<P> 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 from 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 . </P> <P> 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 . </P> <P> 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 α . Larger groups of subunits can also the specified, like α β - hexamer and c - ring . </P>

Where do the basic subunits of proteins come from