Characterization of electron transfer and proton translocation activities in trypsin-treated bovine heart mitochondrial cytochrome c oxidase.

Bovine heart mitochondrial cytochrome c oxidase has been treated with trypsin in order to investigate the role of components a, b, and c (nomenclature of Capaldi) in cytochrome c binding, electron transfer, and proton-pumping activities. Cytochrome c oxidase was dispersed in nondenaturing detergent solution (B. Ludwig, N. W. Downer, and R. A. Capaldi (1979) Biochemistry 18, 1401) and treated with trypsin. This treatment inhibited electron transfer activity by 9% when compared to a similarly treated control in a polarographic assay (493 s-1) and had no large effect on the high affinity (Km = 6.1 X 10(-8) M) or low affinity (Km = 2.2 X 10(-6) M) sites of cytochrome c interaction with cytochrome c oxidase. Direct thermodynamic binding experiments with cytochrome c showed that neither the high affinity (1.04 +/- 0.06 mol cytochrome c/mol cytochrome c oxidase) nor the high-plus-low affinity (2.21 +/- 0.15 mol cytochrome c/mol cytochrome c oxidase) binding sites of cytochrome c on the enzyme were perturbed by the trypsin treatment. Control and trypsin-treated enzyme incorporated into phospholipid vesicles (prepared by the cholate dialysis method) exhibited respiratory control ratios of 6.5 +/- 0.7 and 6.3 +/- 0.6, respectively. The vectorial proton translocation activity in the phospholipid vesicles was unaffected by trypsin treatment with proton translocated to electron transferred ratios being equivalent to the control. NaDodSO4-PAGE showed that components a, b, and c were completely removed by the trypsin treatment. [14C]Iodoacetamide labeling experiments showed that the content of component c in the enzyme was depleted by 85% and that greater than 50% of component a was cleaved upon the trypsin treatment. These results suggest that components a, b, and c are not required for maximum electron transfer and proton translocation activities in the isolated enzyme.