Single protein molecule that assembles with other protein molecules to form a protein complex
Rendering ofHLA-A11 showing theα (A*1101 gene product) andβ (Beta-2 microglobulin) subunits. This receptor has abound peptide (in thebinding 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.[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 asoligomeric.[8] For example,hemoglobin is a symmetrical arrangement of two identical α-globin subunits and two identical β-globin subunits.[3][9] Longermultimeric proteins such asmicrotubules and othercytoskeleton 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 enzymaticallycatalyzes a reaction, whereas a "regulatory subunit" will facilitate or inhibit the activity.[11] Althoughtelomerase hastelomerase 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 aholoenzyme. 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 onepolypeptide chain. A polypeptide chain has onegene coding for it – meaning that a protein must have one gene for each unique subunit.
^abAlberts, Bruce; Johnson, Alexander; Lewis, Julian; Raff, Martin; Roberts, Keith; Walter, Peter (2002).The Shape and Structure of Proteins. New York: Garland Science. Retrieved15 April 2022.
^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.ISBN978-81-7371-254-8. Retrieved15 April 2022.
