peptide bond
/ PEP-tide bond /
To build a protein, the cell has to clip amino acids together in a long chain, and the clip it uses is the peptide bond. It is the specific link that joins one amino acid to the next, the coupling between the train cars of a protein.
A peptide bond forms by dehydration synthesis: the acidic carboxyl group of one amino acid joins the basic amino group of the next, and as they bond a water molecule is released. The result is a covalent bond — a strong, stable connection, far stronger than a hydrogen bond. A short string of amino acids joined this way is called a peptide; a long one is a polypeptide, which folds up into a protein. Crucially, the chain always has direction: a free amino end and a free carboxyl end, like a sentence read from start to finish.
The peptide bond matters because it is both strong and uniform. Strong, so a protein chain holds together reliably through everything the cell throws at it. Uniform, so the same machinery can join any amino acid to any other in the order the gene specifies. The backbone of every protein you contain is a chain of identical peptide bonds, with the twenty different side chains hanging off it like charms on a bracelet.
Insulin, the hormone that controls blood sugar, is just two short chains of amino acids held together by peptide bonds — 51 letters spelling out a molecule that keeps you alive.
A 51-amino-acid chain, every link a peptide bond — that is the hormone insulin.
Forming a peptide bond releases water (dehydration synthesis); breaking one — as in digestion — adds water back (hydrolysis). The same bond is read forwards and backwards by these two opposite reactions.