A novel carbohydrate-binding module from sugar cane soil metagenome featuring unique structural and carbohydrate affinity properties

Abstract Carbohydrate-binding modules (CBMs) are appended to glycoside hydrolases and can contribute to the degradation of complex recalcitrant substrates such as the plant cell wall. For application in bioethanol production, novel enzymes with high catalytic activity against recalcitrant lignocellulosic material are being explored and developed. In this work, we report the functional and structural study of CBM_E1, discovered through a metagenomics approach, which is the founding member of a novel CBM family, CBMxx. CBM_E1, which is linked to an endoglucanase, displayed affinity for mixed linked beta-1,4-beta-1,3-glucans, xyloglucan, avicel and cellooligosaccharides. The crystal structure of CBM_E1 in complex with cellopentaose displayed a canonical beta-sandwich fold comprising two beta sheets. The planar ligand binding site, observed in a parallel orientation with the beta strands, is a typical feature of Type A CBMs. On the other hand, affinity for bacterial crystalline cellulose was not detected, a ligand recognized by Type A CBMs, while binding to soluble glucans was enthalpically driven, typical of Type B modules. These unique properties of CBM_E1 are at the interface between Type A and Type B CBMs.