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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: oligomeric: hemoglobin, DNA polymerase, nucleosomes and multimeric: ion channels, 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 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 polypeptide chain has one gene coding for it - meaning that a protein must 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 α. Three of these are present in the ATP synthase molecule, and is therefore designated α3. Larger groups of subunits can also the specified, like α3β3-hexamer and c-ring.