EVIDENCE FOR AN NO-REBOUND MECHANISM FOR PRODUCTION OF N2O FROM NITRITE BY THE COPPER-CONTAINING NITRITE REDUCTASE FROM ACHROMOBACTER-CYCLOCLASTES

Reduction of NO2- by the Cu-containing nitrite reductase from Achromobacter cycloclastes produces NO as the primary product initially, but as NO accumulates, NO production levels-off and N2O production becomes significant. Reaction of the enzyme with NO2- in the presence of NO increases the amount of N2O product significantly, while trapping the NO product as nitrosylhemoglobin or rapid removal of NO by sparging results in no detectable N2O production. Reaction of the enzyme with (NO2-)-N-15 in the presence of (NO)-N-14 results in rapid formation of the mixed isotope product (N-14, N-15)O in ca. 45% yield. In contrast, the presence or absence of NO has no effect on N2O production by a prototypical heme cd1-containing nitrite reductase. These results are consistent with formation of a labile Cu+-NO+ species in the copper enzyme, which normally decomposes to NO. Production of N2O requires that the released NO must rebind to the enzyme to combine with a second NO2- or a species derived therefrom.