SAM1 semiempirical calculations on the catalytic cycle of nitric oxide reductase from Fusarium oxysporum

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The reaction path for nitric-oxide (NO) reduction in nitric oxide reductase (NOR) was obtained using the semiempirical method SAM1. The active site model consisted of non-residue porphyrin, iron, and methylthiolate. We obtained a reaction path consisting of seven reaction steps and forming a cycle. In obtaining the reaction path, we used information about the activation conditions of NOR. The initial structures of all states for optimization calculation were determined in consideration of the activation conditions of NOR. The structures of all complexes in all possible spin-states throughout the all reaction path candidates were calculated and extensive conformational searches were performed. The change in energy, atomic charge, and thermodynamic data of the obtained reaction paths and optimized structures of each complex were examined. Our reaction path required two NO molecules, two electrons, and two protons. The first proton combined the oxygen atom of the NO ligand bound with the heme iron. On our reaction path, an NO ligand molecule was eventually transformed into an N 2O molecule and an H 2O molecule. The same final products were obtained from NOR in an in vitro experiment. Our reaction path is similar to the path based on the experimental results proposed by Shiro et al.