Reconstitution of monomeric cytochrome c oxidase into phospholipid vesicles yields functionally interacting cytochrome aa3 units.

When the carbon monoxide complex of fully reduced cytochrome c oxidase, reconstituted into liposomes, is mixed with oxygen-containing buffer, complex kinetic progress curves are observed. This pattern is seen irrespective of whether the oxidase used in reconstitution is the dimeric or monomeric (subunit III-depleted) enzyme. These findings are interpreted in the light of similar experiments on the detergent-solubilized enzyme reported by Gibson and Greenwood (Gibson, Q.H., and Greenwood, C. (1963) Biochem. J. 86, 541-554) and confirmed by ourselves. We conclude that reconstitution of monomeric (subunit III-less) enzyme yields, preferentially, vesicles containing more than one functional unit, possibly associated as dimers. This result is of significance to our understanding of the relationships between aggregation state and proton pumping capacity of cytochrome oxidase.