protein subunit
{{Short description|Single protein molecule that assembles with other protein molecules to form a protein complex}}
Image:HLA-A11.png 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.{{cite book |last1=Stoker |first1=H. Stephen |title=General, Organic, and Biological Chemistry |date=1 January 2015 |publisher=Cengage Learning |location=Boston, MA |isbn=978-1-305-68618-2 |pages=709–710 |edition=7th |url=https://books.google.com/books?id=IBGdBQAAQBAJ&pg=PA709 |access-date=15 April 2022 |language=en}}{{cite book |last1=Smith |first1=Michael B. |title=Biochemistry: An Organic Chemistry Approach |date=27 April 2020 |publisher=CRC Press |location=Boca Raton |isbn=978-1-351-25807-4 |pages=269–270 |url=https://books.google.com/books?id=0TXfDwAAQBAJ&pg=PA269 |access-date=15 April 2022 |language=en}}{{cite book |last1=Alberts |first1=Bruce |last2=Johnson |first2=Alexander |last3=Lewis |first3=Julian |last4=Raff |first4=Martin |last5=Roberts |first5=Keith |last6=Walter |first6=Peter |title=The Shape and Structure of Proteins |date=2002 |publisher=Garland Science |location=New York |url=https://www.ncbi.nlm.nih.gov/books/NBK26830/ |access-date=15 April 2022 |language=en}}
Large assemblies of proteins such as viruses often use a small number of types of protein subunits as building blocks.{{cite book |editor-last1=Vijayan |editor-first1=M. |editor-last2=Yathindra |editor-first2=N. |editor-last3=Kolaskar |editor-first3=A. S. |author-last1=Kumar |author-first1=A. |author-last2=Evarsson |author-first2=A. |author-last3=Hol |author-first3=W. G. J. |title=Perspectives in Structural Biology: A Volume in Honour of G.N. Ramachandran | chapter=Multi-protein assemblies with point group symmetry |date=1999 |publisher=Universities Press |location=Hyderabad, India |isbn=978-81-7371-254-8 |pages=449–466 |chapter-url=https://books.google.com/books?id=xTy_M3B5sf4C&pg=PA462 |access-date=15 April 2022 |language=en}}{{Cite journal |last1=Witwit |first1=Haydar |last2=de la Torre |first2=Juan C. |date=2025-04-29 |title=Mammarenavirus Z Protein Myristoylation and Oligomerization Are Not Required for Its Dose-Dependent Inhibitory Effect on vRNP Activity |journal=BioChem |language=en |volume=5 |issue=2 |pages=10 |doi=10.3390/biochem5020010 |doi-access=free |issn=2673-6411}}
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.{{cite book |last1=Lesieur |first1=Claire |title=Oligomerization of Chemical and Biological Compounds |date=18 June 2014 |publisher=Intech |location=Croatia |isbn=978-953-51-1617-2 |pages=240–241 |url=https://books.google.com/books?id=vjehDwAAQBAJ&dq=%22protein+subunit%22+oligomer&pg=PA240 |access-date=15 April 2022 |language=en}} 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.{{cite journal |last1=Ahmad |first1=Zulfiqar |last2=Okafor |first2=Florence |last3=Azim |first3=Sofiya |last4=Laughlin |first4=Thomas F. |title=ATP Synthase: A Molecular Therapeutic Drug Target for Antimicrobial and Antitumor Peptides |journal=Current Medicinal Chemistry |date=2013 |volume=20 |issue=15 |pages=1956–1973 |doi=10.2174/0929867311320150003 |pmid=23432591 |pmc=4734648 |issn=0929-8673}}
Naturally occurring proteins that have a relatively small number of subunits are referred to as oligomeric.{{cite journal|title=Glossary of basic terms in polymer science (IUPAC Recommendations 1996)|journal=Pure and Applied Chemistry|year=1996|volume=68|issue=12|pages=2287–2311|doi=10.1351/pac199668122287|last1=Jenkins|first1=A. D.|last2=Kratochvíl|first2=P.|last3=Stepto|first3=R. F. T.|last4=Suter|first4=U. W.|doi-access=free}}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. For example, hemoglobin is a symmetrical arrangement of two identical α-globin subunits and two identical β-globin subunits.{{cite book |last1=Liu |first1=Shijie |title=Bioprocess Engineering: Kinetics, Sustainability, and Reactor Design |date=7 April 2020 |publisher=Elsevier |isbn=978-0-12-822383-3 |page=358 |url=https://books.google.com/books?id=XdPVDwAAQBAJ&dq=oligomeric++hemoglobin&pg=PA358 |access-date=15 April 2022 |language=en}} 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).{{cite journal |last1=Dharan |first1=Adarsh |last2=Campbell |first2=Edward M. |title=Role of Microtubules and Microtubule-Associated Proteins in HIV-1 Infection |journal=Journal of Virology |date=31 July 2018 |volume=92 |issue=16 |pages=e00085–18 |doi=10.1128/JVI.00085-18 |pmid=29899089 |pmc=6069196 |issn=0022-538X}}
The subunits of a protein complex may be identical, homologous or totally dissimilar and dedicated to disparate tasks.
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.{{cite journal |last1=Søberg |first1=Kristoffer |last2=Skålhegg |first2=Bjørn Steen |title=The Molecular Basis for Specificity at the Level of the Protein Kinase a Catalytic Subunit |journal=Frontiers in Endocrinology |date=12 September 2018 |volume=9 |pages=538 |doi=10.3389/fendo.2018.00538 |pmid=30258407 |pmc=6143667 |issn=1664-2392|doi-access=free }} Although telomerase has telomerase reverse transcriptase as a catalytic subunit, regulation is accomplished by factors outside the protein.{{cite journal |vauthors=Daniel M, Peek GW, Tollefsbol TO | title=Regulation of the human catalytic subunit of telomerase (hTERT) | journal= Gene | volume=498 | issue=2 | year=2012 | pages=135–46 | doi=10.1016/j.gene.2012.01.095 | pmc=3312932 | pmid=22381618}}
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.{{cite journal |vauthors=Carpenter CL, Duckworth BC, Auger KR, Cohen B, Schaffhausen BS, Cantley LC |title=Purification and characterization of phosphoinositide 3-kinase from rat liver |journal=J. Biol. Chem. |volume=265 |issue=32 |pages=19704–11 |date=November 1990 |doi=10.1016/S0021-9258(17)45429-9 |pmid=2174051 |doi-access=free }} 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.