The degradation of some Bz-substituted tryptophans by Escherichia coli tryptophanase

Abstract

The degradation of tryptophan and the Bz-methyl-and-chloro-tryptophans by tryptophanase and by washed cell suspensions of E. coli studied; the equilibrium constants, Ka, for the formation of the enzyme-substrate complexes and the first-order velocity constants, k3, for their breakdown have been evaluated. A fairly satisfactory correlation of Ka with the electronic properties of the substituents, as measured by the Hammett substituent constants, [sigma], has been obtained; the formation of the enzyme-substrate complex is facilitated by electron-attracting substituents in the benzene ring of the tryptophan molecule. The velocity constants k3 do not show satisfactory correlation with [sigma] ; however, both electron-attracting and electron-repelling substituents diminish k3. The results are discussed in the light of current views on the mechanism of the degradation of tryptophan by tryptophanase. The affinity constants Ka are about 100 times as great for washed cells of E. coli as for cell-free tryptophanase; possible explanations for this unexpected result are discussed.