Pyroglutamic acid is an uncommon amino acid derivative in which the free amino group of glutamic acid or glutamine cyclizes to form a lactam. It is found in many proteins including bacteriorhodopsin. N-terminal glutamic acid and glutamine residues can spontaneously cyclize to become pyroglutamate. This is one of several forms of blocked N-terminals which present a problem for N-terminal sequencing using Edman chemistry, which requires a free primary amino group not present in pyroglutamic acid. The enzyme pyroglutamate aminopeptidase can restore a free N-terminus by cleaving off the pyroglutamate residue. The sodium salt of pyroglutamic acid – known either as sodium pyroglutamate or sodium pidolate is used on the skin to retain moisture. Pyroglutamic acid is sold as a dietary supplement in the United States.
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