No abstract

Thiosphaerupuntorropha, a Gram-negative heterotrophic nitrifying bacterium, expresses a soluble 20 kDa monomeric periplasmic hydroxylamine oxidase that differs markedly from the hydroxylamine oxidase found in autotrophic bacteria. This enzyme can use the periplasmic redox proteins, cytochrome cssl and pseudoazurin as electron acceptors, both of which can also donate electrons to denitrification enzymes. A model of electron transfer is proposed, that suggests a coupling of nitrification to denitrification and provides a mechanism by which nitrification can play a role in dissipating red&ant.

The characterization of the hydroxylamine oxidase from the heterotrophic nitrifier Paracoccus denitrificans GB17 indicates the enzyme to be entirely distinct from the hydroxylamine oxidase from the autotrophic nitrifier Nitrosomonas europaea. Hydroxylamine oxidase from P. denitrificans contains three to five non-haem, non-iron-sulphur iron atoms as prosthetic groups, predominantly co-ordinated by carboxylate ligands. The interaction of the enzyme with the electron-accepting proteins cytochrome C556 and pseudoazurin is mainly hydrophobic. The catalytic mechanism of hydroxylamine oxidase from P. denitrificans is different from the enzyme from N. europaea because the production of nitrite by the former requires molecular oxygen. Under anaerobic conditions the enzyme makes nitrous oxide as a sole product.

Hydroxylamine oxidation was measured in four recently isolated heterotrophic nitrate-reducing bacteria belonging to the genera Pseudomonas, Moraxella, Arthrobacter and Aeromonas. A hydroxylamine-cytochrome c oxidoreductase activity was detected in periplasmic fractions of the Pseudomonas and Aeromonas spp. and in total soluble fractions of the Arthrobacter sp. A monomeric 19-kDa non-haem iron hydroxylamine-cytochrome c oxidoreductase was purified from the Pseudomonas species and shown to be similar to hydroxylamine-cytochrome c oxidoreductase of Paracoccus denitrificans.