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J. Biol. Chem., Vol. 266, Issue 13, 8328-8335, 05, 1991
MG Caligiuri and R Bauerle
The anthranilate synthase-phosphoribosyl transferase complex, a
heterotetrameric enzyme made up of the TrpE and TrpD polypeptides,
catalyzes three reactions comprising the first two steps of tryptophan
biosynthesis in Salmonella typhimurium. All three activities of the complex
are subject to feedback inhibition by tryptophan, which results from
allosteric effects associated with the binding of one molecule of inhibitor
to each of the TrpE subunits of the complex. Random in vitro chemical
mutagenesis of the trpE gene was used to generate a collection of mutant
forms of the complex which displayed varying degrees of resistance to
feedback inhibition. Single amino acid substitutions, identified by DNA
sequencing, were found at 14 different residues within the TrpE
polypeptide. The residues were distributed throughout TrpE, but those that
appeared to be most critical for regulation were found in two clusters, one
at the extreme amino-terminal end, including residues Glu-39, Ser-40, and
Ala-41, and the other in the middle of the polypeptide, including residues
Asn-288, Pro-289, Met-293, Phe-294, and Gly-305. Kinetic and binding
studies of the purified mutant complexes demonstrated that 9 of the 14 had
a marked decrease in affinity for tryptophan with little or no change in
substrate affinity or catalytic capacity. The remaining five enzymes
exhibited more subtle changes, having small decreases in inhibitor affinity
coupled with small increases in substrate affinity. Mutant enzymes that
were not totally feed-back-resistant had a decreased kinetic response to
tryptophan binding. All enzymes exhibited alterations in tryptophan-induced
conformational changes as monitored by dye-ligand chromatography.
Identification of amino acid residues involved in feedback regulation of the anthranilate synthase complex from Salmonella typhimurium. Evidence for an amino-terminal regulatory site
Department of Biology, University of Virginia, Charlottesville 22901.
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