title: Activation and inhibition of dextransucrase by calcium.
abstract: Initial rate kinetics of dextran synthesis by dextransucrase (sucrose:1,6-alpha-D-glucan-6-alpha-D-glucosyltransferase, EC 2.4.1.5) from Leuconostoc mesenteroides NRRL B-512F showed that below 1 mM, Ca2+ activated the enzyme by increasing Vmax and decreasing the Km for sucrose. Above 1 mM, Ca2+ was a weak competitive inhibitor (Ki = 59 mM). Although it was an activator at low concentration, Ca2+ was not required for dextran synthesis, either of main chain or branch linkages. Neither was it required for sucrose hydrolysis, acceptor reactions, or enzyme renaturation after SDS-polyacrylamide gel electrophoresis. A model for dextran synthesis is proposed in which dextransucrase has two Ca2+ sites, one activating and one inhibitory. Ca2+ at the inhibitory site prevents the binding of sucrose.
mesh_terms: D002118:Calcium / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D005964:Glucosyltransferases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D006602:Hexosyltransferases / Q000378:metabolism; D007700:Kinetics; D007934:Leuconostoc / Q000201:enzymology
chemical_list: D005964:Glucosyltransferases; D006602:Hexosyltransferases; C021513:levansucrase; C020025:dextransucrase; D002118:Calcium