Side chains of many amino acid residues are known to participate in nucleophilic catalysis by forming covalent enzyme-substrate intermediates. Enzymes have a choice of many nucleophilic groups but have little to offer in terms of good electrophiles. Therefore, a few small molecules (cofactors and prosthetic groups) are recruited by nature to complement an apoenzyme—resulting in a functional holoenzyme. These small molecules act as temporary electron sinks during catalysis by forming covalent adducts with substrates. Much of enzyme chemistry is carbanion chemistry. The abstraction of a proton or decarboxylation from an sp3 carbon leaves behind a carbanion that is not so stable. Developing carbanions may be stabilized—hence effecting rate accelerations—by suitably placing temporary electron sinks. Coenzymes like pyridoxal phosphate and thiamine pyrophosphate function to stabilize them via their electrophilic adducts. We already listed more commonly encountered electrophilic reagents in Table 29.1 . Decarboxylations involving carboxylic acids (other than amino acids) were covered previously (Chap. 32 ). Amino acid transformations including decarboxylation offer a different chemical challenge and are described in this chapter.