Enhancement of the performance of the GH75 family chitosanases by fusing a carbohydrate binding module and insights into their substrate binding mechanisms

Abstract

Chitooligosaccharides (COSs) with relatively higher degrees of polymerization (DP) exhibited better biological properties than COSs with lower DP. Here, we demonstrated a potential strategy to enhance the affinity, activity, and thermostability of the chitosanase Csn75 from Aspergillus fumigatus CJ22-326 through fusion of a carbohydrate binding module (CBM) that can specifically bind to chitosan. Compared with the Csn75, the specific enzyme activity of the Csn75-CBM32 and the Csn75-2CBM32 had increased by 59.18% and 14.29%, respectively. The free energy (ΔG), enthalpy (ΔH), and entropy (-TΔS) of the three enzymes (Csn75, Csn75-CBM32, and Csn75-2CBM32) toward (GlcN)6 was (−7.19, −7.54, and −8.08 kcal/mol), (−5.37, 1.75, and 0.86 kcal/mol), and (1.45, 9.27, and 8.03 kcal/mol), respectively. The substrate binding of the Csn75 and the fusion enzymes (Csn75-CBM32 and Csn75-2CBM32) were mainly driven by the hydrophobic (entropy effect) and favorable intermolecular forces (enthalpy contribution), respectively. Moreover, the DP of COSs in the final hydrolysate of Csn75 was 2–4, but that of Csn75-CBM32 and Csn75-2CBM32 was 3–5. This strategy provides a potential alternative for the bioproduction of desirable COSs to meet the requirement of application in food systems.