On the reaction mechanism of flavin-sensitized photoregulation of Monoraphidium braunii nitrate reductase

Abstract

Nitrate reductase from the green alga Monoraphidium braunii is reversibly inactivated by reduction on irradiation in the presence of flavins and an electron donor; the process is markedly accelerated by cyanide anions. When the enzyme is depleted of its endogenous flavin adenine dinucleotide (FAD), a significant lag period of 1–2 min is observed before photoinactivation takes place. This lag period is not observed if flavins (except FAD) are previously photoreduced in the reaction mixture; in this case the inactivation kinetics depend on the type of flavin used. The photoinactivation process is accompanied by reduction of the haem prosthetic groups. The flavin-sensitized photoinactivation of nitrate reductase is dependent on ionic strength; this appears to be due to complementary electrostatic interactions between the reduction site of the protein and the exogenous flavins. Flavins can also photosensitize the reactivation of the cyanide-inactivated reduced enzyme (either native or FAD-depleted nitrate reductase) in the absence of added electron donors. The mechanism of flavin-mediated nitrate reductase photoregulation is discussed.