Rapid kinetic studies link tetrahydrobiopterin radical formation to heme-dioxy reduction and arginine hydroxylation in inducible nitric-oxide synthase

To understand how heme and (6R)-5,6,7,8-tetrahydro-L-biopterin (H4B) participate in nitric-oxidle synthesis, we followed ferrous-dioxy heme ((FeO2)-O-II,) formation and disappearance, H4B radical formation, and Arg hydroxylation during a single catalytic turnover by the inducible nitric-oxide synthase oxygenase domain (iNOSoxy), In all cases, prereduced (ferrous) enzyme was rapidly mixed with an O-2-containing buffer to start the reaction, A ferrous-dioxy intermediate formed quickly (53 s(-1)) and then decayed with concurrent buildup of ferric iNOSoxy, The buildup of the ferrous-dioxy intermediate preceded both H4B radical formation and Arg hydroxylation, However, the rate of ferrous-dioxy decay (12 s(-1)) was equivalent to the rate of H4B radical formation and s(-1) ) and the rate of Arg hydroxylation (9 s(-1)). Practically all bound H4B was oxidized to a radical during the reaction and was associated with hydroxylation of 0.6 mol of Arg/mol of heme, In dihydrobiopterin-containing iN-OSoxy, ferrous-dioxy decay was much slower and was not associated with Arg hydroxylation. These results establish kinetic and quantitative links among ferrous-dioxy disappearance, H4B oxidation, and Arg hydroxylation and suggest a mechanism whereby H4B transfers an electron to the ferrous-dioxy intermediate to enable the formation of a heme-based oxidant that rapidly hydroxylates Arg.