Characterization of key residues in the subdomain encoded by exons 8 and 9 of human inducible nitric oxide synthase: A critical role for Asp-280 in substrate binding and subunit interactions

D. K. Ghosh, M. B. Rashid, B. Crane, V. Taskar, M. Mast, M. A. Misukonis, J. B. Weinberg, N. T. Eissa

Research output: Contribution to journalArticle

34 Scopus citations

Abstract

Human inducible nitric oxide synthase (iNOS) is active as a dimer of two identical subunits. Each subunit has an amino-terminal oxygenase domain that binds the substrate L-Arg and the cofactors heme and tetrahydrobiopterin and a carboxyl-terminal reductase domain that binds FMN, FAD, and NADPH. We previously demonstrated that a subdomain in the oxygenase domain encoded by exons 8 and 9 is important for dimer formation and NO synthesis. Further, we identified Trp-260, Asn-261, Tyr-267, and Asp-280 as key residues in that subdomain. In this study, using an Escherichia coli expression system, we produced, purified, and characterized wild-type iNOS and iNOS-Ala mutants. Using H2O2-supported oxidation of Nω-hydroxy-L-Arg, we demonstrate that the iNOS mutants' inabilities to synthesize NO are due to selective defects in the oxygenase domain activity. Detailed characterization of the Asp-280-Ala mutant revealed that it retains a functional reductase domain, as measured by its ability to reduce cytochrome c. Gel permeation chromatography confirmed that the Asp-280-Ala mutant exists as a dimer, but, in contrast to wild-type iNOS, urea-generated monomers of the mutant fail to reassociate into dimers when incubated with L-Arg and tetrahydrobiopterin, suggesting inadequate subunit interaction. Spectral analysis reveals that the Asp-280-Ala mutant does not bind L-Arg. This indicates that, in addition to dimerization, proper subunit interaction is required for substrate binding. These data, by defining a critical role for Asp-280 in substrate binding and subunit interactions, give insights into the mechanisms of regulation of iNOS activity.

Original languageEnglish (US)
Pages (from-to)10392-10397
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Volume98
Issue number18
DOIs
StatePublished - Aug 28 2001

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