Electron transfer during the oxidation of ammonia by the chemolithotrophic bacterium Nitrosomonas europaea

The combined action of ammonia monooxygenase, AMO, (NH3+2e(-)+O-2 --> NH2OH) and hydroxylamine oxidoreductase, HAO, (NH2OH+H2O --> HNO2+4e(-)+4H(+)) accounts for ammonia oxidation in Nitrosomonas europaea. Pathways for electrons from HAO to O-2, nitrite, NO, H2O2 or AMO are reviewed and some recent advances described. The membrane cytochrome c(M)552 is proposed to participate in the path between HAO and ubiquinone. A brl complex is shown to mediate between ubiquinol and the terminal oxidase and is shown to be downstream of HAO. A novel, red, low-potential, periplasmic copper protein, nitrosocyanin, is introduced. Possible mechanisms for the inhibition of ammonia oxidation in cells by protonophores are summarized. Genes for nitrite- and NO-reductase but not N2O or nitrate reductase are present in the genome of Nitrosomonas. Nitrite reductase is not repressed by growth on O-2, the flux of nitrite reduction is controlled at the substrate level. (C) 2000 Elsevier Science B.V. All rights reserved.