Membrane-bound cytochrome c is an alternative electron donor for cytochrome aa3 in Nitrobacter winogradskyi.

We purified membrane-bound cytochrome c-550 [cytochrome c-550(m)] to an electrophoretically homogeneous state from Nitrobacter winogradskyi. The cytochrome showed peaks at 409 and 525 nm in the oxidized form and peaks at 416, 521, and 550 nm in the reduced form. The molecular weight of the cytochrome was estimated to be 18,400 on the basis of protein and heme c contents and 18,600 by gel filtration. The N-terminal amino acid sequence of cytochrome c-550(m) was determined to be A-P-T-S-A-A-D-A-E-S-F-N-K-A-L-A-S-A-?-A-E-?-G-A-?-L-V-K-P. We previously purified soluble cytochrome c-550 cytochrome c-550(s)] from N. winogradskyi and determined its complete amino acid sequence (Y. Tanaka, Y. Fukumori, and T. Y. Yamanaka, Biochim. Biophys. Acta 707:14-20, 1982). Although the sequence of cytochrome c-550(m) was completely different from that of cytochrome c-550(s), ferrocytochrome c-550(m) was rapidly oxidized by the cytochrome c oxidase of the bacterium. Furthermore, the liposomes into which nitrite cytochrome c oxidoreductase, cytochrome c oxidase, and nitrite were incorporated showed nitrite oxidase activity in the presence of cytochrome c-550(m). These results suggest that cytochrome c-550(m) may be an alternative electron mediator between nitrite cytochrome c oxidoreductase and cytochrome c oxidase.