Pyroglutamic acid, also known as PCA, 5-oxoproline, pidolic acid, or pyroglutamate, is an uncommon amino acid derivative where the free amino group of glutamic acid or glutamine cyclizes to form a lactam. It plays a role in the glutathione cycle, being converted to glutamate by 5-oxoprolinase, and is found in many proteins, including bacteriorhodopsin. N-terminal glutamic acid and glutamine residues can spontaneously cyclize to become pyroglutamate, complicating N-terminal sequencing with Edman chemistry due to the absence of a free primary amino group. The enzyme pyroglutamate aminopeptidase can remove the pyroglutamate residue, restoring a free N-terminus. Pyroglutamic acid exists as two distinct enantiomers.