Biochemical evidence for the involvement of tyrosine in epoxide activation during the catalytic cycle of epoxide hydrolase

Epoxide hydrolases (EH) catalyze the hydrolysis of epoxides and arene oxides to their corresponding diols. The crystal structure of murine soluble EH suggests that Tyr⁴⁶⁵ and Tyr³⁸¹ act as acid catalysts, activating the epoxide ring and facilitating the formation of a covalent intermediate between the epoxide and the enzyme. To explore the role of these two residues, mutant enzymes were produced and the mechanism of action was analyzed. Enzyme assays on a series of substrates confirm that both Tyr⁴⁶⁵ and Tyr³⁸¹ are required for full catalytic activity. The kinetics of chalcone oxide hydrolysis show that mutation of Tyr⁴⁶⁵ and Tyr³⁸¹ decreases the rate of binding and the formation of an intermediate, suggesting that both tyrosines polarize the epoxide moiety to facilitate ring opening. These two tyrosines are, however, not implicated in the hydrolysis of the covalent intermediate. Sequence comparisons showed that Tyr⁴⁶⁵ is conserved in microsomal EHs. The substitution of analogous Tyr³⁷⁴ with phenylalanine in the human microsomal EH dramatically decreases the rate of hydrolysis of cis-stilbene oxide. These results suggest that these tyrosines perform a significant mechanistic role in the substrate activation by EHs.