Functional and structural analysis of a cyclization domain in a cyclic beta-1,2-glucan synthase
Cyclic {beta}-1,2-glucan synthase (CGS) is a key enzyme in production of cyclic {beta}-1,2-glucans (C{beta}Gs) which are involved in bacterial infection or symbiosis to host organisms. Nevertheless, a mechanism of cyclization, the final step in the CGS reaction, has not been fully understood. Here we performed functional and structural analyses of the cyclization domain of CGS alone from Thermoanaerobacter italicus (TiCGSCy). We first found that {beta}-glucosidase-resistant compounds are produced by TiCGSCy with linear {beta}-1,2-glucans as substrates. The 1H-NMR analysis revealed that these products are C{beta}Gs. Next, action pattern analyses using {beta}-1,2-glucooligosaccharides revealed a unique reaction pattern: exclusive transglycosylation without hydrolysis and a hexasaccharide as the minimum length of the substrate. Nevertheless, longer substrate {beta}-1,2-glucooligosaccharides are preferred, being consistent with the fact that CGSs generally produce C{beta}Gs with degrees of polymerization of around 20. Finally, the overall structure of the cyclization domain of TiCGSCy was found to be similar to those of {beta}-1,2-glucanases in phylogenetically different groups. Meanwhile, the identified catalytic residues indicated clear difference in the reaction pathways between these enzymes. Therefore, we propose a novel reaction mechanism of TiCGSCy. Thus, we propose that the present group of CGSs should be defined as a new glycoside hydrolase family, GHxxx. Statements and DeclarationsCompeting Interests: The authors declare no competing interests. Key PointsIt was clearly evidenced that cyclization domain alone produces cyclic {beta}-1,2-glucans. The domain exclusively catalyzes transglycosylation without hydrolysis. The present catalytic domain should be defined as a new glycoside hydrolase family.