Information AboutChaperonin |
| CATEGORIES ABOUT CHAPERONIN | |
| proteins | |
| cell biology | |
| wikiproject molecular and cellular biology articles | |
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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 Chaperone s. 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. 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. 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 an 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. 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. REFERENCES
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