The Intricate Mechanism of Nitric Oxide Synthase.

The biological formation of NO is performed by nitric oxide synthase. For the first half of the mechanism, a consensus understanding has been reached, but not for the second half. Several attempts to reach a mechanism in agreement with experimental observations have been reported, but without success. Therefore, another study of that half-reaction has been made here. The main difficulties have been to explain the observed non-stoichiometric action of the cofactor H4B, and also why only one outside reduction from NADP is enough. A key new finding here is that the first step involves a protonation of the proximal cysteine of the heme. O2 then binds to iron and forms O2H with a proton from the NOH ligand of the substrate NHA. A long-range electron transfer from H4B leads to the formation of the heme bound H2O2, which is followed by a dissociation of the OO bond of H2O2. The last step studied here was a release of NO and a reverse electron transfer back to H4B. The results are in very good agreement with experimental observations, which is in line with previous findings for other redox enzymes using the present methodology.