One-electron transfer reactions in biochemical systems. VI. Changes in electron transfer mechanism of lipoamide dehydrogenase by modification of sulfhydryl groups

Abstract

In the presence of NADH, lipoamide dehydrogenase (NADH: lipoamide oxidoreductase, EC 1.6.4.3) catalyzes reduction of p-benzoquinone by way of a mixed mechanism involving one- and two-electron transfers. The ratio of rates of semiquinone formation and NADH oxidation is 0.3, the two-electron transfer mechanism being dominant. When the enzyme is treated with cupric ions, phenylmercuric acetate, N-ethylmaleimide and NADH + arsenite the electron transfer mechanism is shifted to a one-electron transfer type accompanied with remarkable changes in the pH-activity curve of diaphorase activity. Upon treatment of the enzyme with Cu 2+ a typical alkaline shift of pH optima for NADH-ferricyanide and NADH-benzoquinone reductase activities is observed. Changes taking place in the early phase of copper treatment can be mostly reversed by treatment with cysteine. It appears that the type of electron transfer mechanism is closely related to the extent of modification of lipoamide dehydrogenase by sulfhydryl reagents.