Allosteric effects acting over a distance of 20-25 A in the Escherichia coli tryptophan synthase bienzyme complex increase ligand affinity and cause redistribution of covalent intermediates.
The reactions of L-histidine (L-His) and L-tryptophan (L-Trp) with the alpha 2 beta 2 complex of Escherichia coli tryptophan synthase are introduced as probes both of beta-subunit catalysis and of ligand-mediated alpha-beta allosteric interactions. Binding of DL-alpha-glycerol 3-phosphate (GP), an analogue of 3-indole-D-glycerol 3'-phosphate (IGP), to the alpha-catalytic site increases the affinity of alpha 2 beta 2 for L-His 4.5-fold and the affinity for L-Trp 17-fold and brings about a redistribution of beta-bound intermediates that favors the quinonoids derived from each amino acid. Inorganic phosphate (Pi) (presumably via binding to the alpha-catalytic site) influences the distribution of L-His intermediates as does GP. Previous binding studies [Heyn, M. P., & Weischet, W. O. (1975) Biochemistry 14, 2962-2968] indicate that when the phosphoryl group subsite of the alpha-catalytic site is occupied by GP or Pi, a high-affinity indole subsite is induced at the alpha-catalytic site. Interaction of benzimidazole (BZ), an analogue of indole, with this site also shifts the distribution of beta-bound L-His intermediates in favor of the L-His quinonoid. In the absence of Pi or GP, BZ interacts primarily at the beta-catalytic site and competes with L-His for the beta-subunit indole subsite. Since L-His and GP (or Pi) are substrate analogues and L-Trp is the physiological product, these allosteric effects likely take place with the natural substrates.(ABSTRACT TRUNCATED AT 250 WORDS)
Houben
K F
KF
Department of Biochemistry, University of California, Riverside 92521.
Dunn
M F
MF
D000494:Allosteric Regulation; D001562:Benzimidazoles / Q000494:pharmacology; D001665:Binding Sites; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D004926:Escherichia coli / Q000201:enzymology*; D005994:Glycerophosphates / Q000378:metabolism; D006639:Histidine / Q000378:metabolism; D007700:Kinetics; D008024:Ligands; D010710:Phosphates / Q000378:metabolism; D013057:Spectrum Analysis; D014364:Tryptophan / Q000378:metabolism; D014367:Tryptophan Synthase / Q000378:metabolism*
D001562:Benzimidazoles; D005994:Glycerophosphates; D008024:Ligands; D010710:Phosphates; D006639:Histidine; D014364:Tryptophan; C029620:alpha-glycerophosphoric acid; D014367:Tryptophan Synthase