Anthranilate synthetase-anthranilate 5-phosphoribosylpyrophosphate phosphoribosyl-transferase from Salmonella typhimurium. Inactivation of glutamine-dependent anthranilate synthetase by agarose-bound anthranilate

Anthranilate synthase can generate sufficient phosphoribosyl amine for thiamine synthesis in Salmonella enterica

In bacteria, the biosynthetic pathway for the hydroxymethyl pyrimidine moiety of thiamine shares metabolic intermediates with purine biosynthesis. The two pathways branch after the compound aminoimidazole ribotide. Past work has shown that the first common metabolite, phosphoribosyl amine (PRA), can be generated in the absence of the first enzyme in purine biosynthesis, PurF. PurF-independent PRA synthesis is dependent on both strain background and growth conditions. Standard genetic approaches have not identified a gene product singly responsible for PurF-independent PRA formation. This result has led to the hypothesis that multiple enzymes contribute to PRA synthesis, possibly as the result of side products from their dedicated reaction. A mutation that was able to restore PRA synthesis in a purF gnd mutant strain was identified and found to map in the gene coding for the TrpD subunit of the anthranilate synthase (AS)-phosphoribosyl transferase (PRT) complex. Genetic analyses indicated that wild-type AS-PRT was able to generate PRA in vivo and that the P362L mutant of TrpD facilitated this synthesis. In vitro activity assays showed that the mutant AS was able to generate PRA from ammonia and phosphoribosyl pyrophosphate. This work identifies a new reaction catalyzed by AS-PRT and considers it in the context of cellular thiamine synthesis and metabolic flexibility.

Metal ion requirement and tryptophan inhibition of normal and variant anthranilate synthase-anthranilate 5-phosphoribosylpyrophosphate phosphoribosyltransferase complexes from Salmonella tyrhimrium

1. Both Mn2+ and Co2+ can replace Mg2+ as the required divalent cation for all activities of the enzyme complex between anthranilate synthase (chorismate pyruvate-lyase (amino-accepting), EC 4.1.3.27) and anthranilate-5-phosphoribosylpyrophosphate phosphoribosyltransferase (N(5'-phosphoribosyl)-anthranilate:pyrophosphate phosphoribosytransferase, EC 2.4.2.18) from Salmonella typhimurium. They have much lower apparent Km values than Mg2+, both for glutamine-dependent anthranilate synthase (Mn2+ = 1.1 muM, Co2+ - 2.6 muM, Mg2+ = 83 muM) and for phosphoribosyltransferase (Mn2+ = 16 muM, Co2+ = 14.6 muM, Mg2+ = 133 muM). The ratio of total Mg2+ to total Mn2+ found in a cell extract of S. typhimurium trpE2 , the source of normal enzyme complex, was found to be 350, suggesting that Mg2+ is probably utilized by the enzyme complex in vivo under our growth conditions. 2. An enzyme complex has been isolated from a mutant strain of S. typhimurium (SO-515) that has a variation in the anthranilate synthase subunit which is thought to be a single amino acid substitution. This variation causes glutamine-dependent anthranilate synthase to be hypersensitive to feedback inhibition by tryptophan (Ki = 0.4 muM compared to Ki = 20 muM for normal enzyme complex). The phosphoribosyltransferase in the variant enzyme complex is also hypersensitive to tryptophan but the kinetics are complex and involve activation by tryptophan in the presence of low amounts of 5-phosphoribosyl 1-pyrophosphate. 3. In the variant enzyme complex the apparent Km for Mg2+ is elevated to 360 muM for glutamine-linked anthranilate synthase but reduced to 75 muM for phosphoribosyltransferase. 4. These results suggest that the variant enzyme complex has altered tertiary and quaternary structures and that regulation of both activities is effected by tryptophan binding to only anthranilate synthase.

Anthranilate synthase-anthranilate 5-phosphoribosylpyrophosphate phosphoribosyltransferases from Salmonella typhimurium. Purification of the enzyme complex and analysis of multiple forms

1. The anthranilate synthase-anthranilate 5-phosphoribosylpyrophosphate phosphoribosyltransferase enzyme complex (chorismate pyruvatelyase (amino-accepting), EC 4.1.3.27) - (N-(5'-phosphoribosyl)-anthranilate: pyrophosphate phosphoribosyltransferase, EC 2.4.2.18), from Salmonella typhimurium has been purified with high yields to homogeneity. Sodium dodecyl sulfate gel electrophoresis of the purified enzyme complex revealed one major band containing 96% of the protein. The final yield of enzyme complex activity ranged from 30 to 60%. The absorbance spectrum of enzyme complex showed a peak at 280 nm and fine structure with peaks at 253, 259, 266 and 269 nm. These latter wavelengths correspond closely with the known absorbance maxima of phenylalanine. 2. When purified enzyme complex was subjected to standard gel electrophoresis, a four band pattern of protein peaks was consistently observed. The major enzyme complex band was apparently the native tetramer, having a molecular weight 280 000 and containing ammonia- and glutamine-dependent anthranilate synthase activity. The other three bands were molecular weight isomers of the major enzyme complex band. Two forms of molecular weight isomers were present: dimers and an aggregate of the native enzyme complex. The molecular weight isomers of the enzyme complex may represent forms generated by aggregation and denaturation of the native enzyme complex. 3. A new and highly sensitive spectrophotometric assay for phosphoribosyl-transferase is described. The method is based upon the difference in extinction coefficients between anthranilate and N-(5'-phosphoribosyl)anthranilate.