Localization and concentration of hydroxylamine oxidoreductase and cytochromes c-552, c-554, cm-553, cm-552 and a in Nitrosomonas europaea

Abstract

Two membrane-associated cytochromes, cytochrome c m-553 and cytochrome c m-552, were derived from Nitrosomonas europaea. The major c-type cytochrome, cytochrome c m-553, accounted for 92% of the c heme found in the membrane. It had absorption maxima at 410 nm in the oxidized form and at 417, 523 and 553 nm in the dithionite reduced form. Cytochrome c m-552 possessed absorption maxima at 409 nm in the oxidized form, at 421, 522 and 552 in the dithionite reduced form, and at 418 in the dithionite reduced plus CO form. The concentration and cellular distribution of the two c-type membrane cytochromes, hydroxylamine oxidoreductase and cytochromes c-552, c-554, and a were determined. Over 95% of the soluble cytochromes (hydroxylamine oxidoreductase cytochromes and c-552 and c-554) were periplasmic, whereas cytochrome c m-553, cytochrome c m-552 and cytochrome a were associated with the cell membrane. The outer membrane and cytoplasm were devoid of cytochromes. The extracytoplasmic location of the proton-yielding hydroxylamine oxidizing system (NH 2OH ™ HNO + 2H + + 2e −) may contribute to an energy-linked proton gradient. The heme concentrations of hydroxylamine oxidoreductase and cytochromes c-552, c-554, c m-553, c m-552 and a were approx. 2.4, 1.2, 0.3, 1.3, 0.1 and 1.1 nmol/mg cell protein, respectively. The corresponding molar ratios of heme were 22:11:2.9:12:1.0:10. The enzyme or cytochrome concentrations for hydroxylamine oxidoreductase and cytochromes c-552, c-554, c m-553, c m-552 and a were approx. 0.13, 1.05, 0.09, 0.63, 0.055 and 0.56 nmol/mg cell protein, respectively. The corresponding molar ratios were 0.24:2.2:0.16:1.2:0.1:1.0.