Studies on the electron transfer pathway, topography of iron-sulfur centers, and site of coupling in NADH-Q oxidoreductase.

Abstract

Electron transfer activities and steady state reduction levels of Fe-S centers of NADH-Q oxidoreductase were measured in mitochondria, submitochondrial particles (ETP_H), and complex I after treatment with various reagents. p-Chloromercuribenzenesulfonate destroyed the signal from center N-4 (g_x = 1.88) in ETP_H but not in mitochondria, showing that N-4 is accessible only from the matrix side of the inner membrane. N-Bromosuccinimide also destroyed the signal from N-4 but without inhibiting rotenone-sensitive electron transfer to quinone, suggesting a branched pathway for electron transfer. Diethylpyrocarbonate caused oxidation of N-3 and N-4 in the steady state without changing N-1, suggesting N-1 is before N-3 and N-4. Difluorodinitrobenzene and dicyclohexylcarbodiimide inhibited oxidation of all Fe-S centers and tetranitromethane inhibited reduction of all Fe-S centers. Titrations of the rate of superoxide (O₂^-) generation in rotenone-treated submitochondrial particles were similar with the ratio [NADH]/[NAD] and that of 3-acetyl pyridine adenine nucleotide in spite of different midpoint potentials of the two couples. On reaction with inhibitors the inhibition of O₂^- formation was similar to that of ferricyanide reductase rather than quinone reductase. The rate of O₂^- formation during ATP-driven reverse electron transfer was 16% of the rate observed with NADH. The presence of NAD increased the rate to 83%. The results suggest that bound, reduced nucleotide, probably E-NAD., is the main source of O₂^- in NADH dehydrogenase. The effect of ATP on the reduction levels of Fe-S centers in well-coupled ETP_H was measured by equilibrating with either NADH/NAD or succinate/fumarate redox couples. With NADH/NAD none of the Fe-S centers showed ATP induced changes, but with succinate/fumarate all centers showed ATP-driven reduction with or without NAD present. The effect on N-2 was smaller than that on N-1, N-3, and N-4. These observations indicate that the major coupling interaction is between N-2 and the low potential centers, N-1, N-3, and N-4. Possible schemes of coupling in this segment are discussed.