Immunochemical evidence for conformational flexibility and its modulation by specific ligands in the beta 2 subunit...

The immunochemical reactivity of unfractionated antibodies elicited by denatured beta 2 subunits of Escherichia coli tryptophan synthase [L-serine hydro-lyase (adding indole) EC 4.2.1.20] with the homologous antigen and with the native enzyme is examined. These antibodies recognize the native apoenzyme nearly as well as the denatured protein. On the contrary, after binding of its cofactor, pyridoxal 5'-phosphate, the protein exhibits a much lower immunoreactivity toward these antibodies. This decrease of affinity becomes even more pronounced when the beta 2 protein interacts with the alpha subunit. Similarly, reduction of the Schiff base formed between the cofactor and the protein leads to a strong decrease of immunoreactivity. To account for these results, it is proposed that apo-beta 2 must be a dynamic flexible structure that easily exposes to the solvent regions of its polypeptide chain that normally are buried in its interior. The increase in rigidity of this structure upon binding of the cofactor, reduction of Schiff base, and formation of the alpha 2 beta 2 complex would then account for the decreased immunoreactivity of these various states of the native beta 2 protein.