The acid base chemistry that drives catalysis in pyridoxal-5'-phosphate (PLP)-dependent enzymes has been the subject of intense interest and investigation since the initial identification of PLP's role as a coenzyme in this extensive class of enzymes. It was first proposed over 50 years ago that the initial step in the catalytic cycle is facilitated by a protonated Schiff base form of the holoenzyme in which the linking lysine epsilon-imine nitrogen, which covalently binds the coenzyme, is protonated. Here we provide the first N-15 NAIR chemical shift measurements of such a Schiff base linkage in the resting holoenzyme form, the internal aldimine state of tryptophan synthase. Double-resonance experiments confirm the assignment of the Schiff base nitrogen, and additional C-13, N-15, and P-31 chemical shift measurements of sites on the PLP coenzyme allow a detailed model of coenzyme protonation states to be established.