Protein subunit

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

Rendering of 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) of heterologous origin that also contributes to function.

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.[5] For example, 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.[6]

Naturally-occurring proteins that have a relatively small number of subunits are referred to as oligomeric.[7] For example, hemoglobin is a symmetrical arrangement of two identical α-globin subunits and two identical β-globin subunits.[3][8] Longer multimeric proteins such as microtubules and other cytoskeleton proteins may consist of very large numbers of subunits. For example, 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).[9]

The subunits of a protein complex may be identical, homologous or totally dissimilar and dedicated to disparate tasks.[1] In some protein assemblies, one subunit may be a "catalytic subunit" that enzymatically catalyzes a reaction, whereas a "regulatory subunit" will facilitate or inhibit the activity.[10] Although telomerase has telomerase reverse transcriptase as a catalytic subunit, regulation is accomplished by factors outside the protein.[11]

An enzyme composed of both regulatory and catalytic subunits when assembled is often referred to as a holoenzyme. For example, class I phosphoinositide 3-kinase is composed of a p110 catalytic subunit and a p85 regulatory subunit.[12] One subunit is made of one polypeptide chain. A polypeptide chain has one gene coding for it – meaning that a protein must have one gene for each unique subunit.

See also


  1. Stoker, H. Stephen (1 January 2015). General, Organic, and Biological Chemistry (7th ed.). Boston, MA: Cengage Learning. pp. 709–710. ISBN 978-1-305-68618-2. Retrieved 15 April 2022.
  2. Smith, Michael B. (27 April 2020). Biochemistry: An Organic Chemistry Approach. Boca Raton: CRC Press. p. 269–270. ISBN 978-1-351-25807-4. Retrieved 15 April 2022.
  3. Alberts, Bruce; Johnson, Alexander; Lewis, Julian; Raff, Martin; Roberts, Keith; Walter, Peter (2002). The Shape and Structure of Proteins. New York: Garland Science. Retrieved 15 April 2022.
  4. Kumar, A.; Evarsson, A.; Hol, W. G. J. (1999). "Multi-protein assemblies with point group symmetry". 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 978-81-7371-254-8. Retrieved 15 April 2022.
  5. Lesieur, Claire (18 June 2014). Oligomerization of Chemical and Biological Compounds. Croatia: Intech. pp. 240–241. ISBN 978-953-51-1617-2. Retrieved 15 April 2022.
  6. Ahmad, Zulfiqar; Okafor, Florence; Azim, Sofiya; Laughlin, Thomas F. (2013). "ATP Synthase: A Molecular Therapeutic Drug Target for Antimicrobial and Antitumor Peptides". Current Medicinal Chemistry. 20 (15): 1956–1973. doi:10.2174/0929867311320150003. ISSN 0929-8673. PMC 4734648. PMID 23432591.
  7. Jenkins, A. D.; Kratochvíl, P.; Stepto, R. F. T.; Suter, U. W. (1996). "Glossary of basic terms in polymer science (IUPAC Recommendations 1996)". Pure and Applied Chemistry. 68 (12): 2287–2311. doi:10.1351/pac199668122287.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.
  8. Liu, Shijie (7 April 2020). Bioprocess Engineering: Kinetics, Sustainability, and Reactor Design. Elsevier. p. 358. ISBN 978-0-12-822383-3. Retrieved 15 April 2022.
  9. Dharan, Adarsh; Campbell, Edward M. (31 July 2018). "Role of Microtubules and Microtubule-Associated Proteins in HIV-1 Infection". Journal of Virology. 92 (16): e00085–18. doi:10.1128/JVI.00085-18. ISSN 0022-538X. PMC 6069196. PMID 29899089.
  10. Søberg, Kristoffer; Skålhegg, Bjørn Steen (12 September 2018). "The Molecular Basis for Specificity at the Level of the Protein Kinase a Catalytic Subunit". Frontiers in Endocrinology. 9: 538. doi:10.3389/fendo.2018.00538. ISSN 1664-2392. PMC 6143667. PMID 30258407.
  11. Daniel M, Peek GW, Tollefsbol TO (2012). "Regulation of the human catalytic subunit of telomerase (hTERT)". Gene. 498 (2): 135–46. doi:10.1016/j.gene.2012.01.095. PMC 3312932. PMID 22381618.
  12. Carpenter CL, Duckworth BC, Auger KR, Cohen B, Schaffhausen BS, Cantley LC (November 1990). "Purification and characterization of phosphoinositide 3-kinase from rat liver". J. Biol. Chem. 265 (32): 19704–11. doi:10.1016/S0021-9258(17)45429-9. PMID 2174051.
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