The Formation and Properties of Dimers of the Tryptophan Synthetase αSubunit of Escherichia coli

The normally monomeric α subunit of Escherichia coli tryptophan synthetase forms dimers and higher order aggregates following exposure to high urea concentrations and removal of the urea by dialysis. The maximum yield of α chain dimers is approximately 2%. Dimers of certain combinations of different enzymatically inactive mutant α monomers exhibit the missing enzymatic activity. In such heterologous dimers, the reconstituted active site has at least 50% of the activity of the active site of the native wild type α monomer. α chain dimers form an enzymatically active complex with the tryptophan synthetase β2 subunit. However, an α chain dimer can bind only a single β2 molecule. The α chain dimer dissociates into α monomers when heated at 51°. A tentative model of the structure of an α chain dimer is presented.