title: Fragment-based screening of the donor substrate specificity of human blood group B galactosyltransferase using saturation transfer difference NMR.
abstract: Saturation transfer difference NMR experiments on human blood group B alpha-(1,3)-galactosyltransferase (GTB) for the first time provide a comprehensive set of binding epitopes of donor substrate analogs in relation to the natural donor UDP-Gal. This study revealed that the enzyme binds several UDP-activated sugars, including UDP-Glc, UDP-GlcNAc, and UDP-GalNAc. In all cases, UDP is the dominant binding epitope. To identify the minimum requirements for specific binding, a detailed analysis utilizing a fragment-based approach was employed. The binding of donor substrate to GTB is essentially controlled by the base as a "molecular anchor." Uracil represents the smallest fragment that is recognized, whereas CDP, AMP, and GDP do not exhibit any significant binding affinity for the enzyme. The ribose and beta-phosphate moieties increase the affinity of the ligands, whereas the pyranose sugar apparently weakens the binding, although this part of the molecule controls the specificity of the enzyme. Accordingly, UDP represents the best binder. The binding affinities of UDP-Gal, UDP-Glc, and UMP are about the same, but lower than that of UDP. Furthermore, we observed that beta-D-galactose and alpha-D-galactose bind weakly to GTB. Whereas beta-D-galactose binds to the acceptor and donor sites, it is suggested that alpha-D-galactose occupies a third hitherto unknown binding pocket. Finally, our experiments revealed that modulation of enzymatic activity by metal ions critically depends on the total enzyme concentration, raising the question as to which of the bivalent metal cations Mg(2+) and Mn(2+) is more relevant under physiological conditions.
authors.affiliation: Institute of Chemistry, University of Luebeck, Ratzeburger Allee 160, 23538 Luebeck, Germany.
authors.forename: Astrid
authors.initials: A
authors.lastname: Blume
authors.forename: Jesus
authors.initials: J
authors.lastname: Angulo
authors.forename: Thorsten
authors.initials: T
authors.lastname: Biet
authors.forename: Hannelore
authors.initials: H
authors.lastname: Peters
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Benie
authors.forename: Monica
authors.initials: M
authors.lastname: Palcic
authors.forename: Thomas
authors.initials: T
authors.lastname: Peters
mesh_terms: D000017:ABO Blood-Group System; D000939:Epitopes; D004926:Escherichia coli / Q000235:genetics; D005690:Galactose / Q000737:chemistry / Q000378:metabolism; D005700:Galactosyltransferases / Q000032:analysis / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D006801:Humans; D008956:Models, Chemical; D015394:Molecular Structure; D019906:Nuclear Magnetic Resonance, Biomolecular; D011994:Recombinant Proteins / Q000378:metabolism; D012016:Reference Values; D013379:Substrate Specificity; D014530:Uridine Diphosphate / Q000737:chemistry / Q000378:metabolism; D014531:Uridine Diphosphate Galactose / Q000737:chemistry / Q000378:metabolism; D014532:Uridine Diphosphate Glucose / Q000737:chemistry / Q000378:metabolism; D014542:Uridine Monophosphate / Q000737:chemistry / Q000378:metabolism
chemical_list: D000017:ABO Blood-Group System; D000939:Epitopes; D011994:Recombinant Proteins; D014531:Uridine Diphosphate Galactose; D014530:Uridine Diphosphate; D014542:Uridine Monophosphate; D005700:Galactosyltransferases; C047006:N-acetyllactosaminide alpha-1,3-galactosyltransferase; D014532:Uridine Diphosphate Glucose; D005690:Galactose