Tryptophan synthase: structure and function of the monovalent cation site

The monovalent cation (MVC) site of the tryptophan synthase from Salmonella typhimurium plays essential roles in catalysis and in the regulation of substrate channeling. In vitro, MVCs affect the equilibrium distribution of intermediates formed in the reaction of l-Ser with the alpha(2)beta(2) complex; the MVC-free, Cs(+)-bound, and NH(4)(+)-bound enzymes stabilize the alpha-aminoacrylate species, E(A-A), while Na(+) binding stabilizes the l-Ser external aldimine species, E(Aex(1)). Two probes of beta-site reactivity and conformation were used herein, the reactive indole analogue, indoline, and the l-Trp analogue, l-His. MVC-bound E(A-A) reacts rapidly with indoline to give the indoline quinonoid species, E(Q)(indoline), which slowly converts to dihydroiso-l-tryptophan. MVC-free E(A-A) gives very little E(Q)(indoline), and turnover is strongly impaired; the fraction of E(Q)(indoline) formed is