[Interaction of tryptophanase with substrate analogs].

Tryptophanase from Escherichia coli was studied with respect to its interactions with L-alanine, beta-chloro-L-alanine, L-phenylalanine, L-methionine, L-threonine, beta-phenyl-DL-serine (threo form) and also with a new tryptophan analog oxindolyl-L-alanine. Slow transamination of L-alanine in the active site of the enzyme was observed. Some evidence is presented which indicates that the side transamination reaction occurs during incubation of tryptophanase with an adequate substrate, beta-chloro-L-alanine. Absorption and circular dichroism (CD) spectra of the enzyme-quasisubstrate complexes have been recorded. Addition of beta-phenylserine and threonine to the enzyme induces a decrease of absorbance at 337 nm and an increase of absorbance at 420 nm. The spectral changes are associated with inversion of the CD sign, i.e. with disappearance of positive CD in the 420 nm band and appearance of negative CD in this band. It is inferred that beta-phenylserine and threonine form an external coenzyme-substrate aldimine which undergoes slow conversion to give a keto acid and the free enzyme. Addition of oxindolylalanine to tryptophanase results in the formation of an intense narrow absorption band at 504 nm with a shoulder at about 475 nm. This band belongs to a quinonoid intermediate. A positive CD is seen in the 504 nm band; the dissymmetry factor (delta A/A) in this band is much smaller than that in the absorption bands of the free enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)