Proteins are heteropolymers made up of amino acids, and the linear sequence of these amino acids constitutes the primary structure. The long polypeptide chain folds upon itself into a compact, globular form, which is referred to as the tertiary structure. However, the statement that the first amino acid is called the C-terminus and the last amino acid is regarded as the N -terminus is not true, because in reality the first amino acid has a free amino group (N-terminus) and the last amino acid has a free carboxyl group (C-terminus).