What are the 4 levels of protein structure?

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.