Dissecting regulation mechanism of the FMN to heme interdomain electron transfer in nitric oxide synthases

• NO biosynthesis by NOS is tightly regulated under physiological conditions.
• It is important to study how electron transfer is coupled to NO production by NOS.
• The control mechanism of FMN-heme electron transfer remains unclear.
• A combined spectroscopic and rapid kinetics approach has been recently developed.
• Advances in understanding mechanism of the FMN-heme electron transfer is reviewed.

Nitric oxide synthase (NOS), a flavo-hemoprotein, is responsible for biosynthesis of nitric oxide (NO) in mammals. Three NOS isoforms, iNOS, eNOS and nNOS (inducible, endothelial, and neuronal NOS), achieve their biological functions by tight control of interdomain electron transfer (IET) process through interdomain interactions. In particular, the FMN−heme IET is essential in coupling electron transfer in the reductase domain with NO synthesis in the heme domain by delivery of electrons required for O2 activation at the catalytic heme site. Emerging evidence indicates that calmodulin (CaM) activates NO synthesis in eNOS and nNOS by a conformational change of the FMN domain from its shielded electron-accepting (input) state to a new electron-donating (output) state, and that CaM is also required for proper alignment of the FMN and heme domains in the three NOS isoforms. In the absence of a structure of full-length NOS, an integrated approach of spectroscopic, rapid kinetic and mutagenesis methods is required to unravel regulation mechanism of the FMN−heme IET process. This is to investigate the roles of the FMN domain motions and the docking between the primary functional FMN and heme domains in regulating NOS activity. The recent developments in this area that are driven by the combined approach are the focuses of this review. A better understanding of the roles of interdomain FMN/heme interactions and CaM binding may serve as a basis for the rational design of new selective modulators of the NOS enzymes.

Recent advances in understanding the mechanism of the FMN to heme interdomain electron transfer in nitric oxide synthases, which are driven by an integrated approach of laser flash photolysis and pulsed EPR, are reviewed.Figure optionsDownload as PowerPoint slide