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Scientists Determine Structure of Active Form of the Enzyme Producing Nitric Oxide; New Knowledge May Assist in Rational Drug Design for Several Human Diseases

La Jolla, CA. March 27, 1998 -- Scientists in the Department of Molecular Biology and The Skaggs Institute for Chemical Biology at The Scripps Research Institute (TSRI) and their colleagues at the Lerner Institute at the Cleveland Clinic have determined the molecular structures of the complete active site of the enzyme responsible for generating nitric oxide (NO). Knowledge of these structures advances understanding of how the essential intercellular signal and protective cytotoxin NO is produced and regulated in mammalian cells. According to the researchers, moreover, this structural information will provide important information in enabling rational drug design for the potential control of many NO-dependent processes in humans that include blood pressure regulation, digestion, muscular contraction, the immune response, learning and memory.

The paper, "Structure of Nitric Oxide Synthase Oxygenase Dimer with Pterin and Substrate," by Brian R. Crane, Andrew S. Arvai, Dipak K. Ghosh, Chaoqun Wu, Elizabeth D. Getzoff, Dennis J. Stuehr, and John A. Tainer, appears in today's issue of Science.

NO has been implicated in myriad biological processes in the cardiovascular, digestive, muscular, immune and nervous systems. Its properties make it an ideal signal for communication and toxin for defense. Its availability is controlled solely at synthesis by three closely-related forms of the enzyme nitric oxide synthase (NOS), each of which produces NO for carefully designed purposes. Uncontrolled synthesis of nitric oxide can lead to such diseases as juvenile diabetes, arthritis, aneurysms, impotence, neurodegenerative diseases and septic shock.

In this work the scientists describe specific requirements for NO synthesis to occur. The study reveals how two NOS oxygenase domains must associate in a dimeric structure and bind two ancillary molecules of tetrahydrobiopterin, a cofactor whose role in NO synthesis until this time had not been well understood. Only after this assembly will each NOS oxygenase domain accept electrons from its reductase domain partner, bind its substrate -- the amino acid arginine -- and react arginine with molecular oxygen to form NO.

Commented John Tainer, Ph.D., "Perhaps the most important implication for medicine is that these structures provide a complete three-dimensional template for chemists to design synthetic molecules for the selective inhibition of the three different forms of NOS and thereby alleviate the destructive, unwanted NO production that is associated with many human pathologies."

This new research builds on previous studies by the same group in late 1997 that resulted in the structural determination of a NOS oxygenase domain monomer in isolation. This discovery revealed a novel enzyme configuration that, for the first time, placed structural constraints on NOS activity and differentiated it from other enzymes that catalyze related chemistry.

According to Brian Crane, Ph.D., "However, this structure only hinted at the roles for dimerization, the cofactor tetrahydrobiopterin, interaction with the reductase domain and arginine binding in the catalytic mechanism. In contrast, these new dimeric structures reveal how binding of the pterin cofactors and dimerization act together to remodel and sequester the enzyme's catalytic center, generate the arginine binding site, and provide a suitable chemical environment for NO synthesis."

The structure rules out pterin acting directly in arginine oxidation, implicating instead a heme cofactor, but suggests that pterin may participate indirectly in catalysis by influencing the heme's reactivity toward oxygen. The structure of the NOS dimer in complex with its substrate arginine implies that arginine itself may differentiate multiple chemical steps in NO synthesis by catalyzing formation of an activated form of molecular oxygen at the heme center.

The study was funded by the National Institutes of Health, a Helen Hay Whitney Fellowship and the American Heart Association.

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