N-Terminus

The N-terminus, or amino terminus, is the end of a peptide where an alpha-amino group sits unbonded to any preceding residue. Every convention in the field starts here: sequences are written left to right from N to C, and residues are numbered from this end outward. The terminal amine has a pKa near 9, so at blood pH it is protonated and carries a positive charge, which contributes to the molecule's net charge and to how it binds ion-exchange resins and anionic membranes.

It is also the exposed flank. Aminopeptidases remove residues one at a time from this end, and dipeptidyl peptidase-4 clips the first two residues from chains that present a proline or alanine in the second position. Native GLP-1 has exactly that arrangement, His-Ala at its first two positions, and is cut within a couple of minutes in circulation. The long-acting analogs answer at the same site: semaglutide replaces the alanine with the non-natural residue aminoisobutyric acid, which the enzyme cannot process. Worth noting that solid-phase synthesis builds a chain in the opposite direction, from C to N, so the N-terminal residue is the last one coupled.

Knowing which end is modified is what lets you read a structure. An acetyl cap, a pyroglutamate, a fatty acid attached at the front rather than to a side chain further along, a substituted second residue: all are N-terminal engineering aimed at the same short list of enzymes.

The mistake is assuming that losing a residue or two from this end is a minor trim. It frequently is not. The DPP-4 product of GLP-1 loses its insulinotropic activity at the receptor, so an assay that measures total immunoreactive peptide rather than the intact form can report abundant hormone that is doing nothing.