The Role of a Conserved Serine Residue within Hydrogen Bonding Distance of FAD in Redox Properties and the Modulation of Catalysis by Ca2+/Calmodulin of Constitutive Nitric-oxide Synthases

Authors

Satya Prakash Panda, University of Texas at San Antonio
Ying Tong Gao, University of Texas at San Antonio
Linda J. Roman, University of Texas at San Antonio
Pavel Marta´sek, Charles University School of Medicine
John C. Salerno, Kennesaw State UniversityFollow
Bettie Masters, University of Texas at San Antonio

Department

Molecular and Cellular Biology

Document Type

Article

Publication Date

11-2006

Abstract

The crystal structure of the neuronal nitric-oxide synthase (nNOS) NADPH/FAD binding domain indicated that Ser-1176 is within hydrogen bonding distance of Asp-1393 and the O4 atom of FAD and is also near the N5 atom of FAD (3.7Å). This serine residue is conserved in most of the ferredoxin-NADP⁺ reductase family of proteins and is important in electron transfer. In the present study, the homologous serines of both nNOS (Ser-1176) and endothelial nitric-oxide synthase (eNOS) (Ser-942) were mutated to threonine and alanine. Both substitutions yielded proteins that exhibited decreased rates of electron transfer through the flavin domains, in the presence and absence of Ca²⁺/CaM, as measured by reduction of potassium ferricyanide and cytochrome c. Rapid kinetics measurements of flavin reduction of all the mutants also showed a decrease in the rate of flavin reduction, in the absence and presence of Ca²⁺/CaM, as compared with the wild type proteins. The serine to alanine substitution caused both nNOS and eNOS to synthesize NO more slowly; however, the threonine mutants gave equal or slightly higher rates of NO production compared with the wild type enzymes. The midpoint redox potential measurements of all the redox centers revealed that wild type and threonine mutants of both nNOS and eNOS are very similar. However, the redox potentials of the FMN/FMNH· couple for alanine substitutions of both nNOS and eNOS are >100 mV higher than those of wild type proteins and are positive. These data presented here suggest that hydrogen bonding of the hydroxyl group of serine or threonine with the isoalloxazine ring of FAD and with the amino acids in its immediate milieu, particularly nNOS Asp-1393, affects the redox potentials of various flavin states, influencing the rate of electron transfer.

Journal Title

The Journal of Biological Chemistry

Journal ISSN

0021-9258

Volume

281

First Page

34246

Last Page

34257

Digital Object Identifier (DOI)

10.1074/jbc.M601041200