What separates a peptide from a protein is above all the length of the amino acid chain. Peptides usually have 2 to 50 amino acids, proteins more, often several hundred. The US Food and Drug Administration has written this boundary into its regulations: for the FDA, a protein is an amino acid polymer with a specific sequence that has more than 40 amino acids. Everything else follows from length: the shape of the molecule, how it is made and what job it does in the body.
What are peptides and what are proteins?
Both are built from the same bricks. Amino acids join into a chain through the peptide bond, and that chemistry is identical in both groups. The difference starts with the number of links.
- Peptides are short chains. The body makes them itself, but peptides also reach us from food, cosmetics and medicines.
- Proteins are long chains, single or made of several. They fold into a complex three-dimensional form and only in that form do they start to work.
The practical side of both groups is covered in the beginner's guide to peptides.
Where is the boundary?
Textbooks usually put the boundary at 50 amino acids. US regulations put it at 40, and chains that are joined together as they are in nature are counted together. Both numbers are a convention, not a law of nature.
| Feature | Peptides | Proteins |
|---|---|---|
| Chain length | usually 2 to 50 amino acids | over 50, often hundreds |
| Boundary in FDA regulations | up to 40 amino acids | more than 40 amino acids |
| Shape | simple, little folding | complex three-dimensional structure |
| How they are made | in the body or by chemical synthesis | in the cell, on ribosomes |
| Typical job | signal, hormone, defence | enzyme, transport, scaffolding |
What follows from size?
A short chain rarely folds into a complicated shape. A protein, by contrast, takes on several levels of structure, from the sequence up to several chains arranged side by side. That shape decides whether an enzyme catches its target and whether an antibody recognises a bacterium.
Size also decides how the molecule is made. Proteins are produced in the cell by translation on ribosomes, and then folded and modified. A short peptide can be assembled chemically in a laboratory, one amino acid at a time. That is why the research peptides in this shop are synthetic and arrive as a lyophilisate, a dry powder in a vial.
How do peptides work as signals?
Most known peptides work as messengers. They act in small amounts and trigger a response in a cell that has a receptor for them.
- Peptide hormones. One example is GLP-1, a peptide released by gut cells after a meal. It boosts insulin secretion, suppresses glucagon secretion, slows gastric emptying and reduces food intake.
- Neuropeptides. In the nervous system they act as transmitters or modulators. Endorphins, linked to the perception of pain, belong to this group.
- Defence peptides. Defensins are the natural antibiotics of innate immunity. They protect the skin, the small intestine and other mucosal surfaces against microbes.
A whole group of medicines for type 2 diabetes and obesity was built on the GLP-1 receptor. Molecules from this family, as research material, are collected in the weight loss peptides category.
What do proteins do?
Proteins do most of the heavy work in the cell. Many of them are enzymes that speed up chemical reactions. Haemoglobin carries oxygen, and albumin carries other molecules in the blood. Collagen, keratin and actin form the scaffolding of tissues and muscles, and antibodies recognise pathogens.
Collagen shows well how the two groups overlap. Collagen itself is a protein, but the collagen peptides sold as supplements are its short fragments. Peptides that prompt the skin to produce collagen are described in the article on peptides for skin.
Is the boundary strict?
No. Peptides can combine into larger assemblies that behave like proteins. Modifications after synthesis, for example attaching a sugar or a fatty acid, change the size, stability and properties of the molecule. That is why the same substance is called a peptide in one source and a small protein in another.
Why did peptides become medicines and research tools?
A peptide combines two advantages. It acts selectively on one receptor, like a hormone, and at the same time it can be made chemically at high purity. Hence the medicines of the GLP-1 family, and hence the peptides studied for tissue repair, growth hormone and skin.
All peptides available in the shop as research material are collected in the shop catalogue. Shipping from Poland costs 30 EUR paid in advance or 80 EUR cash on delivery, the same for every country.
Frequently asked questions
What is the difference between a peptide and a protein?
Chain length and job. Peptides usually have 2 to 50 amino acids and work as signals, while proteins are longer and serve as enzymes, transporters and scaffolding.
How many amino acids does a protein have according to the FDA?
More than 40. That is the definition in the US regulations on biological products, and shorter amino acid polymers are peptides under those rules.
Are peptides the building blocks of proteins?
Yes, because both are chains of amino acids joined by the same peptide bond. A protein is simply a much longer chain, folded in space.
Is collagen a peptide or a protein?
Collagen is a structural protein of the skin and connective tissue. Collagen peptides in supplements are its short fragments produced by breaking it down.
Sources
- 21 CFR 600.3(h)(6), definition of protein in the regulations of the US Food and Drug Administration (FDA). https://www.law.cornell.edu/cfr/text/21/600.3
- Drucker DJ. Mechanisms of Action and Therapeutic Application of Glucagon-like Peptide-1. Cell Metabolism, 2018, 27(4): 740-756. https://doi.org/10.1016/j.cmet.2018.03.001
- Ganz T. Defensins: antimicrobial peptides of innate immunity. Nature Reviews Immunology, 2003, 3(9): 710-720. https://doi.org/10.1038/nri1180