Peptide vs protein: what's the difference?

They're all made of the same building blocks. The difference is mostly size, and whether the chain folds into a working shape.

	What it is	Size
Amino acid	A single building block	1 unit
Peptide	A short chain of amino acids	About 2 to 50
Polypeptide	A longer single, unbranched chain	Dozens to hundreds
Protein	One or more polypeptides folded into a 3D shape	Usually 50+, often hundreds
Enzyme	A protein (occasionally RNA) that speeds up a chemical reaction	Usually hundreds

A simple way to picture it

Think of amino acids as beads. A few beads on a string is a peptide. A long string is a polypeptide. When that long string folds and twists into a specific shape that does a job, it's a protein. If the protein's job is to speed up a chemical reaction, it's an enzyme.

Insulin: peptide or protein?

Insulin is a good example of the grey area. It has 51 amino acids across two chains of 21 and 30, held together by disulfide bridges. It sits right on the line, so you'll see it called both a peptide hormone and a small protein.

Peptide hormones vs protein hormones

Both are chains of amino acids that carry signals around the body. Shorter ones like oxytocin (9 amino acids) and vasopressin (9) are called peptide hormones. Larger ones, such as growth hormone at 191 amino acids, are usually called protein hormones.

Try it yourself: paste a sequence into the molecular weight calculator to see how long it is and how much it weighs.