title: Photoaffinity labeling of the indole sites on the Escherichia coli tryptophan synthase alpha-subunit.
abstract: The alpha subunit of the Escherichia coli tryptophan synthase catalyzes the reversible aldolytic reaction: Indole-3-glycerol phosphate in equilibrium indole + glyceraldehyde 3-phosphate. The use of 5-azidoindole as a photoaffinity label has made the generation of a number of enzyme-substrate complexes possible, each with a given degree of saturation of the two postulated indole sites. When assayed in the reverse reaction (indole-3-glycerol phosphate synthesis), samples of alpha subunit treated at concentrations of 5-azidoindole less than or equal to 2 mM show a progressive 30-40% activation. A gradual inactivation occurs only in samples irradiated at concentrations in excess of 2 mM 5-azidoindole, and this inactivation is complete at 8-10 mM. A quantitatively similar activation occurs in the forward reaction (indole synthesis), however inactivation in this case is incomplete, with complexes treated at 8-12 mM 5-azidoindole retaining 30-40% relative activity in this reaction. When treated alpha subunits were assayed for their abilities to complement the beta 2-subunit in the reactions indole + L-serine leads to L-tryptophan + H2O and indole-3-glycerol phosphate + L-serine leads to L-tryptophan + glyceraldehyde 3-phosphate, quantitatively lesser amounts of activation followed by total inactivation are observed over a similar range of 5-azidoindole concentrations.
mesh_terms: D000345:Affinity Labels; D001386:Azides; D001665:Binding Sites; D055598:Chemical Phenomena; D002621:Chemistry; D004926:Escherichia coli / Q000201:enzymology*; D007211:Indoles / Q000378:metabolism*; D007700:Kinetics; D010777:Photochemistry; D014367:Tryptophan Synthase / Q000302:isolation & purification*
chemical_list: D000345:Affinity Labels; D001386:Azides; D007211:Indoles; C037179:5-azidoindole; D014367:Tryptophan Synthase