Ttryptophan synthase (TRPS) alpha subunit (TSA). TPRS is a bifunctional tetrameric enzyme (2 alpha and 2 beta subunits) that catalyzes the last two steps of L-tryptophan biosynthesis. Alpha and beta subunit catalyze two distinct reactions which are both strongly stimulated by the formation of the complex. The alpha subunit catalyzes the cleavage of indole 3-glycerol phosphate (IGP) to indole and d-glyceraldehyde 3-phosphate (G3P). Indole is then channeled to the active site of the beta subunit, a PLP-dependent enzyme that catalyzes a replacement reaction to convert L-serine into L-tryptophan.