Abstract
The nitric oxide synthases (NOS), although unrelated to the cytochromes P450 in terms of sequence, exhibit spectroscopic and catalytic properties strongly reminiscent of those of the P450 system. One important difference is the requirement of the NOS enzymes for tetrahydrobiopterin. The biopterin cofactor is shown by chemical studies to bind close to pyrrole ring D of the prosthetic heme group, a position confirmed recently for inducible NOS and endothelial NOS by crystal structures. The only plausible role so far for the tetrahydrobiopterin is as a transient electron donor for the activation of molecular oxygen. NADPH-derived electrons are provided to the heme by the NOS flavin domain, but the biopterin may be required to provide an electron at a faster rate than that supported by the flavin groups. Chimeras in which the reductase domains of the isoforms have been exchanged indicate that the overall rate of catalytic turnover is directly governed by the ability of the flavin domain to deliver electrons. Electron transfer from the flavin to the heme domain, and within the flavin and heme domains, is thus a critical determinant of the catalytic turnover of NOS.
Footnotes
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Send reprint requests to: Dr. Paul R. Ortiz de Montellano, School of Pharmacy, University of California, San Francisco, CA 94143-0446. e-mail: ortiz{at}cgl.ucsf.edu
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The work in the author’s laboratory was supported by National Institutes of Health grant GM25515.
- Abbreviations used are::
- NOS
- nitric oxide synthase
- CaM
- Ca2+-dependent calmodulin
- P450
- cytochrome P450
- l-Arg
- l-arginine
- FAD
- flavin adenine dinucleotide
- FMN
- flavin mononucleotide
- H4B
- 5,6,7,8-tetrahydrobiopterin
- nNOS
- neuronal NOS
- iNOS
- inducible NOS
- eNOS
- endothelial NOS
- heme
- iron protoporphyrin IX regardless of iron coordination and oxidation state
- PDZ
- domains of ∼80 amino acids found in structural proteins of the cytoskeleton and in enzymes that associate with the cytoskeleton, and therefore thought to be involved in protein-protein interaction (also called GLGF repeats or DHRs)
- E/N
- chimera consisting of the eNOS heme and CaM-binding domains and the nNOS flavin domain
- I/N
- chimera consisting of the iNOS heme and CaM-binding domains and the nNOS flavin domain
- The American Society for Pharmacology and Experimental Therapeutics
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