Dextransucrase and the mechanism for dextran biosynthesis.

Remaud-Simeon and co-workers [Moulis, C.; Joucla, G.; Harrison, D.; Fabre, E.; Potocki-Veronese, G.; Monsan, P.; Remaud-Simeon, M. J. Biol. Chem., 2006, 281, 31254-31267] have recently proposed that a truncated Escherichia coli recombinant B-512F dextransucrase uses sucrose and the hydrolysis product of sucrose, D-glucose, as initiator primers for the nonreducing-end synthesis of dextran. Using (14)C-labeled D-glucose in a dextransucrase-sucrose digest, it was found that <0.02% of the D-glucose appears in a dextran of M(n) 84,420, showing that D-glucose is not an initiator primer, and when the dextran was treated with 0.01 M HCl at 80 degrees C for 90 min and a separate sample with invertase at 50 degrees C for 24h, no D-fructose was formed, indicating that sucrose is not present at the reducing-end of dextran, showing that sucrose also was not an initiator primer. It is further shown that both d-glucose and dextran are covalently attached to B-512FMC dextransucrase at the active site during polymerization. A pulse reaction with [(14)C]-sucrose and a chase reaction with nonlabeled sucrose, followed by dextran isolation, reduction, and acid hydrolysis, gave (14)C-glucitol in the pulsed dextran, which was significantly decreased in the chased dextran, showing that the D-glucose moieties of sucrose are added to the reducing-ends of the covalently linked growing dextran chains. The molecular size of dextran is shown to be inversely proportional to the concentration of the enzyme, indicating a highly processive mechanism in which D-glucose is rapidly added to the reducing-ends of the growing chains, which are extruded from the active site of dextransucrase. It is also shown how the three conserved amino acids (Asp551, Glu589, and Asp 622) at the active sites of glucansucrases participate in the polymerization of dextran and related glucans from a single active site by the addition of the D-glucose moiety of sucrose to the reducing-ends of the covalently linked glucan chains in a two catalytic-site, insertion mechanism.