Substrate interactions with the alpha-subunit of the Escherichia coli tryptophan synthase. A study of the activity of mutant alpha-subunits.

Extracts of 19 trpA mutant strains of Escherichia coli were examined for their relative activity in the reversible aldolytic reaction catalyzed by the trpA gene product, the α-subunit of tryptophan synthase, in combination with the β-subunit of this enzyme. The specific activities in this reaction, indoleglycerol-P (InGP) ? indole + glyceraldehyde-3-P, were determined for both the forward reaction (InGP to indole) and the reverse reaction (indole to InGP). The majority of the mutant α-subunits had <10% of the wild-type activity in the forward reaction, as expected since these mutant strains were selected for defects in this reaction. In contrast, the majority of these mutant enzymes had >50% of the wild-type activity in the reverse reaction. Several had 5 to 15% of wild-type specific activity in the forward reaction but 60 to 100% of wild-type specific activity in the reverse reaction. Spontaneous revertant strains, selected for their increased ability to catalyze the forward reaction effectively, contained α-subunits with the expected higher specific activities in the forward reaction but without parallel changes in the reverse reaction activity.