The tryptophan synthase alpha 2 beta 2 multienzyme complex is formed by the association of two alpha subunits and one beta 2 subunit. The chemical nature of the subunit interaction sites, of the active site, and the factors influencing subunit association are being studied. The cofactor, pridoxal 5'-phosphate and active site sulfhydryl and histidyl residues in the beta 2 subunit are found to become partially shielded from the aqueous environment in the alpha 2 beta 2 complex. Subunit association is favored by salts and by the cofactor, pyridoxal 5'-phosphate. The substrate analog, trans-L-2-amino-4-methoxy-3-butenoic acid irreversibly inactivates the beta 2 subunit by reacting with an active site nucleophilic group. Tryptophan synthase catalyzes two new pyridoxal 5'-phosphate dependent reactions in which, beta, gamma-unsaturated amino acids are converted to saturated alpha-keto acids.