C-Terminal Amidation

C-terminal amidation converts a peptide's terminal carboxylic acid into a primary amide, removing the negative charge a free acid carries at physiological pH. In the body it is a biosynthetic step rather than a synthetic trick: peptidylglycine alpha-amidating monooxygenase acts on a precursor extended by one glycine, consuming that glycine to donate the amide nitrogen. In chemical synthesis the outcome is decided at the outset, by whether the first residue is loaded onto an amide-releasing or an acid-releasing resin.

Many natural peptide hormones are amidated, and for several the modification is not optional. Oxytocin, vasopressin, calcitonin, gastrin and alpha-melanocyte-stimulating hormone all carry a C-terminal amide, and the GHRH analogues sermorelin and tesamorelin reproduce the 1-29 fragment as its amide. Removing the amide from calcitonin or alpha-MSH costs most of the receptor potency, because the reintroduced charge disrupts contacts the C-terminal region makes in the binding site.

Beyond potency, amidation blunts one degradation route. Carboxypeptidases recognise a free terminal carboxylate, so capping it removes that handle just as N-terminal acetylation removes the aminopeptidase handle at the other end. It does nothing about endopeptidase cleavage in the middle of the chain, so amidation is not general stabilisation.

The commercial trap is the free-acid impurity. An amide and its corresponding acid differ by 0.98 daltons, which most low-resolution mass spectrometry cannot resolve at several kilodaltons, yet the two can differ substantially in activity and co-elute closely on reversed-phase chromatography. A certificate reporting a mass consistent with the amide, without stating instrument resolution or an assay distinguishing the forms, has not excluded material that is largely the less active acid.