A 3D diagram of the protein myoglobin showing turquoise alpha helices. This protein was the first to have its structure solved by X-ray crystallography. Towards the right-center among the coils, a heme group (shown in grey) with a bound oxygen molecule (red)

Proteins are long-chain molecules built from small units known as amino acids. They are joined with peptide bonds.

They are biochemical compounds.They have one or more polypeptides folded into a round or fibrous shape.

A polypeptide is a single linear polymer chain of amino acids. The sequence of amino acids in a polypeptide comes from the DNA sequence of a gene. The genetic code specifies 20 standard amino acids. Shortly after synthesis, some amino acids are chemically modified. This alters the folding, stability, activity, and function of the protein. Sometimes proteins have non-peptide groups attached, as cofactors.