Protein subunit

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 protein complex: 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 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 &alpha;. Three of these are present in the ATP synthase molecule, and is therefore designated &alpha;3. Larger groups of subunits can also the specified, like &alpha;3&beta;3-hexamer and c-ring.

Vaccines
A subunit vaccine presents an antigen to the immune system without introducing viral particles, whole or otherwise. One method of production involves isolation of a specific protein from a virus and administering this by itself. A weakness of this technique is that isolated proteins can be denatured and will then bind to different antibodies than the proteins in the virus. A second method of subunit vaccine is the recombinant vaccine, which involves putting a protein gene from the targeted virus into another virus. The second virus will express the protein, but will not present a risk to the patient. This is the type of vaccine currently in use for hepatitis, and it is experimentally popular, being used to try to develop new vaccines for difficult to vaccinate viruses such as Ebola and HIV.

Vi capsular polysaccharide vaccine (ViCPS) is another subunit vaccine, in this case against typhoid.