Benzocycloarene Hydroxylation by P450 Biocatalysis
M P. Mayhew, A. E. Roitberg, Yadu D. Tewari, Marcia J. Holden, David J. Vanderah, V L. Vilker
Experimental and theorectical studies of the hydroxylation of a family of benzocycloarene compounds (benzocyclobutene, benzocyclopentene (indan), benzocyclohexene (tetralin), and benzocycloheptene) by wild type P450cam and mutant Y96F P450cam were performed in order to understand the factors affecting product distribution, catalytic rate and cofactor utilization. The products of all reactions except that of benzocycloheptene were regiospecifically hydroxylated in the 1-position. Reaction energetics predominated over active site steric constraints in this case so that quantum mechanical calculations (B3LYP/6-31G*) comparing the energetics of all possible radical intermediates successfully predicted hydroxylation at the 1-and 3- positions of benzocycloheptene, and at the 1-position for the other three compounds. The indan and tetralin reaction products were stereoselective to the R-enantiomer (88% and 94%, respectively). Steric constraints of the active site were confirmed by molecular dynamics calculations (Locally Enhanced Sampling Dynamics) to control enantiomer distribution for tetralin hydroxylation. These calculations also indicated that steric factors did not greatly influence regiospecificity. NADH coupling, binding affinity, and product turnover rates were dramatically higher for Y96F P450cam, showing that the removal of the active site hydroxyl group on tyrosine makes the enzyme better suited for oxidation for these hydrophobic compounds. NADH coupling, binding affinity and product turnover rate for each enzyme generally increased with arene ring size. For both enzymes, NADH coupling and product turnover rates were correlated with extent of high spin shift upon substrate binding as determined by the shift in Soret absorption bands at 417 nm and 391 nm.