QM/MM study of the mechanism of enzymatic limonene 1,2-epoxide hydrolysis

Limonene 1,2-epoxide hydrolase (LEH) is completely different from those of classic epoxide hydrolases (EHs) which catalyze the hydrolysis of epoxides to vicinal diols. A novel concerted general acid catalysis step involving the Asp101-Arg99-Asp132 triad is proposed to play an important role in the mechanism. Combined quantum-mechanical/molecular-mechanical (QM/MM) calculations gave activation barriers of 16.9 and 25.1 kcal/mol at the B3LYP/6-31G(d,p)//CHARMM level for nucleophilic attack on the more and less substituted epoxide carbons, respectively. Furthermore, the important roles of residues Arg99, Tyr53 and Asn55 on mutated LEH were evaluated by QM/MM-scanned energy mapping. These results may provide an explanation for site-directed mutagenesis.