Density Functional Studies on Thromboxane Biosynthesis: Mechanism and Role of the Heme-Thiolate System



Reaction mechanisms for the isomerization of prostaglandin H2 to thromboxane A2, and degradation to 12-L-hydroxy-5,8,10-heptadecatrienoic acid (HHT) and malondialdehyde (MDA), catalyzed by thromboxane synthase, were investigated using the unrestricted Becke-three-parameter plus Lee–Yang–Parr (UB3LYP) density functional level theory. In addition to the reaction pathway through FeIV-porphyrin intermediates, a new reaction pathway through FeIII-porphyrin π-cation radical intermediates was found. Both reactions proceed with the homolytic cleavage of endoperoxide O[BOND]O to give an alkoxy radical. This intermediate converts into an allyl radical intermediate by a C[BOND]C homolytic cleavage, followed by the formation of thromboxane A2 having a 6-membered ring through a one electron transfer, or the degradation into HHT and MDA. The proposed mechanism shows that an iron(III)-containing system having electron acceptor ability is essential for the 6-membered ring formation leading to thromboxane A2. Our results suggest that the step of the endoperoxide O[BOND]O homolytic bond cleavage has the highest activation energy following the binding of prostaglandin H2 to thromboxane synthase.