MONOVALENT CATIONS PARTIALLY REPAIR A CONFORMATIONAL DEFECT IN A MUTANT TRYPTOPHAN SYNTHASE ALPHA(2)BETA(2) COMPLEX (BETA-E109A)

We are using the tryptophan synthase alpha(2) beta(2) complex as a mod el system to investigate how ligands, protein-protein interaction, and mutations regulate enzyme activity, reaction specificity, and substra te specificity. The rate of conversion of L-serine and indole to L-try ptophan by the beta(2) subunit alone is quite low, but is activated by certain monovalent cations or by association with alpha subunit to fo rm an alpha(2) beta(2) complex. Since monovalent cations and a subunit appear to stabilize an active conformation of the beta(2) subunit, we have investigated the effects of monovalent cations on the activities and spectroscopic properties of a mutant form of alpha(2) beta(2) com plex having beta(2) subunit glutamic acid 109 replaced by alanine (E10 9A). The E109A alpha(2) beta(2) complex is inactive in reactions with L-serine but active in reactions with beta-chloro-L-alanine. Parallel experiments show effects of monovalent cations on the properties of wi ld type beta(2) subunit and alpha(2) beta(2) complex. We find that CsC l stimulates the activity of the E109A alpha(2) beta(2) complex and of wild type beta(2) subunit with L-serine and indole and alters the equ ilibrium distribution of L-serine reaction intermediates. The results indicate that CsCl partially repairs the deleterious effects of the E1 09A mutation on the activity of the alpha(2) beta(2) complex by stabil izing a conformation with catalytic properties more similar to those o f the wild type alpha(2) beta(2) complex. This conclusion is consisten t with observations that monovalent cations alter the catalytic and sp ectroscopic properties of several pyridoxal phosphate dependent enzyme s by stabilizing alternative conformations.