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The interdomain electron transfer (IET) between the flavin mononucleotide (FMN) and heme domains is essential in the biosynthesis of nitric oxide (NO) by the NO synthase (NOS) enzymes. A conserved tyrosine residue in the FMN domain (Y631 in human inducible NOS) was proposed to be a key part of the electron transfer pathway in the FMN/heme docked complex model. In the present study, the FMN-heme IET kinetics in the Y631F mutant and wild type of a bidomain oxygenase/FMN construct of human inducible NOS were determined by laser flash photolysis. The rate constant of the Y631F mutant is significantly decreased by ∼75% (compared to the wild type), showing that the tyrosine residue indeed facilitates the FMN-heme IET through the protein medium. The IET rate constant of the wild type protein decreases from 345 to 242 s -1 on going from H 2 O to 95% D 2 O, giving a solvent kinetic isotope effect of 1.4. In contrast, no deuterium isotope effect was observed for the Tyr-to-Phe mutant. Moreover, an appreciable change in the wild type iNOS IET rate constant value was observed upon changing pH. These results indicate that the FMN-heme IET is proton coupled, in which the conserved tyrosine residue may play an important role.

Role of a Conserved Tyrosine Residue in the FMN–Heme Interdomain Electron Transfer in Inducible Nitric Oxide Synthase