Mechanism of action of the endo-(1 → 3)-α-glucanase MutAp from the mycoparasitic fungus Trichoderma harzianum

(1 → 3)-α-Glucanases catalyze the hydrolysis of fungal cell wall (1 → 3)-α-glucan, and function during cell division of yeasts containing this cell wall component or act in mycoparasitic processes. Here, we characterize the mechanism of action of the (1 → 3)-α-glucanase MutAp from the mycoparasitic fungus Trichoderma harzianum. We observed that MutAp releases predominantly β-glucose upon hydrolysis of crystalline (1 → 3)-α-glucan, indicating inversion of the anomeric configuration. After having identified (1 → 3)-α-glucan tetrasaccharide as the minimal substrate for MutAp, we showed that reduced (1 → 3)-α-glucan pentasaccharide is cleaved into a trisaccharide and a reduced disaccharide, demonstrating that MutAp displays endo-hydrolytic activity. We propose a model for the catalytic mechanism of MutAp, whereby the enzyme breaks an intrachain glycosidic linkage of (1 → 3)-α-glucan, and then continues its hydrolysis towards the non-reducing end by releasing β-glucose residues in a processive manner.