The four levels of protein structure are primary, secondary, tertiary, and quaternary structure, which are distinguished from one another by the degree of complexity in the polypeptide chain.

• Primary structure: Primary structure describes the sequence of amino acids in the polypeptide chain, which is unique and specific to a particular protein.
• Secondary structure: Secondary structure refers to the highly regular local sub-structures derived from the coiling or folding of a polypeptide chain. There are two main types of secondary structures, the ?-helix and the ?-strand or ?-sheets, who are defined by pattern of hydrogen bonds between the main-chain peptide groups.
• Tertiary structure: Tertiary structure is the comprehensive three-dimensional structure of monomeric and multimeric protein molecules, where ?-helix and ?-sheets are further folded into a compact globular structure.
• Quaternary structure: Quaternary structure is the three-dimensional structure of a protein macromolecule formed by aggregation of two or more individual polypeptide chains. This protein macromolecule operates as a single functional unit, which is referred to as multimer, while each polypeptide chain is called a subunit.