Effect of pH and monovalent cations on the formation of quinonoid intermediates of the tryptophan synthase alpha(2)beta(2) complex in solution and inthe crystal

Quinonoid intermediates play a key role in the catalytic mechanism of pyrid oxal 5'-phosphate-dependent enzymes. Whereas the structures of other pyrido xal 5'-phosphate-bound intermediates have been determined, the structure of a quinonoid species has not yet been reported. Here, we investigate factor s controlling the accumulation and stability of quinonoids formed at the be ta-active site of tryptophan synthase both in solution and the crystal. The quinonoids were obtained by reacting the alpha-aminoacrylate Schiff base w ith different nucleophiles, focusing mainly on the substrate analogs indoli ne and beta-mercaptoethanol, In solution, both monovalent cations (Cs+ or N a+) and alkaline pH increase the apparent affinity of indoline and favor ac cumulation of the indoline quinonoid, A similar pH dependence is observed w hen beta-mercaptoethanol is used. As indoline and beta-mercaptoethanol exhi bit very distinct ionization properties, this finding suggests that nucleop hile binding and quinonoid stability are controlled by some ionizable prote in residue(s). In the crystal, alkaline pH favors formation of the indoline quinonoid as in solution, but the effect of cations is markedly different. In the absence of monovalent metal ions the quinonoid species accumulates substantially, whereas in the presence of sodium ions the accumulation is m odest, unless alpha-subunit Ligands are also present, alpha-Subunit Ligands not only favor the formation of the intermediate, but also reduce signific antly its decay rate. These findings define experimental conditions suitabl e for the stabilization of the quinonoid species in the crystal, a critical prerequisite for the determination of the three-dimensional structure of t his intermediate.