Crucial Role of Lys⁴²³ in the Electron Transfer of Neuronal Nitric-oxide Synthase*

Abstract

Nitric-oxide synthase (NOS) is composed of an oxygenase domain having cytochrome P450-type heme active site and a reductase domain having FAD- and FMN-binding sites. To investigate the route of electron transfer from the reductase domain to the heme, we generated mutants at Lys⁴²³ in the heme proximal site of neuronal NOS and examined the catalytic activities, electron transfer rates, and NADPH oxidation rates. A K423E mutant showed no NO formation activity (<0.1 nmol/min/nmol heme), in contrast with that (72 nmol/min/nmol heme) of the wild type enzyme. The electron transfer rate (0.01 min⁻¹) of the K423E on addition of excess NADPH was much slower than that (>10 min⁻¹) of the wild type enzyme. From the crystal structure of the oxygenase domain of endothelial NOS, Lys⁴²³ of neuronal NOS is likely to interact with Trp⁴⁰⁹ which lies in contact with the heme plane and with Cys⁴¹⁵, the axial ligand. It is also exposed to solvent and lies in the region where the heme is closest to the protein surface. Thus, it seems likely that ionic interactions between Lys⁴²³ and the reductase domain may help to form a flavin to heme electron transfer pathway.