title: Thermodynamics of the reactions catalyzed by the multifunctional enzyme complex tryptophan synthase.
abstract: The intrinsic enthalpy changes (corrected for hydration of D-glyceraldehyde 3-phosphate) for the reactions catalyzed by the alpha and beta 2 subunits of tryptophan synthase from Escherichia coli have been determined calorimetrically. Cleavage of indoleglycerol phosphate (alpha reaction) was found to be associated with a delta H value of 54.0 +/- 2.5 kJ mol-1, while condensation of indole with L-serine (beta reaction) involved -80.3 +/- 4.6 kJ mol-1'. By direct determination of the enthalpy concomitant with the overall synthesis of tryptophan from indoleglycerol phosphate and L-serine an enthalpy value of -13.4 +/- 5.6 kJ mol-1 was observed. In view of the uncertainties of the literature data used for calculation of the hydration contribution, the agreement between the directly measured delta H value of the overall reaction and the sum of the enthalpies of the alpha and beta reactions is fair. Deamination of L-serine, a side reaction catalyzed preferentially by the isolated beta 2 pyridoxal 5'-phosphate2 subunit, was shown to be associated with an enthalpy change of -7.3 +/- 0.4 kJ mol-1.
mesh_terms: D002021:Buffers; D004926:Escherichia coli / Q000201:enzymology*; D007211:Indoles / Q000378:metabolism; D007700:Kinetics; D046911:Macromolecular Substances; D012694:Serine / Q000378:metabolism; D013816:Thermodynamics; D014364:Tryptophan / Q000378:metabolism; D014367:Tryptophan Synthase / Q000378:metabolism*
chemical_list: D002021:Buffers; D007211:Indoles; D046911:Macromolecular Substances; D012694:Serine; C030374:indole; D014364:Tryptophan; D014367:Tryptophan Synthase