In structural biology, a protein subunit or subunit protein is a single protein molecule that assembles (or "coassembles") with other protein molecules to form a multimeric or oligomeric protein. Many naturally occurring proteins and enzymes are multimeric. Examples include hemoglobin, ion channels, DNA polymerase, nucleosomes and microtubules. 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 referred to as a "regulatory subunit" and another as a "catalytic subunit." An enzyme composed of both regulatory and catalytic subunits when assembled is often referred to as a holoenzyme.
One subunit is made of one polypeptide chain. A polypedtide chain have one gene coding for it, that means a protein needs to have one gene for each 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 &alpha. Three of these are present in the ATP synthase molecule, and is therefore designated &alpha3. Larger groups of subunits can also the specified, like &alpha3&beta3-hexamer and c-ring.
See also: quaternary structure -- allostery -- cooperativity
