Mutations at the putative active cavity of styrene monooxygenase: enhanced activity and reversed enantioselectivity.

Styrene monooxygenase (SMO) catalyzes the first step of styrene degradation, and also serves as an important enzyme for the synthesis of enantiopure epoxides. To enhance its activity, molecular docking of styrene was performed based on the X-ray crystal structure of the oxygenase subunit of SMO to identify three amino acid residues (Tyr73, His76 and Ser96) being adjacent to the phenyl ring of styrene. Variants at those positions were constructed and their enzymatic activities were analyzed. Three mutants (Y73V, Y73F, and S96A) were found to exhibit higher enzymatic activities than the wild-type in the epoxidation of styrene, while retaining excellent stereoselectivity. The specific epoxidation activity of the most active mutant S96A toward styrene and trans-ß-methyl styrene were 2.6 and 2.3-fold of the wild-type, respectively. In addition, the Y73V mutant showed an unexpected reversal of enantiomeric preference toward 1-phenylcyclohexene.

Rational design of styrene monooxygenase mutants with altered substrate preference.

Styrene monooxygenase catalyzes the enantioselective epoxidation of styrene but displays significantly decreased activity toward styrene derivatives with an α- or ß-substituent. Based on the X-ray crystal structure of the oxygenase subunit of styrene monooxygenase, molecular docking of α-ethylstyrene was performed to identify adjacent residues. Four amino acid substitutions (R43A, L44A, L45A, and N46A) were introduced into the enzyme by site-directed mutagenesis. All four mutations led to a change of substrate preference. The mutant L45A, in particular, exhibited an altered substrate preference toward the bulkier substrate α-ethylstyrene.