Enzyme catalyzed reactions involve one or more proton transfers. Acid-base chemistry permeates most of enzyme chemical mechanisms. Ionizable amino acid side chains of the enzyme protein are typically involved in such catalysis. Each ionizable group can be viewed as an acid and a conjugate base. A general exposure to acid-base chemistry and catalysis may be found in Chap. 28 of the book. The reader is well advised to read that background material to better appreciate the subject covered in this chapter.