Effects of Monovalent Cations on Functional Properties of the Tryptophan Synthase α₂β₂ Complex in Solution and in the Crystal

Summary

Synthesis of L-tryptophan, catalyzed by tryptophan synthase from Salmonella typhimurium, at 10°C is activated by monovalent cations in the order Cs⁺ > Rb⁺ > Li⁺ > K⁺ > Na⁺. The most efficient cations increase V_max and decrease K_M for indole, Na⁺ only affects K_M. In the absence of indole, the equilibrium distribution of the external aldimine and the α-aminoacrylate Schiff base depends on monovalent cations both in solution and in the crystalline state. The least activating ions, Na⁺ and K⁺, stabilize the external aldimine, whereas Cs⁺, Rb⁺ and Li⁺ favor the accumulation of a species absorbing at 470 nm, tentatively identified as a tautomer of α-aminoacrylate. Activation of the enzyme might be associated with the stabilization of this more reactive intermediate.