Protein subunit

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 of 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 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 be 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), as 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 Typhiserotype 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

References

  1. Daniel M, Peek GW, Tollefsbol TO (2012). "Regulation of the human catalytic subunit of telomerase (hTERT)". Gene. 498 (2): 135–46. doi:10.1016/j.gene.2012.01.095. PMC 3312932. PMID 22381618.
  2. Carpenter CL, Duckworth BC, Auger KR, Cohen B, Schaffhausen BS, Cantley LC (November 1990). "Purification and characterization of phosphoinositide 3-kinase from rat liver". J. Biol. Chem. 265 (32): 19704–11. PMID 2174051.
  3. "Recombivax". Retrieved May 5, 2013.
  4. "Recombivax". Retrieved May 5, 2013.
  5. "Recombivax". Retrieved May 5, 2013.
  6. Department of Veterinary Science & Microbiology at The University of Arizona Archived 2003-06-10 at the Wayback Machine Vaccines by Janet M. Decker, PhD
  7. Manuela Raffatellu, Daniela Chessa, R. Paul Wilson, Richard Dusold, Salvatore Rubino, Andreas J. Bäumler (June 2005). "The Vi Capsular Antigen of Salmonella enterica Serotype Typhi Reduces Toll-Like Receptor-Dependent Interleukin-8 Expression in the Intestinal Mucosa". Infect. Immun. 73 (6). pp. 3367–74. doi:10.1128/IAI.73.6.3367-3374.2005.CS1 maint: uses authors parameter (link)
  8. Brenda A. Wilson, Abigail A. Salyers, Dixie D. Whitt, Malcolm E. Winkler. Bacterial Pathogenesis, A Molecular Approach, Third Edition. ASM Press, American Society for Microbiology, 1752 N St. NW, Washington, D.C. 20036-2904, copyrightyear=2011.CS1 maint: uses authors parameter (link)
  • Dilip Gore; Reecha Pandit (2011). "In silico Identification of Cell Surface Antigens in Neisseria meningitidis". Biomirror. 2: 1–5.
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