Location of three active site residues in the NH2-terminal sequence of the beta 2 subunit tryptophan synthase from Escherichia coli.

The three known active site residues of the tryptophan synthase beta 2 subunit from Escherichia coli are shown to fall within 25 residues of each other in the primary sequence of the NH2-terminal region of the beta 2 subunit. These residues are: lysine-86, which forms a Schiff's base with pyridoxal phosphate; histidine-81 or histidine-85, which removes the alpha proton of L-serine; and cysteine-61, which reacts with bromoacetylpyridoxamine phosphate, an affinity label for the beta 2 subunit. The sequence of the first 78 residues of a single cyanogen bromide fragment containing these active site residues has been determined by automatic Edman degradation and by sequence analysis of daughter peptides. This 79-residue cyanogen bromide fragment, which contains the 22-residue pyridoxyl peptide sequenced earlier by Fluri et al. (Fluri, R., Jackson, L. E., Lee, W. E., and Crawford, I. P. (1971) J. Biol. Chem. 246, 6620-6624), was placed at the NH2-terminal end of the beta chain beginning at residue 22. Thus, the primary sequence of residues 1 to 99 of the beta 2 subunit is reported.