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Encyclopedia > Chaperonin

Contents


Protein folding

A proportion of all newly-made proteins require assistance to convert from a linear chain of amino acids to a functional three-dimensional entity. This process is called protein folding. Chaperonins are protein complexes that assist the folding of these nascent, non-native polypeptides into their native, functional state. These proteins belong to a large class of molecules that assist protein folding, called molecular chaperones. The structure of these chaperonins resemble two donuts stacked on top of one another to create a barrel. Each ring is composed of either 7, 8 or 9 subunits depending on the organism in which the chaperonin is found. These molecular machines use chemical energy, in the form of Adenosine triphosphate (ATP), to promote protein folding in all cells. Protein folding is the process by which a protein structure assumes its functional shape or conformation. ... In biology, chaperones are proteins whose function is to assist other proteins in achieving proper folding. ... Adenosine 5-tripenis (ATP) is the nucleotide known in biochemistry as the molecular currency of intracellular energy transfer; that is, ATP is able to store and transport chemical energy within cells. ...


Categories of Chaperonins

Group I chaperonins are found in prokaryotes as well as organelles of endosymbiotic origin: chloroplasts and mitochondria. The GroEL/GroES complex in E. coli is a Group I chaperonin and the best characterized large (~ 1 MDa) chaperonin complex. GroEL is a double-ring 14mer with a greasy hydrophobic patch at its opening; it is so large it can accommodate native folding of 54-kDa GFP in its lumen. GroES is a single-ring heptamer that binds to GroEL in the presence of ATP or ADP. GroEL/GroES may not be able to undo previous aggregation, but it does compete in the pathway of misfolding and aggregation. See (Fenton and Horwich, 2003), and the articles on GroEL/GroES for more information. GroEL is a protein chaperone required for the proper folding of many proteins in prokaryotes. ... In chemistry, hydrophobic or lipophilic species, or hydrophobes, tend to be electrically neutral and nonpolar, and thus prefer other neutral and nonpolar solvents or molecular environments. ... The initialism GFP may refer to Green fluorescent protein, a fluorescent marker frequently used in biology. ... GroEL is a protein chaperone required for the proper folding of many proteins in prokaryotes. ... GroEL is a protein chaperone required for the proper folding of many proteins in prokaryotes. ... GroEL is a protein chaperone required for the proper folding of many proteins in prokaryotes. ...


Group II chaperonins, found in the eukaryotic cytosol and in archaebacteria, are more poorly characterized. TRiC (TCP-1 Ring Complex, also called CCT), the eukaryotic chaperonin, is composed of eight different though related subunits, each thought to be represented once per eight-membered ring. TRiC was originally thought to fold only the cytoskeletal proteins actin and tubulin but is now known to fold dozens of substrates. Group II chaperonins are not thought to utilize a GroES-type cofactor to fold their substrates. They instead contain a "built-in" lid that closes in an ATP-dependent manner to encapsulate its substrates, a process that is required for the proteins to fold.


Mechanism of action

Chaperonins undergo large conformational changes during a folding reaction as a function of the enzymatic hydrolysis of ATP. These conformational changes allow the chaperonin to bind a unfolded or misfolded protein, encapsulate that protein within one of the cavities formed by the two rings, and release the protein back into the cytosol. Upon release, the newly-made 'substrate' protein will either be folded or will require further rounds of folding, in which case it can again be bound by a chaperonin. Hydrolysis is a chemical process in which a molecule is cleaved into two parts by the addition of a molecule of water. ... ÃThe cytosol (as opposed fatty cytoplasm, which also includes the organelles) is the internal fluid of the cell, and a large part of cell metabolism occurs here. ...


Structural and functional homology is conserved

As mentioned, all cells contain chaperonins. In bacteria the archetype is the well-characterized chaperonin GroEL from E. coli, in archaea the chaperonin is called the thermosome and in eukarya the chaperonin is called CCT (also called TRiC or c-cpn). These protein complexes appear to be essential for life in E. coli, Saccharomyces cerevisiae and higher eukaryotes. While there are differences between eukaryotic, bacterial and archaeal chaperonins the general structure and mechanism are conserved. GroEL is a protein chaperone required for the proper folding of many proteins in prokaryotes. ... Phyla / Classes Phylum Crenarchaeota Phylum Euryarchaeota     Halobacteria     Methanobacteria     Methanococci     Methanopyri     Archaeoglobi     Thermoplasmata     Thermococci Phylum Korarchaeota Phylum Nanoarchaeota The Archaea (also called Archaebacteria) are a major division of living organisms. ... Kingdoms Eukaryotes are organisms with complex cells, in which the genetic material is organized into membrane-bound nuclei. ... Binomial name Escherichia coli T. Escherich, 1885 Escherichia coli (usually abbreviated to E. coli) is one of the main species of bacteria that live in the lower intestines of warm-blooded animals (including birds and mammals) and are necessary for the proper digestion of food. ... Binomial name Saccharomyces cerevisiae Meyen ex E.C. Hansen Saccharomyces cerevisiae is a species of budding yeast. ... Kingdoms Eukaryotes are organisms with complex cells, in which the genetic material is organized into membrane-bound nuclei. ...


References

  • Fenton and Horwich, Q Rev Biophys 36(2): 229-256, 2003.

See also

  • Heat shock protein
  • Molecular crowding

  Results from FactBites:
 
txt001jrl: Chaperonins are cell-signalling proteins: the unfolding biology of molecular chaperones (7205 words)
When ATP is bound to chaperonin 60, the chaperonin 10 forms a lid on top of the chaperonin 60 barrel (Refs 19, 20), and causes the central cavity to enlarge, thus aiding protein folding.
Chaperonin 60 preparations, even after purification by high-performance liquid chromatography (HPLC), can contain substantial amounts of other contaminating proteins (Ref. 51); thus, the reported bioactivity of the chaperonin 60 proteins could actually be due to such contaminating proteins.
Chaperonins are released from cells and/or are expressed on external cell membranes (of both eukaryotic and prokaryotic cells), and thereby become part of the population of proteins that act as cell-to-cell signals.
  More results at FactBites »

 
 

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