C-terminal

The C-terminal end (or C-terminus) of a peptide designates the polypeptide chain end bearing a free carboxyl group (-COOH), as opposed to the N-terminal end bearing the free amine (-NH₂). By universal convention, a peptide sequence is always written from N-terminus (left) to C-terminus (right), matching the order amino acids are incorporated during ribosomal translation.

The C-terminus is a crucial functional zone in peptide biology. It serves as a recognition site for many exopeptidases (notably carboxypeptidases A, B and E) that progressively cleave residues from the C-terminal end. It also hosts cellular localization signals (endoplasmic reticulum retention signal KDEL, prenylation signal CAAX) and may undergo major post-translational modifications.

C-terminal amidation is one of the most important modifications: it converts the carboxyl group (-COOH) into an amide (-CONH₂), conferring increased resistance to carboxypeptidases and often essential for biological activity. Many neuropeptides (oxytocin, vasopressin, substance P, neuropeptide Y, α-MSH, TRH) are C-terminally amidated and their activity collapses without this modification.

In peptide research, designing a stable analog frequently involves modifying or protecting the C-terminus: amidation, head-to-tail cyclization, addition of D-isomer amino acids at C-terminal position, lipid grafting. Modern GLP-1 agonists (semaglutide, liraglutide) retain a C-terminal glycinamide critical for receptor affinity. Mastering C-terminal architecture is thus a direct lever to optimize activity and plasma half-life of research peptides.

In SPPS (Solid-Phase Peptide Synthesis) practice, the C-terminal end is the first residue fixed to the resin: Wang resin for free carboxylic acid (-COOH), Rink Amide resin for C-terminal amide (-CONH2). Choice determines final functionality: amidated peptides (GHK-Cu, Ipamorelin, GHRP-2, Melanotan II) show increased resistance to serum carboxypeptidases and extended half-life, while free-acid peptides (GHRH(1-29), CJC-1295) retain native functionality.

ESI-MS mass spectrometry analysis confirms C-terminal nature: an amidated peptide shows a mass 1 Da lower than the acid equivalent (replacement of -OH with -NH2). Notable C-terminal modifications also include: (1) natural C-amidated amino acid (oxytocin, vasopressin), (2) C-terminal methylation (optimised synthetic peptides), (3) head-to-tail cyclisation forming a cyclopeptide, (4) conjugation to a fluorophore (dansyl, fluorescein) or targeting group (biotin, PEG). Any C-terminal deamidation (Asn → Asp or Gln → Glu) during storage is detectable by +1 Da mass increase and characteristic RP-HPLC shift.