the peptide bond
/ PEP-tide /
To build a protein chain, the cell has to clip amino acids together one after another. The clip it uses is a single, specific chemical link called the peptide bond. It is what turns a loose pile of amino-acid 'beads' into a connected necklace, the polypeptide.
A peptide bond forms between the carboxyl group (-COOH) of one amino acid and the amino group (-NH2) of the next. In joining, the pair sheds one water molecule — a reaction called condensation, or dehydration — leaving a -C(=O)-N(H)- linkage between them. This is the same chemistry as an amide bond in ordinary chemistry. Two important features follow. First, although it is drawn as a single bond, the electrons are shared between the C-N bond and the neighboring C=O, giving the peptide bond partial double-bond character: it cannot freely rotate, and the six atoms around it are locked into a flat plane. Second, the linkage is directional — every chain has a free amino end (the N-terminus) and a free carboxyl end (the C-terminus), and by convention we read and write sequences from N to C.
That rigidity and flatness are quietly crucial. Because each peptide bond is a stiff little plate, the backbone can only bend at the joints between plates — which sharply limits how a chain can fold and makes regular structures like the alpha helix and beta sheet possible. The peptide bond is also chemically tough; it does not fall apart on its own at body temperature, which is why proteins are stable, and why breaking a protein back down to amino acids takes either strong acid or a dedicated enzyme.
Two glycines joining release one water and become a dipeptide: H2N-CH2-C(=O)-N(H)-CH2-COOH. Add more amino acids the same way and the chain grows, always one water lost per bond.
Each peptide bond joins two residues and releases one molecule of water.
The peptide bond looks like a plain single bond but behaves like a partial double bond: this is why it is flat and rigid, not freely rotating — a detail that quietly governs all protein folding.