The alpha (2)beta (2) tryptophan synthase complex is a model enzyme for understanding allosteric regulation. We report the functional and regulatory properties of the beta S178P mutant. Ser-178 is located at the end of helix 6 of the beta subunit, belonging to the domain involved in intersubunit signaling. The carbonyl group of beta Ser-178 is hydrogen bonded to Gly-181 of loop 6 of the alpha subunit only when alpha subunit ligands are bound. An analysis by molecular modeling of the structural effects caused by the beta S178P mutation suggests that the hydrogen bond involving alpha Gly-181 is disrupted as a result of localized structural perturbations. The ratio of alpha to beta subunit concentrations was calculated to be 0.7, as for the wild type, indicating the maintenance of a tight alpha-beta complex. Both the activity of the alpha subunit and the inhibitory effect of the alpha subunit ligands indole-3-acetylglycine and D,L-alpha -glycerol-3-phosphate were found to be the same for the mutant and wild type enzyme, whereas the beta subunit activity of the mutant exhibited a 2-fold decrease. In striking contrast to that observed for the wild type, the allosteric effecters indole-3-acetylglycine and D,L-alpha -glycerol-3-phosphate do not affect the beta activity. Accordingly, the distribution of L-serine intermediates at the beta -site, dominated by the alpha -aminoacrylate, is only slightly influenced by alpha subunit ligands. Binding of sodium ions is weaker in the mutant than in the wild type and leads to a limited increase of the amount of the external aldimine intermediate, even at high pH, whereas binding of cesium ions exhibits the same affinity and effects as in the wild type, leading to an increase of the alpha -aminoacrylate tautomer absorbing at 450 nm, Crystals of the beta S178P mutant were grown, and their functional and regulatory properties were investigated by polarized absorption microspectrophotometry. These findings indicate that (i) the reciprocal activation of the alpha and beta activity in the alpha2 beta2 complex with respect to the isolated subunits results from interactions that involve residues different from beta Ser-178 and (ii) beta Ser178 is a critical residue in ligand-triggered signals between alpha and beta active sites.
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