Immunochemical and enzymatic comparisons of the tryptophan synthase alpha subunits from five species of Enterobacteriaceae.

The reactive surface structures of alpha subunits of tryptophan synthase from Escherichia coli, Shigella dysenteriae, Salmonella typhimurium, Aerobacter aerogenes, and Serratia marcescens were compared by measuring (i) their reactivities in micro-complement-fixation assays with antibodies directed specifically to E. coli wild-type alpha subunit, (ii) their reactivities in enzyme neutralization assays with the same antibodies, and (iii) their binding affinities for tryptophan synthase beta(2) subunits. The enzymes from the four heterologous species cross-reacted in the microcomplement-fixation assays with the anti-E. coli alpha subunit antibodies, each to a different degree. However, neutralization titers of the antibodies reacting with the various alpha subunits were comparatively similar, and the beta(2) subunit-binding and -stimulating abilities of the alpha subunits were even more closely alike. The results suggested that the tertiary structure of the beta(2) subunit-binding site of the alpha subunit has been conserved, relative to the rest of the molecule, during the evolutionary divergence of the species of Enterobacteriaceae.