The defining feature of an amino acid is its side chain (at top, blue circle; below, all colored circles). When connected together by a series of peptide bonds, amino acids form a polypeptide ...

Each protein is a molecule made up of different combinations of 20 types of smaller, simpler amino acids. Protein molecules are long chains of amino acids that are folded into a three-dimensional ...

The sequence of amino acids in the chain determines how the chain will fold up to make the protein, so different proteins have different three-dimensional shapes. The three-dimensional shape of a ...