Protein subunit

Rendering of HLA-A11 showing the α (A*1101 gene product) and β (Beta-2 microglobin) subunits. This receptor has a bound peptide (in the binding pocket) of heterologous origin that also contributes to function.

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.

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 of the protein.^[1] 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.^[2] 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 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.

Subunit 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 become associated with antibodies different from the desired antibodies. A second method of making a subunit vaccine involves putting an antigen's gene from the targeted virus or bacterium into another virus (virus vector), yeast (yeast vector in the case of the hepatitis B vaccine^[3] or attenuated bacterium (bacterial vector) to make a recombinant virus or bacteria to serve as the important component of a recombinant vaccine (called a recombinant subunit vaccine). The recombinant vector that is genomically modified will express the antigen. The antigen (one or more subunits of protein) is extracted from the vector.^[4] Just like the highly successful subunit vaccines, the recombinant-vector-produced antigen will be of little to no risk to the patient. This is the type of vaccine currently in use for hepatitis B,^[5] and it is experimentally popular, being used to try to develop new vaccines for difficult-to-vaccinate-against viruses such as ebolavirus and HIV.^[6]

Vi capsular polysaccharide vaccine (ViCPS) is another subunit vaccine (contains the signature polysaccharide linked to the Vi capsular antigen), in this case, against typhoid caused by the Typhi serotype of Salmonella.^[7] It is also called a conjugate vaccine, in which a polysaccharide antigen has been covalently attached to a carrier protein for T-cell-dependent antigen processing (utilizing MHC II).^[8]

See also

• Protein quaternary structure
• Allostery
• Cooperativity
• Monomer

References

• Dilip Gore; Reecha Pandit (2011). "In silico Identification of Cell Surface Antigens in Neisseria meningitidis". Biomirror. 2: 1–5.