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

Abstract

Reaction mechanisms for the isomerization of prostaglandin H(2) to thromboxane A(2), 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 Fe(IV)-porphyrin intermediates, a new reaction pathway through Fe(III)-porphyrin pi-cation radical intermediates was found. Both reactions proceed with the homolytic cleavage of endoperoxide O-O to give an alkoxy radical. This intermediate converts into an allyl radical intermediate by a C-C homolytic cleavage, followed by the formation of thromboxane A(2) 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 A(2). Our results suggest that the step of the endoperoxide O-O homolytic bond cleavage has the highest activation energy following the binding of prostaglandin H(2) to thromboxane synthase.

Mechanistic insights into the catalytic reaction of plant allene oxide synthase (pAOS) via QM and QM/MM calculations

“…Then the reaction further undergoes the epoxide formation with the electron transfer onto Cpd II and the last step is PCET from the substrate carbon adjacent to the epoxide moiety to his residue forming C C double bond onto the substrate. On the contrary to the typical P450s, it was proposed that plant AOS is unusual among P450s in that its reaction mechanism occurs without passing the Feoxo species as found in thromboxane and prostacyclin biosynthase from prostaglandin H 2 [38,[44][45][46].…”

Density Functional Studies on Isomerization of Prostaglandin H2 to Prostacyclin Catalyzed by Cytochrome P450

“…This conclusion was also drawn in our previous DFT studies on TXA 2 biosynthesis from PGH 2 . [27] The present studies also showed that the PGH 2 isomerization to PGI 2 by an atypical P450 enzyme proceeds through PCET to yield a PGI 2 -PGIS complex with a low activation energy. Interestingly, the one-electron transfers coupled with proton transfer and C À O bond formation were observed in both the PGI 2 and TXA 2 biosyntheses [27] catalyzed by atypical cytochrome P450s, respectively.…”

“…[27] The present studies also showed that the PGH 2 isomerization to PGI 2 by an atypical P450 enzyme proceeds through PCET to yield a PGI 2 -PGIS complex with a low activation energy. Interestingly, the one-electron transfers coupled with proton transfer and C À O bond formation were observed in both the PGI 2 and TXA 2 biosyntheses [27] catalyzed by atypical cytochrome P450s, respectively. The one-electron transfer process is thought to be essential for both typical [54] and atypical P450-catalyzed reactions.…”

Quantum-Mechanical Studies of Reactions Performed by Cytochrome P450 Enzymes

“…The suggested mechanism for the formation of three prostaglandin H 2 metabolites illustrated by the model systems used in calculations. 144,145 In addition to the standard reactions described above, a number of studies of more specialized CYPs have been performed. Yanai and Mori have studied the formation of two prostaglandin H 2 (PGH 2 ) metabolites, prostacyclin (PGI 2 ) and thromboxane A 2 (TXA 2 ) by two different CYPs, prostacyclin synthase and thromboxane synthase.…”

“…Yanai and Mori have studied the formation of two prostaglandin H 2 (PGH 2 ) metabolites, prostacyclin (PGI 2 ) and thromboxane A 2 (TXA 2 ) by two different CYPs, prostacyclin synthase and thromboxane synthase. 144,145 The suggested reaction mechanisms are shown in Figure 21. The isomerization of PGH 2 is initiated by the direct binding of one of the two peroxo oxygen atoms to Fe III .…”