A density functional theory investigation on the mechanism of the second half-reaction of nitric oxide synthase

Density functional theory methods have been employed to systematically investigate the overall mechanism of the second half-reaction of nitric oxide synthases. The initial heme-bound hydrogen peroxide intermediate previously identified is found to first undergo a simple rotation about its O-O peroxide bond. Then, via a "ping-pong" peroxidase-like mechanism the -O _inH- proton is transferred back onto the substrate's -NO oxygen then subsequently onto the outer oxygen of the resulting Fe_heme-OOH species. As a result, O_out is released as H₂O with concomitant formation of a compound I-type (Fe_heme-O) species. Formation of the final citrulline and NO products can then be achieved in one step via a tetrahedral transition structure resulting from direct attack of the Fe_heme-O moiety at the substrate's guanidinium carbon center. The possible role of alternative mechanisms involving a protonated compound II-type species or an initial transfer of only the -NH- hydrogen of the =NHOH ⁺ group of N^ω-hydroxy-L-arginine is also discussed.