Theoretical study of the reduction of nitric oxide in an A-type flavoprotein.

The mechanism for the reduction of nitric oxide to nitrous oxide and water in an A-type flavoprotein (FprA) in Moorella thermoacetica, which has been proposed to be a scavenging type of nitric oxide reductase, has been investigated using density functional theory (B3LYP). A dinitrosyl complex, [{FeNO}(7)](2), has previously been proposed to be a key intermediate in the NO reduction catalyzed by FprA. The electrons and protons involved in the reduction were suggested to "super-reduce" the dinitrosyl intermediate to [{FeNO}(8)](2) or the corresponding diprotonated form, [{FeNO(H)}(8)](2). In this type of mechanism the electron and/or proton transfers will be a part of the rate-determining step. In the present study, on the other hand, a reaction mechanism is suggested in which N(2)O can be formed before the protons and electrons enter the catalytic cycle. One of the irons in the diiron center is used to stabilize the formation of a hyponitrite dianion, instead of binding a second NO. Cleaving the N-O bond in the hyponitrite dianion intermediate is the rate-determining step in the proposed reaction mechanism. The barrier of 16.5 kcal mol(-1) is in good agreement with the barrier height of the experimental rate-determining step of 14.8 kcal mol(-1). The energetics of some intermediates in the "super-reduction" mechanism and the mechanism proceeding via a hyponitrite dianion are compared, favoring the latter. It is also discussed how to experimentally discriminate between the two mechanisms.