Streptococcus mutans Dextransucrase: Functioning of Primer Dextran and Endogenous Dextranase in Water-Soluble and Water-Insoluble Glucan Synthesis

Abstract

The extracellular enzyme activities of Streptococcus mutans 6715 that synthesize glucans from sucrose were concentrated and partially purified by ammonium sulfate precipitation and gel permeation column chromatography. Polyacrylamide gel analysis demonstrated that all of the major proteins precipitated by ammonium sulfate were quantitatively recovered in the high-molecular-weight, enzyme-containing aggregates found in the void volume of the gel column. Anion-exchange column chromatography was used to fractionate the aggregates into preparations, α and β, which produced water-insoluble and water-soluble glucans, respectively. Polyacrylamide gel analysis showed that α and β contained unique proteins and dextransucrase (EC 2.4.1.5) activities. Studies on the time course of glucan synthesis by α demonstrated that this enzyme preparation contained dextranase activity, which partially degraded nascent alcohol-insoluble glucan into alcohol-soluble products that were subsequently reincorporated into insoluble product. The β enzyme preparation contained no detectable dextranase activity. Mixing experiments in the absence of primer dextran demonstrated that the dextranase activity present in α could modify glucan production by β. CsCl density gradient analysis of product glucans demonstrated that exogenous primer dextrans were used as acceptor molecules by both the α and β enzyme preparations, and that water-soluble glucans synthesized by β could be converted into water-insoluble glucans by α. It is proposed that the structural heterogeneity of the native glucans produced from sucrose by S. mutans is a result of the concerted action of glucan-forming dextransucrases and endohydrolytic dextranase activity.