In situ kinetics of cytochromes c1 and c2

Product-controlled steady-state kinetics between cytochrome aa(3) from Rhodobacter sphaeroides and equine ferrocytochrome c analyzed by a novel spectrophotometric approach

Cytochrome c oxidase (CcO) catalyzes the reduction of molecular oxygen to water using ferrocytochrome c (cyt c(2+)) as the electron donor. In this study, the oxidation of horse cyt c(2+) by CcO from Rhodobacter sphaeroides, was monitored using stopped-flow spectrophotometry. A novel analytic procedure was applied in which the spectra were deconvoluted into the reduced and oxidized forms of cyt c by a least-squares fitting method, yielding the reaction rates at various concentrations of cyt c(2+) and cyt c(3+). This allowed an analysis of the effects of cyt c(3+) on the steady-state kinetics between CcO and cyt c(2+). The results show that cyt c(3+) exhibits product inhibition by two mechanisms: competition with cyt c(2+) at the catalytic site and, in addition, an interaction at a second site which further modulates the reaction of cyt c(2+) at the catalytic site. These results are generally consistent with previous reports, indicating the reliability of the new procedure. We also find that a 6×His-tag at the C-terminus of the subunit II of CcO affects the binding of cyt c at both sites. The approach presented here should be generally useful in spectrophotometric studies of complex enzyme kinetics. This article is part of a Special Issue entitled: 17th European Bioenergetics Conference (EBEC 2012).

The diheme cytochrome c(4) from Vibrio cholerae is a natural electron donor to the respiratory cbb(3) oxygen reductase

The respiratory chain of Vibrio cholerae contains three bd-type quinol oxygen reductases as well as one cbb(3) oxygen reductase. The cbb(3) oxygen reductase has been previously isolated and characterized; however, the natural mobile electron donor(s) that shuttles electrons between the bc(1) complex and the cbb(3) oxygen reductase is not known. The most likely candidates are the diheme cytochrome c(4) and monoheme cytochrome c(5), which have been previously shown to be present in the periplasm of aerobically grown cultures of V. cholerae. Both cytochromes c(4) and c(5) from V. cholerae have been cloned and expressed heterologously in Escherichia coli. It is shown that reduced cytochrome c(4) is a substrate for the purified cbb(3) oxygen reductase and can support steady state oxygen reductase activity of at least 300 e(-1)/s. In contrast, reduced cytochrome c(5) is not a good substrate for the cbb(3) oxygen reductase. Surprisingly, the dependence of the oxygen reductase activity on the concentration of cytochrome c(4) does not exhibit saturation. Global spectroscopic analysis of the time course of the oxidation of cytochrome c(4) indicates that the apparent lack of saturation is due to the strong dependence of K(M) and V(max) on the concentration of oxidized cytochrome c(4). Whether this is an artifact of the in vitro assay or has physiological significance remains unknown. Cyclic voltammetry was used to determine that the midpoint potentials of the two hemes in cytochrome c(4) are 240 and 340 mV (vs standard hydrogen electrode), similar to the electrochemical properties of other c(4)-type cytochromes. Genomic analysis shows a strong correlation between the presence of a c(4)-type cytochrome and a cbb(3) oxygen reductase within the beta- and gamma-proteobacterial clades, suggesting that cytochrome c(4) is the likely natural electron donor to the cbb(3) oxygen reductases within these organisms. These would include the beta-proteobacteria Neisseria meningitidis and Neisseria gonnorhoeae, in which the cbb(3) oxygen reductases are the only terminal oxidases in their respiratory chains, and the gamma-proteobacterium Pseudomonas stutzeri.

The Q-cycle reviewed: How well does a monomeric mechanism of the bc(1) complex account for the function of a dimeric complex?

Recent progress in understanding the Q-cycle mechanism of the bc(1) complex is reviewed. The data strongly support a mechanism in which the Q(o)-site operates through a reaction in which the first electron transfer from ubiquinol to the oxidized iron-sulfur protein is the rate-determining step for the overall process. The reaction involves a proton-coupled electron transfer down a hydrogen bond between the ubiquinol and a histidine ligand of the [2Fe-2S] cluster, in which the unfavorable protonic configuration contributes a substantial part of the activation barrier. The reaction is endergonic, and the products are an unstable ubisemiquinone at the Q(o)-site, and the reduced iron-sulfur protein, the extrinsic mobile domain of which is now free to dissociate and move away from the site to deliver an electron to cyt c(1) and liberate the H(+). When oxidation of the semiquinone is prevented, it participates in bypass reactions, including superoxide generation if O(2) is available. When the b-heme chain is available as an acceptor, the semiquinone is oxidized in a process in which the proton is passed to the glutamate of the conserved -PEWY- sequence, and the semiquinone anion passes its electron to heme b(L) to form the product ubiquinone. The rate is rapid compared to the limiting reaction, and would require movement of the semiquinone closer to heme b(L) to enhance the rate constant. The acceptor reactions at the Q(i)-site are still controversial, but likely involve a "two-electron gate" in which a stable semiquinone stores an electron. Possible mechanisms to explain the cyt b(150) phenomenon are discussed, and the information from pulsed-EPR studies about the structure of the intermediate state is reviewed. The mechanism discussed is applicable to a monomeric bc(1) complex. We discuss evidence in the literature that has been interpreted as shown that the dimeric structure participates in a more complicated mechanism involving electron transfer across the dimer interface. We show from myxothiazol titrations and mutational analysis of Tyr-199, which is at the interface between monomers, that no such inter-monomer electron transfer is detected at the level of the b(L) hemes. We show from analysis of strains with mutations at Asn-221 that there are coulombic interactions between the b-hemes in a monomer. The data can also be interpreted as showing similar coulombic interaction across the dimer interface, and we discuss mechanistic implications.