title: Purification and characterization of CobT, the nicotinate-mononucleotide:5,6-dimethylbenzimidazole phosphoribosyltransferase enzyme from Salmonella typhimurium LT2.
abstract: We report the purification and biochemical characterization of the cobalamin biosynthetic enzyme nicotinate-mononucleotide:5, 6-dimethylbenzimidazole phosphoribosyltransferase (CobT) from Salmonella typhimurium. cobT was overexpressed and the protein purified to approximately 97% homogeneity. NH2-terminal sequence analysis confirmed that the protein encoded by cobT was purified. Homogeneous CobT catalyzed the synthesis of N1-(5-phospho-alpha-D-ribosyl)-5,6-dimethylbenzimidazole. The identity of high performance liquid chromatography-purified product was confirmed by fast atom bombardment mass spectrometry. CobT activity was optimal at 45 degrees C and pH 10.0. The apparent Km for nicotinate mononucleotide was 680 microM; the apparent Km for 5, 6-dimethylbenzimidazole was less than 10 microM. CobT used nicotinamide mononucleotide as a ribose phosphate donor. CobT phosphoribosylated alternative base substrates including benzimidazole, 4,5-dimethyl-1,2-phenylenediamine, imidazole, histidine, adenine, and guanine in vitro. The resulting ribotides were incorporated into cobamides that were differentially utilized by methionine synthase (EC 2.1.1.13), ethanolamine ammonia-lyase (EC 4.3.1.7), and 1,2-propanediol dehydratase (EC 4.2.1.28) in vivo. The lack of base substrate specificity by CobT may explain the inability to isolate mutants blocked in the synthesis of 5, 6-dimethylbenzimidazole in this bacterium.
authors.affiliation: Department of Bacteriology, University of Wisconsin-Madison, Madison, Wisconsin 53706-1567, USA.
authors.forename: J R
authors.initials: JR
authors.lastname: Trzebiatowski
authors.forename: J C
authors.initials: JC
authors.lastname: Escalante-Semerena
mesh_terms: D007526:Isoelectric Point; D007700:Kinetics; D008956:Models, Chemical; D009097:Multienzyme Complexes; D009713:Nucleotidyltransferases; D010430:Pentosyltransferases / Q000302:isolation & purification* / Q000378:metabolism; D012267:Ribosemonophosphates / Q000378:metabolism; D012486:Salmonella typhimurium / Q000201:enzymology*; D013379:Substrate Specificity
chemical_list: D009097:Multienzyme Complexes; D012267:Ribosemonophosphates; D010430:Pentosyltransferases; C028015:nicotinate-nucleotide-dimethylbenzimidazole phosphoribosyltransferase; D009713:Nucleotidyltransferases