Figure 28-17 Intermediate in the synthesis of tryp tophan. PLP (green)on a p chain o tryptophan synthetase forms a Schiff base with serine, which is then dehy H2C drated to give the schiff base of aminoacrylate (red). This enzyme C-C bound intermediate is attacked by indole, the product of the partial NH reaction catalyzed by the a subunit, to give tryptophan CH O2POH2C O CH 3