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{{short description|Number and arrangement of multiple folded protein subunits in a multi-subunit complex}}
{{Protein structure}}▼
{{about|quaternary structure in protein|the article about quaternary structure in nucleic acid|Nucleic acid quaternary structure}}
{{Use dmy dates|date=December 2016}}
▲{{Protein structure}}
'''Protein quaternary structure'''{{efn|Here ''[[wikt:quaternary|quaternary]]'' means "''fourth-level'' structure", not "''four-way'' interaction". Etymologically ''[[wikt:quartary|quartary]]'' is correct: ''quaternary'' is derived from Latin [[distributive number]]s, and follows ''binary'' and ''ternary''; while ''quartary'' is derived from Latin [[ordinal number]]s, and follows ''secondary'' and ''tertiary''. However, ''quaternary'' is standard in biology.}} 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 [[protein folding|folded]] [[protein subunit]]s in a [[Multiprotein complex|multi-subunit complex]]. It includes organizations from simple [[protein dimer|dimers]] to large [[homooligomer]]s and [[multiprotein complex|complexes]] with defined or variable numbers of subunits.<ref>{{cite book| vauthors = Berg JM, Tymoczko JL, Stryer L |title=Biochemistry|date=2002|publisher=W. H. Freeman|___location=New York, NY [u.a.]|isbn=0-7167-3051-0|edition=5. ed., 4. print.|url=https://archive.org/details/biochemistrychap00jere|chapter=Section 3.5Quaternary Structure: Polypeptide Chains Can Assemble Into Multisubunit Structures|chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK22550/|url-access=registration}}</ref> 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 complex]]es of proteins with [[nucleic acid]]s and other [[Cofactor (biochemistry)|cofactors]].
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