How external perturbations affect the chemoselectivity of substrate activation by cytochrome P450 OleTJE

Abstract

Cytochrome P450 enzymes are versatile biocatalysts found in most forms of life. Generally, the cytochrome P450s react with dioxygen and hence are haem-based mono-oxygenases; however, in specific isozymes, H₂O₂ rather than O₂ is used and these P450s act as peroxygenases. The P450 OleT_JE is a peroxygenase that binds long to medium chain fatty acids and converts them to a range of products originating from C^α-hydroxylation, C^β-hydroxylation, C^α–C^β desaturation and decarboxylation of the substrate. There is still controversy regarding the details of the reaction mechanism of P450 OleT_JE; how the products are formed and whether the product distributions can be influenced by external perturbations. To gain further insights into the structure and reactivity of P450 OleT_JE, we set up a range of large active site model complexes as well as full enzymatic structures and did a combination of density functional theory studies and quantum mechanics/molecular mechanics calculations. In particular, the work focused on the mechanisms leading to these products under various reaction conditions. Thus, for a small cluster model, we find a highly selective C^α-hydroxylation pathway that is preferred over C^β–H hydrogen atom abstraction by at least 10 kcal mol⁻¹. Introduction of polar residues to the model, such as an active site protonated histidine residue or through external electric field effects, lowers the C^β–H hydrogen atom abstraction barriers are lowered, while a full QM/MM model brings the C^α–H and C^β–H hydrogen atom abstraction barriers within 1 kcal mol⁻¹. Our studies; therefore, implicate that environmental effects in the second-coordination sphere can direct and guide selectivities in enzymatic reaction mechanisms.