On the mechanism of action of Escherichia coli tryptophan synthase. Steady-state investigations.
Tryptophan synthase from Escherichia coli (L-serine hydro-lyase (adding indole), EC 4.2.1.20) synthesizes L-trypotophan from indoleglycerol phosphate and L-serine, releasing glyceraldehyde 3-phosphate, or from indole and L-serine. The latter reaction (B reaction), catalyzed either by the beta2 species or by the (alpha2 beta2) complex, has been studied by steady-state methods. A sequential mechanism is indicated. Inhibition experiments with the substrate analogue benzimidazole were carried out in order to distinguish between random and ordered mechanisms. The results are compatible with a random sequential mechanism. The dissociation constants of the enzyme-substrate complexes are evaluated. When catalyzed by the tetrameric complex (alpha2 beta2) the B reaction is inhibited by higher concentrations of the substrate indole. This inhibition does not follow the usual substrate inhibition pattern. The question whether the binding of indole to the alpha-subunit exerts an inhibitory effect on the beta2 species, possibly by reversing the activation by the alpha subunit of the beta2 species, is discussed.
D001562:Benzimidazoles / Q000494:pharmacology; D001665:Binding Sites; D004926:Escherichia coli / Q000201:enzymology*; D007211:Indoles / Q000378:metabolism; D007700:Kinetics; D046911:Macromolecular Substances; D012694:Serine / Q000378:metabolism; D014367:Tryptophan Synthase / Q000378:metabolism*
D001562:Benzimidazoles; D007211:Indoles; D046911:Macromolecular Substances; D012694:Serine; D014367:Tryptophan Synthase