Amino acids: the building blocks
An amino acid is a small molecule with an amino group at one end and a carboxylic acid group at the other, plus a side chain that makes each one different. Living things build their proteins from twenty standard amino acids. Each has a three-letter and a one-letter code: glycine is Gly or G, lysine is Lys or K. Amino acid codes has the full table.
The peptide bond
When the acid group of one amino acid reacts with the amino group of the next, they join and release a molecule of water. The link that forms is an amide bond, and between amino acids it is called a peptide bond.
A chain therefore has a direction. One end keeps a free amino group, the N-terminus. The other keeps a free acid group, the C-terminus. Sequences are always written from N to C, so Gly-His-Lys and Lys-His-Gly are different peptides.
Peptide or protein?
There is no sharp line. By common use:
| Name | Roughly |
|---|---|
| Dipeptide, tripeptide, tetrapeptide | 2, 3 or 4 amino acids |
| Oligopeptide | Up to about 20 |
| Polypeptide | A longer single chain |
| Protein | About 50 or more, folded, sometimes several chains |
Insulin, with 51 amino acids in two chains, sits on the border and is called both.
Types of peptides
By origin. Natural peptides are made by living cells. Synthetic peptides are built in a laboratory, one amino acid at a time, usually on a solid support. A synthetic peptide can copy a natural sequence or be something new.
By shape. Linear peptides are open chains. Cyclic peptides are closed into a ring, either end to end or through their side chains.
By what they are used for, and so by how they are regulated. This matters more than chemistry to anyone buying:
- some peptides are licensed medicines, prescribed and dispensed like any other;
- some are cosmetic ingredients;
- some are food ingredients, such as collagen peptides;
- many exist only as research materials for laboratories.
The same word covers all four. Which category a peptide falls into decides who may sell it and to whom. Are peptides legal in the UAE?
Reading a peptide's name
Suppliers write modified peptides in a shorthand:
Ac-at the start: an acetyl group on the first amino acid.-NH₂at the end: the last amino acid ends in an amide, not an acid.D-before a residue: the mirror-image form of that amino acid.cyclo[...]: the residues inside the brackets form a ring.
So Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln is a seven-residue peptide with an acetylated start.
Are peptides like steroids?
No. Steroids are fats built on a skeleton of four carbon rings. Peptides are chains of amino acids. They are different kinds of molecule, made differently and tested differently.
Questions about taking peptides
We cannot answer them. We supply laboratories and businesses, and we give no advice on use in people. For a medicine, ask a licensed doctor. For the regulator's warnings about unapproved products, see our legal guide.