We are investigating the structure and function of the bacterial tryptophan synthase alpha 282 complex by use of x-ray crystallography, site-specific mutagenesis, and spectrophotometric studies. The tryptophan synthase multiezyme complex catalyzes the final reaction of the biosynthesis of L- tryptophan and has been the subject of many genetic and biochemical studies. Our recent determination of the three- dimensional structure of this multienzyme complex by x-ray crystallography allows us to locate the active sites of both the alpha and beta subunits and to understand how the indole prodcued at the active site of the alpha subunit is channeled a distance of 25 A to the active site of the beta subunit. The pyridoxal phosphate-dependent reaction of indole with L-serine at the active site of the beta subunit can be studied spectrophotometrically. A comparision of the kinetics of reaction and of the spectral properties of the enzyme in solution and in the crystalline state shows that the crystalline form is catalytically active. Site-specific mutagenesis of tryptophan synthase has been initiated in order to investigate the effects of structure on the functional properties of tryptophan synthase. The tryptophan synthase alpha 282 complex in which arginine-179 of the alpha subunit has been changed to leucine was engineered by site- specific mutagenesis, expressed, purified, crystallized, and characterized. The mutant enzyme was partially active but had changed properties in response to ligands, suggesting that the mutation altered the reciprocal transmission of substrate-induced conformational changes between the alpha and beta subunits in the alpha 282 complex. Studies of a subunits in which glutamic acid-49 was changed to 19 different amino acids indicate the glutamic acid-49 is an essential base in reactions catalyzed by the alpha subunit.
