Evidence that glutamic acid 49 of tryptophan synthase alpha subunit is a catalytic residue. Inactive mutant proteins substituted at position 49 bind ligands and transmit ligand-dependent to the beta subunit

Glutamic acid 49 of the alpha subunit of tryptophan synthase from Escherichia coli is an essential residue since 19 mutant proteins substituted at position 49 were found previously to be inactive. Our present findings that five mutants of the alpha subunit, substituted with Asp, Lys, Ala, Phe, or Gly at position 49, bind a substrate analog normally are further evidence that glutamic acid 49 is a catalytic base. Ligands of the alpha subunit also have similar effects on site-site interactions between the beta subunit and the wild type or mutant alpha subunits. These effects include inhibition of the activity of the beta subunit, reduction of the dissociation constant for D-tryptophan, and increase of the equilibrium concentration of a quinonoid intermediate formed with L-tryptophan.