The Reaction Mechanism of Aphid Myrosinase, A Theoretical Study

The aphid Brevicoryne brassicae is a specialist feeding on Brassicaceae plants. The insecthas an intricate defense system involving the myrosinase enzyme that hydrolysis glucosinolatessequestered from the host plant into volatile isothiocyanates [۱].In this study, we investigated thereaction mechanism of aphid myrosinase usinga quantum mechanics/molecular mechanics(QM/MM) method [۲]. A model of the active site of this enzyme was constructed based on thecrystal structure of aphid myrosinase from Brevicoryne brassicae (۱WCG PDB ID) [۱]. Theactive site can be divided into two parts, the glucose and aglyconbinding sites (cf. Figure ۱).Ourresult showed that in the first step of the mechanism (glycosylation), the catalytic residue (Glu-۳۷۴) performs a nucleophilic attack on the anomeric carbon of the substrate, and then the othercarboxyl residue (Glu-۱۶۷), acting as a general acid, protonates the glycosidic sulfur. This leadsto forming of a glycosyl–enzyme intermediate and the aglycon moiety leaves the reactionmedium. In the next step (deglycosylation), Glu-۱۶۷, acting as a general base, activates a watermolecule to attack the anomeric center of the intermediate, and the covalent bond between theenzyme and the glycosyl is broken, and the β-glucose molecule is released.

کلیدواژه ها:

Aphid myrosinase ، Reaction mechanism ، QM/MM