Association of the α and β2 Subunits of the Tryptophan Synthetase of Escherichia coli

Abstract

Abstract The association of the α and β2 subunits of tryptophan synthetase has been studied by means of sucrose gradient centrifugation, Sephadex gel filtration, and enzymatic activity measurements. The results indicate that the fully associated enzyme (α2β2) has a sedimentation coefficient of 6.4S and is in relatively rapid equilibrium with the free subunits. Mass law and kinetic considerations are consistent with the reversible binding of individual α subunits to two identical and independent sites on the β2 subunit, with each combined site contributing the same enzymatic activity. Pyridoxal phosphate and serine together markedly increase the association of the two subunits. Prior incubation of the two subunits with serine and pyridoxal-P apparently increases their association when they are sedimented in sucrose gradients in the absence of serine and pyridoxal-P. Apparent association constants for the subunits have been measured enzymatically under various conditions and found to range from 4 x 106 to 2.6 x 109 m-1. Under the two extreme conditions of affinity of the α and β2 subunits, apparent rate constants for association have been estimated to be 2 x 104 and 6 x 105 sec-1 m-1. Likewise, the dissociation rate constants determined were 4.8 x 10-3 and 1.8 x 10-4 sec-1. Mutations abolishing the enzymatic activity of the β2 subunit can selectively alter the stimulatory effect of pyridoxal-P and serine on the ability of mutant β2 subunit to combine with normal α subunits, without affecting its basal affinity for the α subunits.