Mechanistic Studies of Fatty Acid Activation by CYP152 Peroxygenases Reveal Unexpected Desaturase Activity

The majority of cytochrome P450 enzymes (CYPs) predominantly operate as monooxygenases, but recently a class of P450 enzymes was discovered, that can act as peroxygenases (CYP152). These enzymes convert fatty acids through oxidative decarboxylation, yielding terminal alkenes, and through alpha- and beta-hydroxylation to yield hydroxy-fatty acids. Bioderived olefins may serve as biofuels, and hence understanding the mechanism and substrate scope of this class of enzymes is important. In this work, we report on the substrate scope and catalytic promiscuity of CYP OleT(JE) and two of its orthologues from the CYP152 family, utilizing alpha-monosubstituted branched carboxylic acids. We identify alpha,beta-desaturation as an unexpected dominant pathway for CYP OleT(JE) with 2-methylbutyric acid. To rationalize product distributions arising from alpha/beta-hydroxylation, oxidative decarboxylation, and desaturation depending on the substrate's structure and binding pattern, a computational study was performed based on an active site complex of CYP OleT(JE) containing the heme cofactor in the substrate binding pocket and 2-methylbutyric acid as substrate. It is shown that substrate positioning determines the accessibility of the oxidizing species (Compound I) to the substrate and hence the regio- and chemoselectivity of the reaction. Furthermore, the results show that, for 2-methylbutyric acid, alpha,beta-desaturation is favorable because of a rate-determining alpha-hydrogen atom abstraction, which cannot proceed to decarboxylation. Moreover, substrate hydroxylation is energetically impeded due to the tight shape and size of the substrate binding pocket.