Mutational disruption of the nitric oxide synthase/calmodulin interaction causes increased formation of reactive oxygen species

Abstract

The nitric oxide synthases (NOSs) catalyze the formation of nitric oxide (NO) from L‐arginine. The NOSs are bidomain enzymes, consisting of a heme‐containing oxygenase domain and a flavin‐containing reductase domain, bisected by a calmodulin (CaM) binding site. Occupancy of this site is required for NO formation. Recently, Xia et al. (2009, J. Biol. Chem., 284, 30708) published the structure of human iNOS CaM‐bound FMN domain, showing that the only interactions between the FMN domain and CaM involve R536 of iNOS, which makes a salt bridge with E47 of CaM and forms H‐bonds with N42 of CaM and S562, F565 and A564 in the FMN domain. Mutations of R530, the homologous residue in the murine iNOS reductase domain, and R753 in the nNOS reductase domain, yield enzymes active in electron transfer to cytochrome c and potassium ferricyanide. The rate of ferricyanide reduction is equivalent to that of wild‐type, but cytochrome c reduction is diminished by approximately 60%. NADPH oxidation in the absence of acceptor, however, is elevated by about 40% over that of wild‐type, indicating an increase in reduced oxygen species production by these mutants.Supported by NIH GM052419 to BSSM and LJR.