The Mitochondrial Enzymes of Oxidative Phosphorylation

Abstract

Today, the amino acid sequences of proteins of unknown function are deduced from the DNA before they are isolated, the receptors of unknown hormones are identified, and antibodies that catalyze nonbiological reactions are generated. Forty years ago, when this author began work on the mitochondrial oxidative phosphorylation system, it was necessary to have a well-defined and assayable biological reaction before one could attempt to isolate the enzyme(s) that catalyzed it. The fortunate circumstance that led to the discovery of the respiratory chain enzyme complexes occurred in 1957, when F. L. Crane, R. L. Lester, C. Widmer, and the author isolated coenzyme Q (ubiquinone) from bovine heart mitochondria and obtained evidence for its role as a respiratory chain electron carrier (Hatefi, 1963; Crane et al., 1957). Consequently, one could think of the mitochondrial electron transport system in terms of the following four assayable reactions, and one could attempt to isolate the four enzymes or enzyme systems that catalyzed them:

$${\text{NADH + Q + }}{{{\text{H}}}^{ + }}{\text{ }} \rightleftharpoons {\text{ NA}}{{{\text{D}}}^{ + }}{\text{ + Q}}{{{\text{H}}}_{2}}$$

((1))

$${\text{Succinate + Q }} \rightleftharpoons {\text{ fumarate + Q}}{{{\text{H}}}_{2}}$$

((2))

$$ {\text{Q}}{{{\text{H}}}_{2}}{\text{ + 2 ferricyt}}{\text{. }}c{\text{ }} \rightleftharpoons {\text{ Q + 2 ferrocyt}}{\text{. }}c{\text{ + 2}}{{{\text{H}}}^{ + }}$$

((3))

$$ 2{\text{ ferrocyt}}{\text{. }}c{\text{ + 2 }}{{{\text{H}}}^{ + }}{\text{ + }} \frac{1}{2} {\text{ }}{{{\text{O}}}_{2}}{\text{ }} \rightleftharpoons {\text{ 2 ferricyt}}{\text{. }}c{\text{ + }}{{{\text{H}}}_{2}}{\text{O}}$$

((4))