Local and Global Control Mechanisms in Allosteric Threonine Deaminase
David T. Gallagher, D Chinchilla, Herbert Lau, Edward Eisenstein
Allosteric and cooperative control signals were investigated in the tetrameric enzyme threonine deaminase. The tetramer consists of two dimers that associate at the x dyad. The structure pointed the way to use the Q175E mutation to create hybrid tetramers for the purpose. In the hybrid tetramers, various combinations of defective or activated regulatory and active sites were made; activity measurements then reported allosteric communication among the different subunits. Hybrid tetramers with functional domains on different dimers were insensitive to allosteric ligands, showing that the allosteric signal does not traverse the x dyad. On the other hand, the cooperative effect of binding ligands to the active site was found to traverse the x dyad and to strengthen the tetramer. These experiments enabled measurement of the cooperative free energy (the energy released in the allosteric activation) as -2.08 kcal. The study illustrates the method of dissection and reconstruction of the oligomer for analysis of allosteric signaling.