Mechanism of nitrite reduction to nitrous oxide in a photodenitrifier, Rhodopseudomonas sphaeroide forma sp. denitrificans

Abstract

The mechanism of anaerobic reduction of NO 2 − to N 2 O in a photodenitrifier, Rhodopseudomonas sphaeroides forma sp. denitrificans , was investigated. With ascorbate-reduced phenazine methosulfate (PMS) as the electron donor, the nitrite reductase of this photodenitrifier reduced NO 2 − to NO and a trace amount of N 2 O. With dithionite-reduced benzyl viologen as the electron donor, the major product of NO 2 − reduction was NH 2 OH, and a trace amount of N 2 O was also produced. The nitrate reductase itself had no NO reductase activity with ascorbate-reduced PMS. It was concluded that the essential product of NO 2 − reduction by the purified nitrite reductase is NO. Chromatophore membranes stoichiometrically produced N 2 O from NO 2 − with any electron donor, such as dithionite-redduced benzyl viologen, ascorbate-reduced PMS or NADH/FMN. The membranes also contrained activity of NO reduction of N 2 O with either ascorbate-reduced PMS or duroquinol. The NO reductase activity with duroquinol was inhibited by antimycin A. Stoichiometric production of N 2 O from N 2 − also was observed in the reconstituted NO 2 − reduction system which contained the cytochrome bc 1 complex, cytochrome c 2 , the nitrite reductase and duroquinol as the electron donor. The preparation of the cytochrome bc 1 complex itself contianed NO reductase activity. From these results the mechanism of NO 2 − reduction to N 2 O in this photodenitrifier was determined as the nitrite reductase reducing NO 2 − to NO with electrons from the cytochrome bc 1 complex, and NO subsequently being reduced, without release, to N 2 O with electrons from the cytochrome bc 1 complex by the NO reductase, which is closely associated with the complex.