The external aldimine crystal structure of Salmonella typhimurium Tryptophan Synthase mutant beta-S377A with inhibitor 2-({[4-(trifluoromethoxy)phenyl]sulfonyl}amino)ethyl dihydrogen phosphate (F9F) at the alpha-site, Cesium ion at the metal coordination site, and (E)-N-({3-hydroxy-2-methyl-5-[(phosphonooxy)methyl]pyridin-4-yl}methylidene)-L-serine (KOU) at the beta-site