Characterization of a Bifunctional Glucoamylase AfGA from Aspergillus fumigatus with Dual Hydrolytic Activity on Starch and Chitosan

Glucoamylase is essential for the hydrolysis of starch to glucose and has broad industrial applications. Although its catalytic domain shares similarities with GH8 family chitosanases, which are known for their bifunctional activity, no bifunctional glucoamylase has been reported to date. In this study, we identify and characterize AfGA, a glucoamylase from the pathogenic fungus Aspergillus fumigatus 293, which exhibits a dual hydrolytic activity toward both starch and chitosan. AfGA demonstrated efficient starch hydrolysis at 70 °C with a specific activity of 503.28 ± 1.3 U/mg and chitosan hydrolysis at 90 °C with a specific activity of 3.67 ± 0.1 U/mg. Molecular docking and dynamics simulations revealed that the enhanced catalytic activity and substrate binding of AfGA for starch are attributed to increased interactions within the substrate-binding pocket. The ΔAfga strain exhibited reduced growth, sporulation, and carbon utilization efficiency as well as hypersensitivity to cell wall-disrupting agents. These results highlight Afga's critical role in maintaining cell wall integrity and carbon metabolism in A. fumigatus. Our findings provide new insights into the substrate promiscuity of glycoside hydrolases and underscore the potential of AfGA in both industrial biocatalysis and fungal biology.