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

Abstract

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 N₂O 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.