A few proteins contain two or more polypeptide chains linked covalently. For example, disulfide bonds link the two-polypeptide chains of insulin, covalently. In such cases, the individual polypeptides are not subunits but are chains. Haemoglobin, for example, has four polypeptide subunits: two identical α-chains and two identical β-chains, and all the four held together by noncovalent interactions. Haemoglobin can be considered either a tetramer of four polypeptide subunits or a dimer of αβ-protomers. Protein is understood as having four levels of three-dimensional structure:
- The amino acid sequence in the peptide chains of the protein and the bonding between them -like peptide, hydrogen, sulphide, describe the primary structure.
- Secondary structure refers to stable structural patterns of the peptide chains.
- Tertiary structure describes all aspects of the three-dimensional folding of a polypeptide.
- When a protein has two or more polypeptide subunits, their total arrangement in space is referred to as a quaternary structure.