title: Hydrogen tunneling steps in cyclooxygenase-2 catalysis.
abstract: Cyclooxygenases-1 and -2 are tyrosyl radical (Y·)-utilizing hemoproteins responsible for the biosynthesis of lipid-derived autocoids. COX-2, in particular, is a primary mediator of inflammation and believed to be up-regulated in many forms of cancer. Described here are first-of-a-kind studies of COX-2-catalyzed oxidation of the substrate analogue linoleic acid. Very large (≥20) temperature-independent deuterium kinetic isotope effects (KIEs) on the rate constant for enzyme turnover were observed, due to hydrogen atom abstraction from the bisallylic C-H(D) of the fatty acid. The magnitude of the KIE depends on the O(2) concentration, consistent with reversible H/D tunneling mediated by the catalytic Y·. At physiological levels of O(2), retention of the hydrogen initially abstracted by the catalytic tyrosine results in strongly temperature-dependent KIEs on O-H(D) homolysis, also characteristic of nuclear tunneling.
authors: Department of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States.
authors: Husain H
authors: HH
authors: Danish
authors: Irina S
authors: IS
authors: Doncheva
authors: Justine P
authors: JP
authors: Roth
mesh_terms: D000818:Animals; D051546:Cyclooxygenase 2 / Q000378:metabolism*; D006859:Hydrogen / Q000378:metabolism*; D019787:Linoleic Acid / Q000378:metabolism*; D051379:Mice; D008958:Models, Molecular; D010084:Oxidation-Reduction
chemical_list: D006859:Hydrogen; D019787:Linoleic Acid; D051546:Cyclooxygenase 2