Structural insights into β-1,3-glucan cleavage by a glycoside hydrolase family.

Comprehensive informatic, structural and biochemical characterization of the GH128 family defines subgroups of glycoside hydrolase enzymes with unique recognition and cleavage mechanisms for 1,3-beta-glucan polysaccharide substrates. The fundamental and assorted roles of beta-1,3-glucans in nature are underpinned on diverse chemistry and molecular structures, demanding sophisticated and intricate enzymatic systems for their processing. In this work, the selectivity and modes of action of a glycoside hydrolase family active on beta-1,3-glucans were systematically investigated combining sequence similarity network, phylogeny, X-ray crystallography, enzyme kinetics, mutagenesis and molecular dynamics. This family exhibits a minimalist and versatile (alpha/beta)-barrel scaffold, which can harbor distinguishing exo or endo modes of action, including an ancillary-binding site for the anchoring of triple-helical beta-1,3-glucans. The substrate binding occurs via a hydrophobic knuckle complementary to the canonical curved conformation of beta-1,3-glucans or through a substrate conformational change imposed by the active-site topology of some fungal enzymes. Together, these findings expand our understanding of the enzymatic arsenal of bacteria and fungi for the breakdown and modification of beta-1,3-glucans, which can be exploited for biotechnological applications.