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.
^
abAlberts, 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.
^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. Retrieved 15 April 2022.