PURIFICATION OF HYDROXYLAMINE OXIDASE FROM THIOSPHAERA-PANTOTROPHA - IDENTIFICATION OF ELECTRON ACCEPTERS THAT COUPLE HETEROTROPHIC NITRIFICATION TO AEROBIC DENITRIFICATION

Thiosphaera pantotropha, a Gram-negative heterotrophic nitrifying bact erium, 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 protei ns, cytochrome c(551) and pseudoazurin as electron accepters, both of which can also donate electrons to denitrification enzymes. A model of electron transfer is proposed, that suggests a coupling of nitrificat ion to denitrification and provides a mechanism by which nitrification can play a role in dissipating reductant.