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 oligomer
ic, for example hemoglobin
or DNA polymerase
. Others may consist from a very large number of subunits and therefore described as multimeric
, for example microtubule
s 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 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.
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.
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 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. 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, and it is experimentally popular, being used to try to develop new vaccines for difficult-to-vaccinate-against viruses such as ebolavirus
. Department of Veterinary Science & Microbiology at The University of Arizona Vaccines
by Janet M. Decker, PhD
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. 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).