Protein quaternary structure

Protein primary structureProtein secondary structureProtein tertiary structureProtein quaternary structure
The image above contains clickable links
The image above contains clickable links
This diagram (which is interactive) of protein structure uses PCNA as an example. (PDB: 1AXC​)
This article is about quaternary structure in protein. For the article about quaternary structure in nucleic acid, see Nucleic acid quaternary structure.

Protein quaternary structure[a] is the fourth (and highest) classification level of protein structure. Protein quaternary structure refers to the structure of proteins which are themselves composed of two or more smaller protein chains (also referred to as subunits). Protein quaternary structure describes the number and arrangement of multiple folded protein subunits in a multi-subunit complex. It includes organizations from simple dimers to large homooligomers and complexes with defined or variable numbers of subunits.[1] In contrast to the first three levels of protein structure, not all proteins will have a quaternary structure since some proteins function as single units. Protein quaternary structure can also refer to biomolecular complexes of proteins with nucleic acids and other cofactors.


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  1. ^ Berg JM, Tymoczko JL, Stryer L (2002). "Section 3.5Quaternary Structure: Polypeptide Chains Can Assemble Into Multisubunit Structures". Biochemistry (5. ed., 4. print. ed.). New York, NY [u.a.]: W. H. Freeman. ISBN 0-7167-3051-0.