title: Catalytic function of a tyrosyl residue in tryptophanase.
abstract: Tryptophanase has an essential tyrosyl residue/active site which can be modified by tetranitromethane. Pyridoxal 5'-phosphate can prevent this modification efficiently, whereas pyridoxal 5'-phosphate N-oxide cannot, indicating that the free pyridinium N is required for the interaction of the coenzyme with the tyrosyl residue, probably via a hydrogen bond. The weakened binding of the coenzyme to the modified enzyme was confirmed on gel filtration, the modified enzyme being dissociated from the coenzyme seven-fold faster than the native enzyme. Furthermore, absorption spectral analyses demonstrated that the modified enzyme can catalyze the transaldimination step, but fails to abstract the alpha-H of substrates. The tyrosyl residue, therefore, not only participates in coenzyme binding, but also contributes to alpha-H labilization.
mesh_terms: D001665:Binding Sites; D002384:Catalysis; D004926:Escherichia coli / Q000201:enzymology; D007700:Kinetics; D008190:Lyases / Q000378:metabolism*; D011732:Pyridoxal Phosphate / Q000031:analogs & derivatives / Q000378:metabolism* / Q000494:pharmacology; D013057:Spectrum Analysis; D013329:Structure-Activity Relationship; D013774:Tetranitromethane / Q000494:pharmacology; D014368:Tryptophanase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D014443:Tyrosine / Q000378:metabolism
chemical_list: D014443:Tyrosine; D011732:Pyridoxal Phosphate; D008190:Lyases; D014368:Tryptophanase; D013774:Tetranitromethane