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 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.

Theoretical study on electronic structures of FeOO, FeOOH, FeO(H2O), and FeO in hemes: As intermediate models of dioxygen reduction in cytochrome c oxidase

“…Our proposed mechanism is in contrast with that of FR CcO proposed by Blomberg and Siegbahn [55,56] based on the theoretical calculations. They employed the bigger model that contains the Cu B moiety, Tyr244 and protonated lysine.…”

Nitric oxide generation from heme/copper assembly mediated nitrite reductase activity

“…Some precedent for such a mechanism exists. For instance, oxo transfer to an active site iron has been previously suggested as a mechanistic alternative in NO reductase (NOR) biochemistry [67, 68]. Also, we have previously demonstrated that ligand-copper(I) complexes do react with (porphyrinate)Fe(IV)=O species to generate the corresponding μ-oxo heme/Cu complex [56].…”

A Nonheme, High-Spin {FeNO}8 Complex that Spontaneously Generates N2O

“…Alternative mechanisms have been proposed for the diiron active sites of cNOR and FDP that involve N-N bond formation and N 2 O production occurring at a single nonheme iron center. 4 …”