Peptide

A peptide is a short chain of amino acids linked end to end by peptide bonds. Each peptide bond forms when the carboxyl group of one amino acid connects to the amino group of the next, so the chain grows one link at a time, much like beads on a string.

Peptide vs. protein

There’s no strict cutoff, but the working distinction is length and shape. Peptides are typically shorter chains — anywhere from two amino acids up to roughly 50 — that don’t fold into the large, stable three-dimensional structures proteins do. Proteins are longer chains built from the same peptide-bond chemistry, but they typically fold into defined shapes that let them act as enzymes, structural components, or transporters. Many peptides instead act as signaling molecules: a chain folds just enough to fit a specific receptor and trigger a response, then gets broken down.

Why the amino acid sequence matters

The order of amino acids in the chain determines which receptor a peptide can bind and what happens when it does. Change the sequence and the peptide typically stops fitting its target — this is why lab-made peptides are described by their exact sequence, not just their length. GLP-1 is one example: a 30-amino-acid peptide hormone whose specific sequence is what lets it bind its receptor.

How long a peptide lasts

Once released into the body, a peptide doesn’t last indefinitely — enzymes break the chain apart over time. How quickly that happens is described by its half-life, which varies between peptides depending on their structure and size.