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-α-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 °C and pH 10.0. The apparent K(m) for nicotinate mononucleotide was 680 μM; the apparent K(m) for 5,6-dimethylbenzimidazole was less than 10 μM. 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.