peptide
Meaning of peptide: Where a peptide stops and a protein starts is a matter of convention, not chemistry: the bond and the building blocks are identical.
Definition of peptide
A peptide is a chain of amino acids joined end to end by peptide bonds, the links formed when the acid group of one amino acid bonds to the amine group of the next. The order and number of amino acids set the shape of the chain and what it does. Longer chains are called polypeptides, and longer still, proteins.
How is a peptide different from a protein?
Chemically they are the same thing at different lengths. The convention places the boundary somewhere around fifty amino acids, but no rule enforces it and the same molecule is sometimes described either way.
| Peptide | Protein | |
|---|---|---|
| Length | Two to roughly fifty amino acids | Longer chains, often several hundred |
| Shape | Flexible, little fixed folding | Folded into a stable three-dimensional structure |
| Typical role | Signaling, hormone, antimicrobial defense | Enzymes, structure, transport, antibodies |
| Examples | Oxytocin, vasopressin, glucagon | Hemoglobin, collagen, insulin receptor |
A protein is built as one long peptide chain and then folds, so every protein is a polypeptide even though not every peptide is called a protein.
What do peptides do in the body?
Many hormones are peptides. Insulin, glucagon, oxytocin, vasopressin and the gut hormones that signal fullness after a meal all work by binding receptors on the outside of a cell, because a peptide is too large and too water-loving to cross the membrane the way a steroid hormone does. Other peptides act closer to home, carrying signals between neighboring cells or within the nervous system as neuropeptides. A separate group, the antimicrobial peptides, punch holes in bacterial membranes and form part of the body's immediate defense, which is one of several ways peptides contribute to immune function. Peptides also appear as fragments left over when a larger protein is cut, and some of those fragments are useful markers. C-peptide is the classic example: it is released in equal amount to insulin when proinsulin is split, so measuring it shows how much insulin the pancreas itself is making.
Why are peptides used as medicines?
A synthetic peptide can imitate a natural signal closely enough to switch the same receptor on or off, which gives a precision that small molecules often cannot match. The drawback is delivery. Digestive enzymes break peptides into their amino acids, so most peptide medicines are injected rather than swallowed, and chemists modify the chain to slow its breakdown in the blood. Peptides also serve in the laboratory as standards, as antigens for raising antibodies, and as the pieces read out by mass spectrometry when a protein is identified.
Used in a sentence
The laboratory measured levels of a specific peptide hormone as part of the diagnostic workup.
Good to know
A peptide and a protein are distinguished mainly by chain length rather than by any difference in chemistry, and the boundary between the two names is a convention rather than a fixed rule.
Word origin
Where “peptide” comes from
From Greek peptos, meaning digested, by way of peptone, the mixture of protein fragments produced by digestion.