A Revised Mechanism for Human Cyclooxygenase-2 [Molecular Bases of Disease]

November 12th, 2015 by Liu, Y., Roth, J.

The mechanism of omega-6 polyunsaturated fatty acid oxidation by wild-type COX-2 and the Y334F variant, lacking a conserved tyrosine that hydrogen bond to the catalytic tyrosyl radical/tyrosine, was examined for the first time under physiologically relevant conditions. The enzymes show apparent bimolecular rate constants and deuterium kinetic isotope effects that increase in proportion to co-substrate concentrations before converging to limiting values. The trends exclude multiple dioxygenase mechanisms as well as the proposal that initial hydrogen atom abstraction from the fatty acid is the first irreversible step in catalysis. Temperature-dependent studies reinforce the novel finding that hydrogen transfer from the reduced catalytic tyrosine to a terminal peroxyl radical is likely the first irreversible step that controls regio and stereospecific product formation