Affinity Chromatography of Tryptophan Synthase fromEscherichia coli

Abstract

Inhibition studies and affinity chromatography indicate that derivatives of tryptophanol phosphate are suitable ligands for the affinity chromatography of tryptophan synthase. A phenyl group on the spacer arm strengthens the interaction of immobilized tryptophanol phosphate with the enzyme. The α₂/β₂ complex specifically requires the presence of 0.3–0.5 M phosphate ions for binding. The α subunit binds in dilute Tris buffer, but its binding is also enhanced by the presence of phosphate ions. The β₂ subunit binds unspecifically but strongly to the affinity material and to a variety of other immobilized hydrophobic ligands. Binding studies with suspensions of affinity material show that the α subunit interacts rapidly and reversibly. Indoleglycerol phosphate and indolepropanol phosphate release bound α₂/β₂ complex and a subunit in a competitive manner, indicating that the interaction occurs biospecifically, i.e. via the active site of the α subunit. l‐Serine is a non‐competitive inhibitor of binding. These results are discussed with regard to the composite‐active‐site hypothesis [T. E. Creighton (1970) Eur. J. Biochem. 13, 1–10]. Both the α subunit and the α₂/β₂ complex of tryptophan synthase from Escherichia coli can be obtained with high yields and in homogenous form by absorption to the affinity material from partially purified preparations. Elution is achieved with linear gradients either of indolepropanol phosphate or of indoleglycerol phosphate or, in the case of the complex, of l‐serine. At the low concentrations of the complex found in crude extracts of wild‐type E. coli cells, the unexpectedly high affinity of the β₂ subunit for hydrophobic ligands leads to partial dissociation of the complex.