Pterin-Centered Radical as a Mechanistic Probe of the Second Step of Nitric Oxide Synthase

The enzyme nitric oxide synthase is both medically relevant and of particular interest from a basic sciences perspective due to the complex nature of the chemical mechanism used to generate NO. The enzyme utilizes multiple redox-active cofactors and substrates to catalyze the five-electron oxidation of substrate L-arginine to citrulline and nitric oxide. Two flavins, a cysteine-coordinated heme cofactor and, uniquely, a tetrahydrobiopterin cofactor, are used to deliver electrons from the cosubstrate NADPH to molecular oxygen, analogous to other P450s. The unprecedented involvement of the pterin cofactor as a single electron donor is unique among P450s and pterin utilizing proteins alike and adds to the complexity of this enzyme. In this report, the peroxide shunt with both Mn- and Fe-containing heme domain constructs of iNOS(heme) was used to characterize the formation of HNO as the initial inorganic product produced when oxygen activation occurs without pterin radical formation. To recover NO formation, preturnover of the iron-containing enzyme with L-arginine was used to generate the pterin-centered radical, followed by peroxide shunt chemistry. Comparison of NO produced by this reaction with reactions that do not undergo preturnover, do not have peroxide added, or are performed with a pterin unable to generate a radical shows NO production to be dependent on both a pterin-centered radical and activated oxygen. Finally, the chemical HNO donor, Angeli's salt, was used to form the ferrous nitrosyl in the presence of the pterin radical intermediate. Under these conditions, the rate of pterin radical decay was increased as monitored by EPR spectroscopy. In comparison to pterin that aerobically decays, the Angeli's salt treated sample is also significantly protected from oxidation, suggesting ferrous-nitrosyl-mediated reduction of the radical. Taken together, these results support a dual redox cycling role for the pterin cofactor during NOS turnover of NHA with particular importance for the proper release of NO from a proposed ferrous nitrosyl intermediate.