The Second Step of the Nitric Oxide Synthase Reaction: Evidence for Ferric-Peroxo as the Active Oxidant

摘要

Nitric oxide synthase (NOS) is a P450 mono-oxygenase that catalyzes the oxidation of L-arginine to citrulline and NO through the stable intermediate N~G-hydroxy-L-arginine (NHA). The oxidation of NHA by NOS is unique. There is little direct evidence in support of the nature of the heme bound oxidant [i.e., ferric-peroxo vs Fe~Ⅳ=O(por~(·+))] responsible for this transformation. Previous work characterizing the H_2O_2-driven oxidation of NHA by NOS showed the formation of citrulline and the side product N~δ-cyanoornithine (CN-orn). This led to the proposed involvement of a ferric-peroxo intermediate in the oxidation of NHA to citrulline. To test this hypothesis we used this model reaction to study the effects of pH, heme substitution, active site mutagenesis, and a fluorinated substrate analogue on the product distribution. Further, the oxidation of 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) by H_2O_2 and iNOS_(heme) was used to probe the protein-catalyzed breakdown of peroxide to the Fe~Ⅳ=O(por~(·+)) intermediate. At pH 6.5, 7.5, and 8.5 the peroxide shunt reaction forms 26 ± 2, 36 ± 1, and 51 ± 1% citrulline, respectively. The rate of peroxidase activity, however, was negatively correlated to pH, with a peroxide breakdown rate of 13.1 ± 0.3, 8.3 ± 0.2, and 4.2 ± 0.1 M~(-1) s~(-1) at pH 6.5, 7.5, and 8.5, respectively. Mutation of active site valine 346 to an alanine shifted the product distribution to 5.2 ± 0.5% citrulline while enhancing the peroxide cleavage rate to 14.3 ± 0.7 M~(-1) s~(-1). Substitution of the heme cofactor with iron mesoporphyrin IX (Fe-MPIX) alters the product distribution from 36 ± 1% citrulline to 22 ± 3% citrulline. Metal substitution with Mn results in the formation of 64.7 ± 0.8% citrulline. Conversely, the electrophilic 4,4-difluoro-N~G-hydroxy-L-arginine substrate analogue shifted the product distribution to 68.6 ± 0.6% 4,4-difluorocitrulline. The peroxidase data provide insight into the chemical features of NOS that control the processing of the ferric-peroxo species to the Fe~Ⅳ=O(por~(·+)) intermediate and help interpret the product distributions observed for the peroxide shunt under various conditions. In all cases, the ability of the protein to break down peroxide is negatively correlated with the formation of citrulline by the peroxide shunt. These results support the high valent Fe~Ⅳ=O(por~(·+)) intermediate as the species responsible for CN-orn formation and are consistent with the involvement of the ferric-peroxo intermediate in the oxidation of NHA to citrulline.