Protein subunit

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.

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] [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 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. ^[8] For example, hemoglobin is a symmetrical arrangement of two identical α-globin subunits and two identical β-globin subunits. ^{[3] [9]} 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). ^[10]

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. ^[11] Although telomerase has 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. For example, class I phosphoinositide 3-kinase is composed of a p110 catalytic subunit and a p85 regulatory subunit. ^[13] 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

• Allostery
• Cooperativity
• Fusion protein
• Monomer
• Protein quaternary structure
• Subunit vaccine

References

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2. Smith, Michael B. (27 April 2020). Biochemistry: An Organic Chemistry Approach. Boca Raton: CRC Press. pp. 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.
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6. 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.
7. 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.
8. 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.
9. 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.
10. 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.
11. 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.
12. 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.
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". J. Biol. Chem. 265 (32): 19704–11. doi: 10.1016/S0021-9258(17)45429-9 . PMID   2174051.