LAUSR

Structural and functional studies were conducted of the glucuronoyl esterase (GE) from Cerrena unicolor ( Cu GE), an enzyme catalyzing cleavage of lignin-carbohydrate ester bonds. Cu GE is an α/β-hydrolase belonging to carbohydrate esterase family 15 (CE15). The enzyme is modular, comprised of a catalytic and a carbohydrate-binding domain. SAXS data show Cu GE as an elongated rigid molecule where the two domains are connected by a rigid linker. Detailed structural information of the catalytic domain in its apo- and inactivated form and complexes with aldouronic acids reveal well-defined binding of the 4- O -methyl-a-D-glucuronoyl moiety, not influenced by the nature of the attached xylo-oligosaccharide. Structural and sequence comparisons within CE15 enzymes reveal two distinct structural subgroups. Cu GE belongs to the group of fungal CE15-B enzymes with an open and flat substrate-binding site. The interactions between Cu GE and its natural substrates are explained and rationalized by the structural results, microscale thermophoresis and isothermal calorimetry. Glucuronoyl esterases have the potential to be used in the biocatalytic conversion of lignin-carbohydrate complexes to obtain pure lignin for downstream biofuel conversion. Here the authors present a detailed structural analysis of the glucuronoyl esterase from Cerrena unicolor , providing the basis for its activity on natural substrate and for how lignin can be selectively separated from lignocellulosic materials.