Figure 16-17 Synthesis of acetylcholine (ACh). Choline and acetyl coenzyme A (CoA) bind to the surface of choline acetyltransferase (ChAT). An imidazole on ChAT promotes proton removal and generates a more nucleophilic choline, facilitating condensation with the acetyl group of acetyl-CoA. The products of the reaction are coenzyme A and ACh, which are rapidly packaged in vesicles for immediate release on proper stimulation. ChAT also catalyzes the reverse reaction between ACh and CoASH, although at a much slower rate than the forward reaction. (From Doukas PH: Drugs affecting the parasympathetic nervous system. In Wingard L, Brody T, Larner J, et al [eds]: Human Pharmacology: Molecular to Clinical. St. Louis, Mosby-Year Book, 1991.)