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Vol. 26, Issue 12, 1185-1189, December 1998
Department of Pharmaceutical Chemistry, University of California,
San Francisco
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.
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