cytochrome only slowly dissociates from unactivated reaction centers. An overall kinetic/equilibrium scheme for cytochrome c binding and photooxidation by reaction centers is presented. This is very similar to that described earlier for cytochrome c2 (Overfield, R.E., Wraight, C.A. and DeVault, D. (1979) FEBS Lett. 105, 137-142), but the tight binding site and associated diffusion controlled oxidation is unique to cytochrome c.

Changes in membrane ionic conductance, but not changes in slip, can account for the non-linear dependence of the electrochemical proton gradient upon the electron-transport rate in chromatophores

Decrease in the rate of cyclic electron transport (JE) measured from the absorbance changes associated with reaction centre bacteriochlorophyll led to a less than proportionate decrease in the membrane potential (delta psi) measured by electrochromism. In principle this result can be explained either by a delta psi-dependent slip in the H+/e- coupling ratio (nE) or by a delta psi-dependent change in the membrane ionic conductance. Simultaneous measurement of the membrane ionic current (JDIS) did not reveal any significant changes in the H+/e- ratio (JDIS/JE) and showed that conductance changes (JDIS/delta psi) account quantitatively for the curved dependence of delta psi on JE. Simultaneous recordings of JDIS and the extravesicular pH from cresol-red absorbance changes, suggest that protons are the main current-carrying species across the chromatophore membrane at high values of delta psi in the presence and absence of Fo-ATPase inhibitor. At reduced delta psi the flux of other ions outweighs the hydrogen ion current.

Cytochrome b50 as a proton carrier in the photosynthetic redox chain of purple bacteria

Recent data on the proton-translocating activity of b cytochromes in chromatophores of purple bacteria and their arrangement in the photosynthetic redox chain are discussed. These data appear to support the concept of the b50 and b-90 cytochrome doublet spanning the membrane. Current schemes of H+ transport by b cytochromes are considered, and the scheme of H+ translocation by cytochrome b50 taking up H+ at the outer side of the membrane and a quinone delivering them from this cytochrome to the inner space of the chromatophore is favored as the most probable in the light of recent findings. This scheme is applicable both to Crofts' linear model of the redox chain and to Mitchell's Q cycle. Kinetic discrepancies between H+ uptake and cytochrome b50 reduction at high ambient redox potentials are interpreted in terms of a special, cytochrome b50-independent, yet Rieske FeS-protein-dependent mode of H+ transport.

Catalysis of electron transfer across phospholipid bilayers by iron-porphyrin complexes

Phospholipid vesicles containing K3Fe(CN)6 were prepared form egg yolk phosphatidylcholine. Hemin dimethyl ester was incorporated into these vesicles during preparation in ratios of phospholipid to hemin dimethyl ester that varied from 200 : 1 to 45 000: 1. Electron transfer across the bilayer was measured anaerobically after injecting the vesicles into a solution containing reduced indigotetrasulfonic acid. Vesicles containing hemin dimethyl ester exhibited high rates of electron transfer (240 electrons/molecule hemin dimethyl ester per min). Conditions could be selected where the rate-limiting step for catalysis was either the biomolecular reaction between ferric hemin dimethyl ester and reduced indigotetrasulfonic acid or the movement of hemin dimethyl ester from interface to interface. The hemin dimethyl ester-catalyzed electron transfer went to completion within a few seconds, completely oxidizing the reduced indigotetrasulfonic acid. Valinomycin (in the presence of potassium) and carbonyl cyanide p-trifluoromethoxyphenylhydrazone were without effect on catalyzed electron transport. Thus, the electron transport is not electrogenic but is a coupled, neutral system. By specific assay, neither phosphate nor cyanide was significantly transported during electron transfer but evidence is provided to suggest that a coordinated hydroxide accompanies movement of Fe(III) hemin dimethyl ester from the inside surface to the outside surface of the bilayer. It was also demonstrated in a bulk phase transport system that hemin dimethyl ester readily catalyzes transfer of S14CN- through a chloroform layer separating two aqueous phases. Another more hydrophobic iron-porphyrin complex, Fe(III) tetraphenylporphyrin, was found to be twice as effective as hemin dimethyl ester. Other porphyrin complexes were also tested as control systems. No significant catalysis was found for metal-free protoporphyrin IX dimethyl ester or Ni(II) tetraphenylporphyrin. The results are discussed in comparison with in vivo electron transport and the future usefulness of this model system.

Photosynthetic control and estimation of the optimal ATP: electron stoichiometry during flash activation of chromatophores from Rhodopseudomonas capsulata

(1) When chromatophores from Rhodopseudomonas capsulata Ala pho+ are exposed to a train of high-frequency, saturating flashes the kinetics of the reaction centre bacteriochlorophyll absorption change enter a pseudo steady-state in which the extent of oxidation during the flashes is equal to the extent of reduction in between the flashes. The level of the pseudo steady-state is lowered by the presence of a phosphate acceptor system, raised by further addition of oligomycin, lowered by a combination of nigericin and valinomycin and raised by antimycin A. (2) In the pseudo steady-state, the extent of reaction centre bacteriochlorophyll oxidation taking place during the flash may be estimated by subtraction from the total concentration of reaction centre bacteriochlorophyll. This value is equated with the amount of electrons transported through the photosynthetic chain. Comparison with the measured ATP yield per flash in the pseudo steady-state permits calculation of the ATP: two electron ratio. The value of the ratio is 1.1 for flash frequencies between 3 and 12.5 Hz and declines at lower and higher frequencies. The ATP: two electron ratio is approximately halved in the presence of antimycin A. (3) An alternative estimate of the ATP: two electron ratio, based on the assumption that high-frequency flashes approximate to the condition of continuous illumination, was approx. 0.8.

Cytochrome b oxidation and reduction reactions in the ubiquinone-cytochrome b/c2 oxidoreductase from Rhodopseudomonas sphaeroides

1. The kinetics of cytochrome b reduction and oxidation in the ubiquinone-cytochrome b/c2 oxidoreductase of chromatophores from Rhodopseudomonas sphaeroides Ga have been measured both in the presence and absence of antimycin, after subtraction of contributions due to absorption changes from cytochrome c2, the oxidized bacteriochlorophyll dimer of the reaction center, and a red shift of the antenna bacteriochlorophyll. 2. A small red shift of the antenna bacteriochlorophyll band centered at 589 nm has been identified and found to be kinetically similar to the carotenoid bandshift. 3. Antimycin inhibits the oxidation of ferrocytochrome b under all conditions; it also stimulates the amount of single flash activated cytochrome b reductions 3- to 4-fold under certain if not all conditions. 4. A maximum of approximately 0.6 cytochrome b-560 (Em(7) = 50 mV, n = 1, previously cytochrome b50) hemes per reaction center are reduced following activating flashes. This ratio suggests that there is one cytochrome b-560 heme functional per ubiquinone-cytochrome b/c2 oxidoreductase. 5. Under the experimental conditions used here, only cytochrome b-560 is observed functional in cyclic electron transfer. 6. We describe the existence of three distinct states of reduction of the ubiquinone-cytochrome b/c2 oxidoreductase which can be established before activation, and result in markedly different reaction sequences involving cytochrome b after the flash activation. Poising such that the special ubiquinone (Qz) is reduced and cytochrome b-560 is oxidized yields the conditions for optimal flash activated electron transfer rates through the ubiquinone-cytochrome b/c2 oxidoreductase. However when the ambient redox state is lowered to reduce cytochrome b-560 or raised to oxidize Qz, single turnover flash induced electron transfer through the ubiquinone-cytochrome b/c2 oxidoreductase appears impeded; the points of the impediment are tentatively identified with the electron transfer step from the reduced secondary quinone (QII) of the reaction center to ferricytochrome b-560 and from the ferrocytochrome b-560 to oxidized Qz, respectively.

The role of the Rieske iron-sulfur center as the electron donor to ferricytochrome c2 in Rhodopseudomonas sphaeroides

The Rieske iron-sulfur center in the photosynthetic bacterium Rhodopseudomonas sphaeroides appears to be the direct electron donor to ferricytochrome c2, reducing the cytochrome on a submillisecond timescale which is slower than the rapid phase of cytochrome oxidation (t 1/2 3-5 microseconds). The reduction of the ferricytochrome by the Rieske center is inhibited by 5-n-undecyl-6-hydroxy-4,7-dioxobenzothiazole (UHDBT) but not by antimycin. The slower (102 ms) antimycin-sensitive phase of ferricytochrome c2 reduction, attributed to a specific ubiquinone-10 molecule (Qz), and the associated carotenoid spectral response to membrane potential formation are also inhibited by UHDBT. Since the light-induced oxidation of the Rieske center is only observed in the presence of antimycin, it seems likely that the reduced form of Qz (QzH2) reduces the Rieske Center in an antimycin-sensitive reaction. From the extent of the UHDBT-sensitive ferricytochrome c2 reduction we estimate that there are 0.7 Rieske iron-sulfur centers per reaction center. UHDBT shifts the EPR derivative absorption spectrum of the Rieske center from gy 1.90 to gy 1.89, and shifts the Em,7 from 280 to 350 mV. While this latter shift may account for the subsequent failure of the iron-sulfur center to reduce ferricytochrome c2, it is not clear how this can explain the other effects of the inhibitor, such as the prevention of cytochrome b reduction and the elimination of the uptake of HII(+); these may reflect additional sites of action of the inhibitor.

Membranes of Rhodopseudomonas sphaeroides. VII. Photochemical properties of a fraction enriched in newly synthesized bacteriochlorophyll a-protein complexes

Previous pulse-chase studies have shown that bacteriochlorophyll a-protein complexes destined eventually for the photosynthetic (chromatophore) membrane of Rhodopseudomonas sphaeroides appear first in a distinct pigmented fraction. This rapidly labeled material forms an upper band when extracts of phototrophically grown cells are subjected directly to rate-zone sedimentation. In the present investigation, flash-induced absorbance changes at 605 nm have demonstrated that the upper fraction is enriched two-fold in photochemical reaction center activity when compared to chromotophores; a similar enrichment in the reaction center-associated B-875 antenna bacteriochlorophyll complex was also observed. Although b- and c-type cytochromes were present in the upper pigmented band, no photoreduction of the b-type components could be demonstrated. The endogenous c-type cytochrome (Em = +345 mV) was photooxidized slowly upon flash illumination. The extent of the reaction was increased markedly with excess exogenous ferrocytochrome c but only slightly in chromatophores. Only a small light-induced carotenoid band shift was observed. These results indicate that the rapidly labeled fraction contains photochemically competent reaction centers associated loosely with c-type and unconnected to b-type cytochrome. It is suggested that this fraction arises from new sites of cytoplasmic membrane invagination which fragment to form leaky vesicles upon cell disruption.

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.

Electrogenic events in the ubiquinone-cytochrome b/c2 oxidoreductase of Rhodopseudomonas sphaeroides

The reductant of ferricytochrome c2 in Rhodopseudomonas sphaeroides is a component, Z, which has an equilibrium oxidation-reduction reaction involving two electrons and two protons with a midpoint potential of 155 mV at pH 7. Under energy coupled conditions, the reduction of ferricytochrome c2 by ZH2 is obligatorily coupled to an apparently electrogenic reaction which is monitored by a red shift of the endogeneous carotenoids. Both ferricytochrome c2 reduction and the associated carotenoid bandshift are similarly affected by the concentrations of ZH2 and ferricytochrome c2, pH, temperature the inhibitors diphenylamine and antimycin, and the presence of ubiquinone. The second-order rate constant for ferricytochrome c2 reduction at pH 7.0 and at 24 degrees C was 2 - 10(9) M-1 - s-1, but this varied with pH, being 5.1 - 10(8) M-1 = s-1 at pH 5.2 and 4.3 - 10(9) M-1 - s-1 at pH 9.3. At pH 7 the reaction had an activation energy of 10.3 kcal/mol.