Mechanism of action of cyclic β-1,2-glucan synthetase from Agrobacterium tumefaciens: Competition between cyclization and elongation reactions

We have examined some aspects of the mechanism of cyclic β-1,2-glucan synthetase from Agrobacterium tumefaciens (235-kDa protein, gene product of the chvB region). The enzyme produces cyclic β-1,2-glucans containing 17 to 23 glucose residues from UDP-glucose. In the presence of added cyclic β- 1,2-glucans (>0.5 mg/ml) (containing 17 to 23 glucose residues), the enzyme instead synthesizes larger cyclic β-1,2-glucans containing 24 to 30 glucose residues. This is achieved by de novo synthesis and not by disproportion reactions with the added product. This is interpreted as inhibition of the specific cyclization reaction for the synthesis of cyclic β-1,2-glucans containing 17 to 23 glucose residues but with no concomitant effect on the elongation (polymerization) reaction. Temperature and detergents both affect the distribution of sizes of cyclic β-1,2-glucans, but glucans containing 24 to 30 glucose residues are not produced. We suggest that the size distribution of cyclic β-1,2-glucan products depends on competing elongation and cyclization reactions.