Structural and functional influence of enzyme-antibody interactions: effects of eight different monoclonal antibodies on the enzymatic activity of Escherichia coli tryptophan synthase

Twelve monoclonal antibodies directed against the beta 2 subunit of Escherichia coli tryptophan synthase (EC 4.2.1.20) were produced from hybridoma clones. These monoclonal antibodies are found to recognize at least eight different epitopes on beta 2, and eight classes of monoclonal antibodies are thus defined. The effects of these monoclonal antibodies on the enzymatic activities of beta 2 are studied. The monoclonal antibodies from three classes rapidly inhibit the serine deaminase activity catalyzed by the beta 2 subunit alone; two of them lead to an inhibition plateau under stoichiometric conditions, and their inhibitory effects are cumulative. With the antibodies from two of these three classes, the tryptophan synthase activity of the alpha 2 beta 2 complex is recovered, through a competition between the alpha subunit and the monoclonal antibody. On the contrary, the antibody from the third class is inhibitory even in the presence of an excess of alpha subunit. The antibodies from the five other classes, though binding easily to the coated antigen in the enzyme-linked immunosorbent assay, react only very slowly with beta 2 in solution and, only after a long time of incubation, inhibit the enzymatic activity at different levels.