Cooperative Binding of α Subunits to the Apo‐β₂ Subunit of Tryptophan Synthase from Escherichia coli

The binding parameters for the interaction of α subunits with the dimeric apo‐β 2 subunit of tryptophan synthase from Escherichia coli were determined by gel chromatography, equilibrium dialysis and sedimentation velocity experiments. In 0.1 M pyrophosphate buffer pH 7.5 the binding of two α subunits to the stable apo‐β 2 dimer is cooperative. Fitting the data to the Adair equation yielded the apparent microscopic dissociation constants for the complexes with one and two bound α chains. Since they differ significantly, the apo‐β 2 dimer seems to exist in distinct conformational states depending on the saturation with α subunits. This finding is in agreement with a recently published scheme summarizing the interactions between the apo‐β 2 subunit and its ligands, α subunit and pyridoxal 5′‐phosphate. The binding between the subunits shows a moderate temperature‐dependence, suggesting that hydrophobic effects may be involved in the subunit association to form the apo‐multienzyme complex.