Consideration of a phlorin structure for haem P-460 of hydroxylamine oxidoreductase and its implications regarding reaction mechanism.

Introduction The aerobic, lithotrophic, autotrophic bacteria exemplified by Nitrosomonas europaea derive energy from the activities of hydroxylamine oxidoreductase (HAO; NH20H + H 2 0 + HN02+4e-+4H+) [ l ] and ammonia monooxygenase (AMO; NH3 + O2 + 2H+ + 2e+ NH20H + H20) [2] which are co-dependent for generation of substrate or electrons, respectively [3-51. Electrons pass from HA0 to the tetrahaem cytochrome c-554 [6,7] and then to A M 0 or, by way of the monohaem cytochrome c-552, to an aa3-Cu.,-type terminal oxidase [8], a nitrite[9,10] or NO-reductase [l l] , a cytochrome c peroxidase [ 121 or an NAD reductase. HA0 will also rapidly oxidize hydrazine to dinitrogen, a reaction which is experimentally useful and may be reflected in the production of dinitrogen in the bacterial anaerobic oxidation of ammonia [ 1,131. The periplasmic cytochrome P-460 [ 141, also found in the methylotrophs [15], oxidizes hydroxylamine too but its significance to cellular energy economy is unknown.