Resonance Raman studies of CuA-modified cytochrome c oxidase

Q-band electron nuclear double resonance (ENDOR) and X-band EPR of the sulfobetaine 12 heat-treated cytochrome c oxidase complex

Heat treatment of the bovine cytochrome c oxidase complex in the zwitterionic detergent sulfobetaine 12 (SB-12) results in loss of subunit III and the appearance of a type II copper center as characterized by electron paramagnetic resonance (EPR) spectroscopy. Previous authors (Nilsson, T., Copeland, R. A., Smith, P. A., and Chan, S. I. (1988) Biochemistry 27, 8254-8260) have interpreted this type II copper center as a modified version of the CuA site. By using electron nuclear double resonance spectroscopy, it is found that the CuA proton and nitrogen resonances remain present in the SB-12 heat-treated enzyme and that three new nitrogen resonances appear having hyperfine coupling constants consistent with histidine ligation. These hyperfine coupling constants correlate well with those recently found for the CuB histidines from the cytochrome aa(3)-600 quinol oxidase from Bacillus subtilis (Fann, Y. C., Ahmed, I., Blackburn, N. J., Boswell, J. S., Verkhovskaya, M. L., Hoffman, B. M., and Wikstr9om, M. (1995) Biochemistry 34, 10245-10255). In addition, the total EPR-detectable copper concentration per enzyme molecule approximately doubles upon SB-12 heat treatment. Finally, the observed type II copper EPR spectrum is virtually indistinguishable from the EPR spectrum of CuB of the as-isolated cytochrome bo3 complex from Escherichia coli. These data indicate that the type II copper species that appears results from a breaking of the strong antiferromagnetic coupling of the heme a3-CuB binuclear center.

Kinetics and mechanism for the binding of HCN to cytochrome c oxidase

The kinetics of cyanide binding to cytochrome c oxidase were systematically studied as a function of [HCN], [oxidase], pH, ionic strength, temperature, type and concentration of solubilizing detergent, and monomer-dimer content of oxidase. On the basis of these results a minimum reaction mechanism is proposed in which the spectrally visible rapid and slow cyanide binding reactions are two consecutive first-order reactions, not parallel reactions with different conformers of cytochrome c oxidase. The fast reaction (k'obs) follows saturation type kinetics to form an HCN complex that subsequently undergoes a slow reaction (k'obs). The fast k'obs reaction is independent of ionic strength but is strongly dependent upon pH. Two pK values were evaluated from the bell-shaped rate versus pH profile; one is due to an ionizable group on the protein (pKa = 7.45), while the other is that of HCN (pKHCN = 9.15). Therefore, oxidase is reactive toward HCN only when the group on the protein is unprotonated. The slow k'obs reaction is not a reaction of oxidase with either CN- or HCN; in fact, the product formed by the fast k'obs reaction, the oxidase-HCN complex, still undergoes the slow k" process even if all of the excess KCN is removed. The apparent rate constant of the slower phase (k"obs) is independent of all the variations done in this study, and it probably corresponds to either a slow conformational change in the protein or a change in ligand coordination at one of the metal centers after HCN binds to the bimetallic center of oxidase. Based upon the bell-shaped pH dependence of the fast phase and the pH independence of the slow phase, the mechanism also predicts that a single conformer of cytochrome c oxidase can exhibit either monophasic or biphasic cyanide binding kinetics depending upon the pH. At either very low or very high pH, the two rates become comparable in magnitude, which makes the reaction appear to be monophasic even though both reactions still occur. The amount of monomeric or dimeric oxidase only slightly affects the magnitude of k'obs and k"obs values, and both processes are clearly present in both types of oxidase.

Conformational change due to reduction of cytochrome-c oxidase in lauryl maltoside: picosecond time-resolved tryptophan fluorescence studies on the native and heat modified enzyme

Detailed fluorescence studies on bovine heart cytochrome-c oxidase (CcO) has been carried out in lauryl maltoside solution. Steady-state fluorescence of the tryptophan residues of the enzyme showed that the fluorophores are embedded deep inside the hydrophobic protein cavity. Time resolved studies of tryptophan fluorescence of native and heat treated CcO have been carried out in both reduced and oxidised forms using synchronously pumped pulsed picosecond dye laser and single photon counting technique. Decay of the tryptophan fluorescence have been fitted using discrete four exponential model. Amplitude distribution of lifetimes also showed four distinct regions in the analysis of the decay profiles by maximum entropy method (MEM). The results indicate that controlled heat treatment of CcO affects the conformation of the enzyme near the active centers which makes it incapable of active proton pumping while the electron transfer property is still conserved. Reduction of the native CcO is associated with a large conformation change in lauryl maltoside near the active centers which is not observed in case of CcO encapsulated in vesicles. Reduction of the heat treated enzyme was found to have a conformation different from the reduced native CcO.