title: Isolation and sequence of an active-site peptide containing a catalytic aspartic acid from two Streptococcus sobrinus alpha-glucosyltransferases.
abstract: An active-site peptide containing an aspartic acid implicated in catalysis has been isolated and sequenced from two Streptococcus sobrinus extracellular glucosyltransferases: sucrose:1,3-alpha-D-glucan 3-alpha-D-glucosyltransferase (GTase-I) and sucrose:1,6-alpha-D-glucan 6-alpha-D-glucosyltransferase (GTase-S). The sequenced peptides, tagged with radiolabeled glucose, were isolated from a pepsin digest of a stabilized glucosylenzyme complex prepared by rapidly denaturing a reaction of enzyme and radiolabeled sucrose. The glucosyl linkage had previously been characterized as a beta-anomer bound to an active-site carboxyl group. Purified GTase-I and GTase-S glucosyl-peptides had the following similar but not identical sequences: GTase-I, Asp-Ser-Ile-Arg-Val-Asp-Ala-Val-Asp; and GTase-S, Asp-Gly-Val-Arg-Val-Asp-Ala-Val-Asp. Each has 3 aspartic acids as potential sites of glucose conjugation, but the relevant residue was not identified in sequence analysis because the highly base-labile glucosyl bond was cleaved in the first sequence cycle. As an alternative, the GTase-I glucosyl-peptide was partially digested at the N terminus with cathepsin C and at the C terminus with carboxypeptidase P. Analysis of the truncated products by fast atom bombardment mass spectrometry localized the glucosyl group to Asp-6 i the GTase-I peptide. In the native enzyme, this sequence is found near the N terminus, well-removed from the glucan-binding site located on a 60-kDa domain at the C terminus. The catalysis-dependent method of incorporating a glucosyl label implicates the aspartic acid as the residue involved in stabilizing an oxocarbonium ion transition state. The peptide segment is highly conserved and homologous to a peptide from sucrase-isomaltase labeled by site-directed irreversible inhibition and peptide segments common to a broad array of alpha-glucosidases and related transferases.
authors.affiliation: Department of Basic Sciences, School of Dentistry, University of Southern California, Los Angeles 90089-0641.
authors.forename: G
authors.initials: G
authors.lastname: Mooser
authors.forename: S A
authors.initials: SA
authors.lastname: Hefta
authors.forename: R J
authors.initials: RJ
authors.lastname: Paxton
authors.forename: J E
authors.initials: JE
authors.lastname: Shively
authors.forename: T D
authors.initials: TD
authors.lastname: Lee
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D002851:Chromatography, High Pressure Liquid; D005964:Glucosyltransferases / Q000737:chemistry*; D013058:Mass Spectrometry; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000737:chemistry; D012689:Sequence Homology, Nucleic Acid; D013291:Streptococcus / Q000201:enzymology*
chemical_list: D010446:Peptide Fragments; C036655:1,3-alpha-D-glucan synthase; D005964:Glucosyltransferases