The mechanism of antibody inhibition of proton pumping by cytochrome oxidase vesicles

Modulation of cytochrome oxidase kinetics by indirect antibody action

Polyclonal antibodies raised against isolated subunit V from beef heart cytochrome oxidase or against the intact enzyme increase its apparent affinity for the substrate cytochrome c at the high-affinity site while diminishing the turnover at that site. At the low-affinity site the major action of both types of antibody is to reduce the apparent affinity for cytochrome c. At high ionic strengths the kinetic effect of anti-subunit V is very small although it still binds to the enzyme. The results are interpreted in terms of a model for the enzyme in which antibodies can modulate cytochrome oxidase kinetics by affecting the binding of cytochrome c, even if the antibody-binding site is on a subunit not directly involved in substrate binding.

Functional equivalence of monomeric (shark) and dimeric (bovine) cytochrome c oxidase

Cytochrome c oxidase isolated from hammerhead shark red muscle is monomeric in relation to the dimeric form of isolated bovine cytochrome c oxidase but in other ways bears a close resemblance to the enzyme isolated from mammalian tissue [1, 2]. Comparative studies of shark and bovine cytochrome c oxidase were extended to address the degree of functional similarity between the monomeric (shark) and dimeric (bovine) enzymes in the kinetics of peroxide binding and in the extent to which the catalytic action of the enzymes in vesicles can establish a proton gradient. Although the kinetics of peroxide binding and the proton pumping processes are complex, the dimeric and monomeric forms are quite similar with respect to these functional attributes. The kinetic heterogeneity of the process of peroxide binding is expressed in the shark enzyme as well as in the bovine enzyme, and both types of enzymes in vesicles can generate transmembrane proton gradients. On this basis we conclude that the dimeric state of isolated cytochrome c oxidase from mammalian sources is not essential for its function in vitro.