Pyroglutamate (pGlu) N-Terminus

Pyroglutamate is what an N-terminal glutamine or glutamate becomes when its side chain condenses with its own alpha-amino group, closing a five-membered lactam ring. The product, also called 5-oxoproline, is still residue one but has no free amine and no N-terminal positive charge; forming it from glutamine releases ammonia and drops the residue mass by 17 daltons. In written sequences it appears as pGlu- in place of the H- that marks a free N-terminus.

Several releasing hormones carry it natively. Thyrotropin-releasing hormone is pGlu-His-Pro-NH2, cyclised at one end and amidated at the other, and gonadotropin-releasing hormone begins pGlu-His-Trp-Ser-Tyr. Every agonist built on GnRH keeps the ring, so gonadorelin, leuprolide, goserelin and triptorelin all start the same way, and leuprolide's D-leucine is numbered position six with the pGlu counted as one.

The design value is stability without an acetyl cap. Aminopeptidases need a free alpha-amine to bind, so a cyclised terminus stops them, and the missing charge lowers net charge by one; only a dedicated enzyme, pyroglutamyl peptidase, removes it. Where a receptor recognises the ring, as the GnRH and TRH receptors do, the modification is part of the pharmacophore rather than a shield bolted on afterwards.

The same reaction is a degradation route. A peptide that starts with glutamine slowly cyclises on its own in solution, so a stored product acquires a pGlu variant 17 daltons lighter and less charged than the label claims. The pyroglutamate form of amyloid beta, pE3-Abeta, is the aggregation-prone species the antibody donanemab targets. A persistent minus-17 peak in such a peptide's spectrum is reporting age or heat, not a synthesis error.