Reciprocal Communication between the Lyase and Synthase Active Sites of the Tryptophan Synthase Bienzyme Complex

It is important to understand how the cleavage of indoleglycerol phosphate, which is catalyzed by the α subunits in the α₂ß₂ bienzyme complex of tryptophan synthase, is modulated by the presence of L-serine in the β subunits. Steady-state kinetic data, including the dependence of k_cat on pH, allowed values to be assigned to each of the eight rate constants of the minimal catalytic mechanism. An ionizing group having an apparent pK value near 7.5 must be protonated for activity. The α active site ligands indolepropanol phosphate, glyceraldehyde 3-phosphate, and glycerol 3-phosphate increase both the affinity and the molar absorbance of L-serine and L-tryptophan bound to the β active site. These effects prove that the α sites communicate with the β sites over a distance of 30 Å. 6-Nitroindole readily condenses with glyceraldehyde 3-phosphate, but not with L-serine. The turnover numbers for 6-nitroindoleglycerol phosphate and 6-nitroindole increased about 10-fold in both directions in the presence of L-serine bound to the β₂ subunits. These data prove that the α and β active sites communicate reciprocally and explain why the turnover number for the physiological reaction of indoleglycerol phosphate with L-serine greatly exceeds that of the cleavage reaction of indoleglycerol phosphate.