Role of tryptophan 248 in the active site of tryptophanase from Escherichia coli.

Tryptophan 248, located in the active site of tryptophanase from Escherichia coli, has been replaced with phenylalanine by site-directed mutagenesis. Judging from CD and fluorescence spectra, the global structure of the mutant enzyme was found to be the same as that of the wild-type enzyme. The binding affinity of the mutant enzyme for the coenzyme pyridoxal 5'-phosphate (PLP) was reduced tenfold compared to the wild-type enzyme. Kinetic analyses under PLP-saturated conditions indicated that the Km values of the mutant enzyme for substrates are the same as those of wild-type enzyme but the kcat values are decreased to about 85%, which accounts for the overall activity decrease. These findings suggest that tryptophan 248 interacts closely with PLP and plays an important role in the catalytic reaction.