Abstract
Recent advances in understanding structure-function relationships in cytochrome P450 (P450), nitric-oxide synthase (NOS), and heme oxygenase are summarized. Of particular importance is the role that dynamics plays in P450 function, where the active site undergoes large open/close motions to enable substrates to bind and products to leave. In sharp contrast, the heme-containing active site of NOS is rigid and remains relatively exposed compared with P450s. This difference in dynamics and active site exposure requires that the O2 activation machinery operate somewhat differently in P450 and NOS. Owing to the open NOS active site, the NOS-oxy complex could be subject to nonspecific protonation that short-circuits the normal reaction path. One working hypothesis holds that NOS recruited the cofactor, tetrahydrobiopterin, to bind near the heme for very rapid coupled electron/proton transfer to the oxy complex, which avoids indiscriminate reaction with bulk solvent. Despite these differences, P450, NOS, and also heme oxygenase use a very similar network of H-bonded water molecules in the active site that are required for oxygen activation. Both P450 and NOS are important drug targets. With NOS, the structural basis for isoform-selective inhibition by a class of dipeptide inhibitors has been worked out, thus providing the basis for structure-based drug design.
Footnotes
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Work in the Poulos laboratory was supported by National Institutes of Health Grants GM33688 and GM57353.
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doi:10.1124/dmd.104.002071.
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ABBREVIATIONS: P450, cytochrome P450; NOS, nitric-oxide synthase; HO, heme oxygenase; BH4, tetrahydrobiopterin; eNOS, endothelial NOS; nNOS, neuronal NOS; iNOS, inducible NOS; NO, nitric oxide.
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Thomas L. Poulos received a Ph.D. in Biology from the University of California, San Diego in 1972. During his postdoctoral work in the Chemistry Department at UCSD, he solved the first crystal structure of a heme enzyme, cytochrome c peroxidase. After his postdoctoral work, Dr. Poulos held the position of Director of Protein Engineering at Genex Corp. in Gaithersburg, Maryland and, later, Director of the University of Maryland's Center for Advanced Research in Biotechnology and Professor of Chemistry at the College Park Campus of the University of Maryland, where he was awarded the Presidential Meritorious Service Award. While at Genex, Dr. Poulos completed work on the first structure determination of a cytochrome P450, P450cam. In 1992 he was recruited by the University of California, Irvine, where he now holds the position of Chancellor's Professor of Biochemistry, Biophysics, and Chemistry. His current work centers on structure-function relationships in heme enzymes and related proteins including nitric-oxide synthase, P450s, and heme oxygenase. - Received August 27, 2004.
- Accepted October 7, 2004.
- The American Society for Pharmacology and Experimental Therapeutics
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