Arginine conversion to nitroxide by tetrahydrobiopterin-free neuronal nitric-oxide synthase - Implications for mechanism

We studied catalysis by tetrahydrobiopterin (H4B)-free neuronal nitric oxid e synthase (nNOS) to understand how heme and H4B participate in nitric oxid e (NO) synthesis. H4B-free nNOS catalyzed Arg oxidation to N-omega-hydroxy- L-Arg (NOHA) and citrulline in both NADPH- and H2O2-driven reactions. Citru lline formation was time- and enzyme concentration-dependent but was uncoup led relative to NADPH oxidation, and generated nitrite and nitrate without forming NO. Similar results mere observed when NOHA served as substrate. St eady-state and stopped-Bow spectroscopy with the H4B-free enzyme revealed t hat a ferrous heme-NO complex built up after initiating catalysis in both N ADPH- and H2O2-driven reactions, consistent with formation of nitroxyl as a n immediate product. This differed from the H4B-replete enzyme, which forme d a ferric heme-NO complex as an immediate product that could then release NO. We make the following conclusions. 1) H4B is not essential for Arg oxid ation by nNOS, although it helps couple NADPH oxidation to product formatio n in both steps of NO synthesis. Thus, the NADPH- or H2O2-driven reactions form common heme-oxy species that can react with substrate in the presence or absence of H4B. 2) The sole essential role of H4B is to enable nNOS to g enerate NO instead of nitroxyl, On this basis we propose a new unified mode l for heme-dependent oxygen activation and H4B function in both steps of NO synthesis.