The CO adduct of yeast cytochrome c oxidase. Mössbauer and photolysis studies.

Mössbauer spectra of 57Fe-enriched NADH-reduced yeast cytochrome c oxidase reveal two quadrupole doublets of unequal intensity; one (approximately 33%) is typical of high-spin ferrous heme with histidine coordination and is assigned to heme a3, while the other (approximately 67%) is typical of low-spin heme with two nitrogeneous axial ligands as expected from heme a. The excess intensity (approximately 17%) of the low-spin doublet must therefore be assigned to heme a3 in a modified environment. The Mössbauer spectra of the same sample exposed to CO show that 50% of the heme iron forms a CO adduct, consistent with heme a3 being inhibited by CO. While low-spin hem a has the same Mössbauer parameters as in the reduced sample, its intensity has dropped to 35%. A distinctly new high-spin species (approximately 15%) is observed and assigned to heme a in a modified environment. The comparable size of the unexpected high-spin heme a fraction in the CO adduct and the low-spin heme a3 fraction in the reduced enzyme suggest that they arise from the same material. This material is likely to be the inactive fraction that has been found in all preparations of resting yeast cytochrome c oxidase (Siedow, J.N., Miller, S., and Palmer, G. (1981) J. Bioenerg. Biomembr. 14, 171-179). The kinetics of CO recombination following photolysis of the CO complex further confirms the coexistence of two distinct fractions associated with active and inactive protein. The majority (approximately 74%), presumably active protein, recombines exponentially from 160 to 270 K following an Arrhenius law. The large activation enthalpy, delta H approximately 35 kJ/mol, is comparable to that found in the beef heart enzyme, suggesting that the flashed-off CO is bound by the nearby CuB as in the mammalian system (Fiamingo, F.G., Altschuld, R.A., Moh, P.P., and Alben, J.O. (1982) J. Biol. Chem. 250, 1639-1650). In the minority, presumably inactive, fraction the CO recombination has fast nonexponential kinetics with a distribution of activation enthalpies peaking near delta Hp = 13 kJ/mol reminiscent of CO binding to myoglobin. In this inactive fraction CuB is apparently not accessible to the flashed-off CO.