# Protein subunit

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Structural unit of a protein complex

Rendering of [HLA-A11](/source/HLA-A11) showing the α (A*1101 gene product) and β (Beta-2 microglobulin) subunits. This receptor has a bound peptide (in the [binding pocket](/source/Binding_pocket)) of heterologous origin that also contributes to function.

In [structural biology](/source/Structural_biology), a **protein subunit** is a [polypeptide chain](/source/Polypeptide_chain) or single [protein](/source/Protein) molecule that assembles (or "*coassembles*") with others to form a [protein complex](/source/Protein_complex).[1][2][3] Large assemblies of proteins such as [viruses](/source/Viruses) often use a small number of types of protein subunits as building blocks.[4][5]

A subunit is often named with a Greek or Roman letter, and the numbers of this type of subunit in a protein is indicated by a subscript.[6] For example, [ATP synthase](/source/ATP_synthase) has a type of subunit called α. Three of these are present in the ATP synthase molecule, leading to the designation α3. Larger groups of subunits can also be specified, like α3β3-hexamer and c-ring.[7]

Naturally occurring proteins that have a relatively small number of subunits are referred to as [oligomeric](/source/Oligomer).[8] For example, [hemoglobin](/source/Hemoglobin) is a symmetrical arrangement of two identical α-globin subunits and two identical β-globin subunits.[3][9] Longer [multimeric](/source/Oligomer) proteins such as [microtubules](/source/Microtubule) and other [cytoskeleton](/source/Cytoskeleton) proteins may consist of very large numbers of subunits. For example, [dynein](/source/Dynein) is a multimeric protein complex involving two heavy chains (DHCs), two intermediate chains (ICs), two light-intermediate chains (LICs) and several light chains (LCs).[10]

The subunits of a protein complex may be identical, [homologous](/source/Homology_(biology)) or totally dissimilar and dedicated to disparate tasks.[1] In some protein assemblies, one subunit may be a "catalytic subunit" that enzymatically [catalyzes](/source/Catalysis) a reaction, whereas a "regulatory subunit" will facilitate or inhibit the activity.[11] Although [telomerase](/source/Telomerase) has [telomerase reverse transcriptase](/source/Telomerase_reverse_transcriptase) as a catalytic subunit, regulation is accomplished by factors outside the protein.[12]

An enzyme composed of both regulatory and catalytic subunits when assembled is often referred to as a [holoenzyme](/source/Holoenzyme). For example, [class I phosphoinositide 3-kinase](/source/Phosphoinositide_3-kinase#Class_I) is composed of a p110 catalytic subunit and a p85 regulatory subunit.[13] One subunit is made of one [polypeptide](/source/Polypeptide) chain. A polypeptide chain has one [gene](/source/Gene) coding for it – meaning that a protein must have one gene for each unique subunit.

## See also

- [Allostery](/source/Allostery)

- [Cooperativity](/source/Cooperative_binding)

- [Fusion protein](/source/Fusion_protein)

- [Monomer](/source/Monomer)

- [Protein quaternary structure](/source/Protein_quaternary_structure)

- [Subunit vaccine](/source/Subunit_vaccine)

## References

1. ^ [***a***](#cite_ref-Stoker_1-0) [***b***](#cite_ref-Stoker_1-1) Stoker, H. Stephen (1 January 2015). [*General, Organic, and Biological Chemistry*](https://books.google.com/books?id=IBGdBQAAQBAJ&pg=PA709) (7th ed.). Boston, MA: Cengage Learning. pp. 709–710. [ISBN](/source/ISBN_(identifier)) [978-1-305-68618-2](https://en.wikipedia.org/wiki/Special:BookSources/978-1-305-68618-2). Retrieved 15 April 2022.

1. **[^](#cite_ref-Smith_2-0)** Smith, Michael B. (27 April 2020). [*Biochemistry: An Organic Chemistry Approach*](https://books.google.com/books?id=0TXfDwAAQBAJ&pg=PA269). Boca Raton: CRC Press. pp. 269–270. [ISBN](/source/ISBN_(identifier)) [978-1-351-25807-4](https://en.wikipedia.org/wiki/Special:BookSources/978-1-351-25807-4). Retrieved 15 April 2022.

1. ^ [***a***](#cite_ref-Alberts_3-0) [***b***](#cite_ref-Alberts_3-1) Alberts, Bruce; Johnson, Alexander; Lewis, Julian; Raff, Martin; Roberts, Keith; Walter, Peter (2002). [*The Shape and Structure of Proteins*](https://www.ncbi.nlm.nih.gov/books/NBK26830/). New York: Garland Science. Retrieved 15 April 2022.

1. **[^](#cite_ref-Kumar_4-0)** Kumar, A.; Evarsson, A.; Hol, W. G. J. (1999). ["Multi-protein assemblies with point group symmetry"](https://books.google.com/books?id=xTy_M3B5sf4C&pg=PA462). In Vijayan, M.; Yathindra, N.; Kolaskar, A. S. (eds.). *Perspectives in Structural Biology: A Volume in Honour of G.N. Ramachandran*. Hyderabad, India: Universities Press. pp. 449–466. [ISBN](/source/ISBN_(identifier)) [978-81-7371-254-8](https://en.wikipedia.org/wiki/Special:BookSources/978-81-7371-254-8). Retrieved 15 April 2022.

1. **[^](#cite_ref-5)** Witwit, Haydar; de la Torre, Juan C. (2025-04-29). ["Mammarenavirus Z Protein Myristoylation and Oligomerization Are Not Required for Its Dose-Dependent Inhibitory Effect on vRNP Activity"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12163724). *BioChem*. **5** (2): 10. [doi](/source/Doi_(identifier)):[10.3390/biochem5020010](https://doi.org/10.3390%2Fbiochem5020010). [ISSN](/source/ISSN_(identifier)) [2673-6411](https://search.worldcat.org/issn/2673-6411). [PMC](/source/PMC_(identifier)) [12163724](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12163724).

1. **[^](#cite_ref-Lesieur_6-0)** Lesieur, Claire (18 June 2014). [*Oligomerization of Chemical and Biological Compounds*](https://books.google.com/books?id=vjehDwAAQBAJ&dq=%22protein+subunit%22+oligomer&pg=PA240). Croatia: Intech. pp. 240–241. [ISBN](/source/ISBN_(identifier)) [978-953-51-1617-2](https://en.wikipedia.org/wiki/Special:BookSources/978-953-51-1617-2). Retrieved 15 April 2022.

1. **[^](#cite_ref-Ahmad_7-0)** Ahmad, Zulfiqar; Okafor, Florence; Azim, Sofiya; Laughlin, Thomas F. (2013). ["ATP Synthase: A Molecular Therapeutic Drug Target for Antimicrobial and Antitumor Peptides"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4734648). *Current Medicinal Chemistry*. **20** (15): 1956–1973. [doi](/source/Doi_(identifier)):[10.2174/0929867311320150003](https://doi.org/10.2174%2F0929867311320150003). [ISSN](/source/ISSN_(identifier)) [0929-8673](https://search.worldcat.org/issn/0929-8673). [PMC](/source/PMC_(identifier)) [4734648](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4734648). [PMID](/source/PMID_(identifier)) [23432591](https://pubmed.ncbi.nlm.nih.gov/23432591).

1. **[^](#cite_ref-IUPAC1966_8-0)** Jenkins, A. D.; Kratochvíl, P.; Stepto, R. F. T.; Suter, U. W. (1996). ["Glossary of basic terms in polymer science (IUPAC Recommendations 1996)"](https://doi.org/10.1351%2Fpac199668122287). *[Pure and Applied Chemistry](/source/Pure_and_Applied_Chemistry)*. **68** (12): 2287–2311. [doi](/source/Doi_(identifier)):[10.1351/pac199668122287](https://doi.org/10.1351%2Fpac199668122287).Quote: *Oligomer molecule: A molecule of intermediate relative molecular mass, the structure of which essentially comprises a small plurality of units derived, actually or conceptually, from molecules of lower relative molecular mass.*

1. **[^](#cite_ref-Liu_9-0)** Liu, Shijie (7 April 2020). [*Bioprocess Engineering: Kinetics, Sustainability, and Reactor Design*](https://books.google.com/books?id=XdPVDwAAQBAJ&dq=oligomeric++hemoglobin&pg=PA358). Elsevier. p. 358. [ISBN](/source/ISBN_(identifier)) [978-0-12-822383-3](https://en.wikipedia.org/wiki/Special:BookSources/978-0-12-822383-3). Retrieved 15 April 2022.

1. **[^](#cite_ref-Dharan_10-0)** Dharan, Adarsh; Campbell, Edward M. (31 July 2018). ["Role of Microtubules and Microtubule-Associated Proteins in HIV-1 Infection"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6069196). *Journal of Virology*. **92** (16): e00085–18. [doi](/source/Doi_(identifier)):[10.1128/JVI.00085-18](https://doi.org/10.1128%2FJVI.00085-18). [ISSN](/source/ISSN_(identifier)) [0022-538X](https://search.worldcat.org/issn/0022-538X). [PMC](/source/PMC_(identifier)) [6069196](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6069196). [PMID](/source/PMID_(identifier)) [29899089](https://pubmed.ncbi.nlm.nih.gov/29899089).

1. **[^](#cite_ref-Søberg_11-0)** Søberg, Kristoffer; Skålhegg, Bjørn Steen (12 September 2018). ["The Molecular Basis for Specificity at the Level of the Protein Kinase a Catalytic Subunit"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6143667). *Frontiers in Endocrinology*. **9**: 538. [doi](/source/Doi_(identifier)):[10.3389/fendo.2018.00538](https://doi.org/10.3389%2Ffendo.2018.00538). [ISSN](/source/ISSN_(identifier)) [1664-2392](https://search.worldcat.org/issn/1664-2392). [PMC](/source/PMC_(identifier)) [6143667](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6143667). [PMID](/source/PMID_(identifier)) [30258407](https://pubmed.ncbi.nlm.nih.gov/30258407).

1. **[^](#cite_ref-pmid22381618_12-0)** Daniel M, Peek GW, Tollefsbol TO (2012). ["Regulation of the human catalytic subunit of telomerase (hTERT)"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3312932). *[Gene](/source/Gene_(journal))*. **498** (2): 135–46. [doi](/source/Doi_(identifier)):[10.1016/j.gene.2012.01.095](https://doi.org/10.1016%2Fj.gene.2012.01.095). [PMC](/source/PMC_(identifier)) [3312932](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3312932). [PMID](/source/PMID_(identifier)) [22381618](https://pubmed.ncbi.nlm.nih.gov/22381618).

1. **[^](#cite_ref-13)** Carpenter CL, Duckworth BC, Auger KR, Cohen B, Schaffhausen BS, Cantley LC (November 1990). ["Purification and characterization of phosphoinositide 3-kinase from rat liver"](https://doi.org/10.1016%2FS0021-9258%2817%2945429-9). *J. Biol. Chem*. **265** (32): 19704–11. [doi](/source/Doi_(identifier)):[10.1016/S0021-9258(17)45429-9](https://doi.org/10.1016%2FS0021-9258%2817%2945429-9). [PMID](/source/PMID_(identifier)) [2174051](https://pubmed.ncbi.nlm.nih.gov/2174051).

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Adapted from the Wikipedia article [Protein subunit](https://en.wikipedia.org/wiki/Protein_subunit) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Protein_subunit?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.