Principles of Biochemistry

Many proteins are composed of two or more polypeptide chains referred to as subunits. The spatial arrangement of its subunits is called the quaternary structure. The subunits are associated by noncovalent forces and called multimeric protein. In these multimeric proteins, the individual polypeptide chains are termed promoters or subunits. Hydrogen bonds and electrostatic bonds formed between surface residues of adjacent subunits, stabilize the association of subunits. Proteins composed of two or four subunits are called dimeric or tetrameric proteins, respectively.

Quaternary structure: The four separate chains of haemoglobin assembled into a tetrameric protein subunit.