Identification of enzymatic functions of osmo-regulated periplasmic glucan biosynthesis proteins from Escherichia coli reveals a novel glycoside hydrolase family

Discovery of Anomer-Inverting Transglycosylase: Cyclic Glucohexadecaose-Producing Enzyme from Xanthomonas, a Phytopathogen

Various Xanthomonas species cause well-known plant diseases. Among various pathogenic factors, the role of α-1,6-cyclized β-1,2-glucohexadecaose (CβG16α) produced by Xanthomonas campestris pv. campestris was previously shown to be vital for infecting model organisms, Arabidopsis thaliana and Nicotiana benthamiana. However, enzymes responsible for biosynthesizing CβG16α are essentially unknown, which limits the generation of agrichemicals that inhibit CβG16α synthesis. In this study, we discovered that OpgD from X. campestris pv. campestris converts linear β-1,2-glucan to CβG16α. Structural and functional analyses revealed OpgD from X. campestris pv. campestris possesses an anomer-inverting transglycosylation mechanism, which is unprecedented among glycoside hydrolase family enzymes.

Functional and structural analysis of a cyclization domain in a cyclic β-1,2-glucan synthase

Cyclic β-1,2-glucan synthase (CGS) is a key enzyme in production of cyclic β-1,2-glucans (CβGs) 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 β-glucosidase-resistant compounds are produced by TiCGSCy with linear β-1,2-glucans as substrates. The 1H-NMR analysis revealed that these products are CβGs. Next, action pattern analyses using β-1,2-glucooligosaccharides revealed a unique reaction pattern: exclusive transglycosylation without hydrolysis and a hexasaccharide being the minimum length of the substrate. These analyses also showed that longer substrate β-1,2-glucooligosaccharides are preferred, being consistent with the fact that CGSs generally produce CβGs 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 β-1,2-glucanases in phylogenetically different groups. Meanwhile, the identified catalytic residues indicated clear differences in the reaction pathways between these enzymes. Overall, we propose a novel reaction mechanism of TiCGSCy. Thus, the present group of CGSs defines a new glycoside hydrolase family, GH189. KEY POINTS: • It was clearly evidenced that cyclization domain alone produces cyclic β-1,2-glucans. • The domain exclusively catalyzes transglycosylation without hydrolysis. • The present catalytic domain defines as a new glycoside hydrolase family 189.