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A protein is a chain of amino acids, compounds that contain amine and carboxyl groups, along with different side chains denoted as R, attached to a core carbon.

Amino acids are linked through covalent peptide bonds, and the repeating series of atoms along the core is called the polypeptide backbone.

The sequence begins at the N-terminus, with a free amino group, NH₃⁺, and ends with the C terminus, a free carboxyl group, COO^-.

Each protein is unique, depending on the number of amino acids and their side chains.

There are 20 different side chains, which fall into one of four groups. Acidic amino acids, like aspartic and glutamic acids, have side chains with a carboxyl group. Because of this, they act like acids at physiological pH.

Basic amino acids have an amino group in their side chain and act like bases.

Several amino acids are polar but uncharged. They tend to be hydrophilic and make up the outer portion of a folded protein, where they are free to interact with the aqueous environment.

Lastly, a large group of amino acids contains nonpolar side chains. They can be simple, as in glycine, or complex, like proline and tryptophan.