A homodimeric bacterial exo-β-1,3-glucanase derived from moose rumen microbiome shows a structural framework similar to yeast exo-β-1,3-glucanases

The impact of various beta-giticans on the gut microbiome and immune system of vertebrates is becoming increasingly recognized. Besides the fundamental interest in understanding how beta-glucans support human and animal health, enzymes that metabolize beta-glucans are of interest for hemicellulose biopmcessing. Our earlier metagenomic analysis of the moose rumen microbiome identified a gene coding for a bacterial enzyme with a possible role in beta-glucan metabolization. Here, we report that the enzyme, mrbExg5, has exo-beta-1,3-glucanase activity on beta-1,3-linked glucooligosaccharides and laminarin, but not on beta-1,6- or beta-1,4-glycosidic bonds. Longer oligosaccharides are good substrates, while shorter substrates are readily transglycosylated into longer products. The enzyme belongs to glycoside hydrolase subfamily GH5_44, which is a close phylogenetic neighbor of the subfamily GH5_9 exo-beta-1,3-glucanases of the yeasts Saccharomyces cerevisiae and Candida albicans. The crystal structure shows that unlike the eukaryotic relatives, mrbExg5 is a functional homodimer with a binding region characterized by: (i) subsite +1 can accommodate a branched sugar on the beta-1,3-glucan backbone; (ii) subsite +2 is restricted to exclude backbone substituents; and (iii) a fourth subsite (+3) formed by a unique loop. mrbExg5 is the first GH5_44 enzyme to be structurally characterized, and the first bacterial GH5 with exo-beta-1,3-glucanase activity.

Automated summary

The bacterial enzyme mrbExg5 has been found to have exo-beta-1,3-glucanase activity on beta-1,3-linked glucooligosaccharides and laminarin, with a unique structural characteristic different from its yeast counterparts in glycoside hydrolase subfamily GH5_44.