Difference in mode of inhibition between alpha-D-xylosyl beta-D-fructoside and alpha-isomaltosyl beta-D-fructoside in synthesis of glucan by Streptococcus mutans D-glucosyltransferase.

Both alpha-isomaltosyl beta-D-fructoside and alpha-D-xylosyl beta-D-fructoside show strong inhibition of the synthesis of water-insoluble and water-soluble D-glucans from sucrose by a partially purified preparation of a D-glucosyltransferase (GTase) from Streptococcus mutans 6715; however, the inhibitory modes differ substantially. In the presence of alpha-isomaltosyl beta-D-fructoside, the production of reducing sugars and the consumption of sucrose are remarkably enhanced, compared with a control of sucrose alone. Under these conditions, a large proportion of low-molecular-weight glycan (lmwg) and a series of nonreducing oligosaccharides (both containing D-fructosyl groups or residues) are produced. In contrast, in the presence of alpha-D-xylosyl beta-D-fructoside, the production of reducing sugars and the sucrose consumption are strikingly suppressed, and no lmwg or oligosaccharides are produced. Thus, it may be concluded that alpha-isomaltosyl beta-D-fructoside acts as an alternative acceptor for the D-glucosyl and/or D-glucanosyl transfer reactions of the enzyme, and serves to lessen the formation of insoluble and soluble D-glucan, although it stimulates the transferring activity of the enzyme. On the other hand, alpha-D-xylosyl beta-D-fructoside competitively inhibits the sucrose-splitting activity of the enzyme as an analog to sucrose, and thereby diminishes the synthesis of D-glucan.