In structural biology, a protein subunit is a single protein molecule that assembles (or "coassembles") with other protein molecules to form a protein complex.
Some naturally occurring proteins have a relatively small number of subunits and therefore described as oligomeric, for example hemoglobin or DNA polymerase.
Others may consist of a very large number of subunits and therefore described as multimeric, for example microtubules and other cytoskeleton proteins.
The subunits of a multimeric protein 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.
Although telomerase has telomerase reverse transcriptase as a catalytic subunit, regulation is accomplished by factors outside the protein.
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.
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.
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.
For example, ATP synthase has a type of subunit called α.
Three of these are present in the ATP synthase molecule, and is therefore designated α3.
Larger groups of subunits can also be specified, like α3β3-hexamer and c-ring.
Fusion protein
Subunit vaccines
See also
Protein quaternary structure
Allostery
Cooperativity
Monomer
References
