A comparative study of the thermal inactivation of the isolated and associated domains within the beta 2 subunit of Escherichia coli tryptophan synthetase. Evidence for strong interdomain interactions.

The irreversible inactivation by heat of the F1 and F2 regions of the beta 2 subunit of Escherichia coli tryptophan synthetase (L-serine hydrolyase (adding indole) EC 4.2.1.20) has been studied. By comparing the kinetics of inactivation of the F1 and F2 fragments, either isolated by proteolysis or associated within the beta 2 protein, it is shown that the F1 fragment is protected considerably against heat inactivation by its interactions with the complementary F2 fragment. An important stabilization of the protein conformation by the coenzyme, pyridoxal 5'-phosphate, is also observed. By using a hybrid enzyme preparation, it is shown that the stabilization by a single pyridoxal 5'-phosphate molecule of the two protomers within beta 2 is likely to be caused by a strengthening of interdomain interactions upon binding of the coenzyme. These results point to a strong energetic coupling between the two domains of the beta chain.