Evidence that the essential, photosensitive histidyl residue in the beta2 subunit of tryptophan synthetase is in the pyridoxyl peptide

The β₂ subunit of tryptophan synthetase of Escherichia coli is photoinactivated in the presence of pyridoxal 5′-phosphate and L-serine as a result of the destruction of one histidyl residue per chain (1). Two tryptic peptides are found in much lower amounts in the photoinactivated enzyme than in the control enzyme. These peptides have been identified from their amino acid composition as the 9 or 10 residue peptides which terminate with the lysyl residue which forms a Schiff base with pyridoxal 5′-phosphate. These peptides contain two histidyl residues, one of which appears to be photosensitive. Thus pyridoxal 5′-phosphate sensitizes the photooxidation of a nearby, essential histidyl residue.