Protein subunit

In 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 occuring 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.