Bacterial chaperonins, such as GroEL, ensure proper folding of proteins by providing optimal micro conditions for folding, or provide aid and recovery of already incorrectly folded proteins and toxic protein aggregates. Improper translation of protein, mutations amongst the newly transcribed mRNA, or unfavorable cellular conditions (such as excessive heat) can result in partially incorrectly folded or completely incompletely folded proteins. These partially and completely nonfunctional proteins then often aggregate with each other, and form clumps within the cell. These protein aggregates are often highly toxic to the cell, as the often interfere with vital cellular process, and can disrupt vital cellular structures (such as the cell membrane.) Bacterial chaperonin GroEL provides a proper micro-environment that is able to alleviate protein aggregation, as well as restoring proper structure to proteins, consequently restoring protein function.
Structure
GroEL consists primarily of two 57 KiloDalton heptameric subunits. The first group of heptameric subunits creates the space where misfolded proteins can first associate with. The second group of heptameric proteins allows a specific microenvironment large enough to force improperly folded proteins to assume their correct structure. Each of the subunits involved in GroEL consists of three domains: an apical, intermediate, and equatorial domains. In the apical domain, facing the cavity of the protein, we find both alpha helices and beta pleated sheets. The amino acids in the interior of the first heptameric ring are essential for protein-GroEL interactions. As such, there is a ring of specific hydrophobic amino acids that form a ring on the interior of this first heptamer, namely Leucine, Isoleucine, Valine, Methionine, Phenylalanine, and Tyrosine. This ring of hydrophobic amino acids interacts with exposed hydrophobic regions on the improperly folding protein, which decreases free energy of the protein, effectively “coaxing” the protein into the GroEL. The intermediate domain connects the apical domain to the equatorial domain. The equatorial domain, which is found primarily on the outside of the GroEL, consist largely of hydrophilic amino acids. It is also here where ATP is able to bind. Upon ATP binding to the equatorial domain, GroES, a roughly 10 kDa cochaperone, can bind to GroEL. Once ATP and the GroES have bound to GroEL, a conformational change happens which forces the improperly folded protein into the second larger cavity within the protein. The “hydrophobic lip” in the first heptameric subunit then retracts away from the core protein, and the substrate of interest is essentially trapped and forced to interact with the amino acids in the apical domains found in the second heptameric subunit. Since these amino acids in the second heptameric ring are hydrophilic, the substrate of interest is then forced to correct its shape and fold properly (hydrophobic amino acids being buried within the protein.) Once this is complete, ATP hydrolysis allows the GroES chaperone to dissociate. It is at this point that the substrate of interest can leave the GroEL. The GroES co-chaperone can also bind to the first heptameric subunit, thus forcefully ejecting the properly folded protein from GroEL and into the cytosol.
Energetics
ATP is a crucial allosteric regulator for the GroEL protein. Each equatorial domain has the capacity to host an APT, allowing up to 7 ATP to bind to GroEL. Binding of ATP to the first heptameric ring creates conformational changes within GroEL, allowing the protein of interest to tall into the larger cavity and fold correctly. ATP hydrolysis (due to intrinsic ATPase activity within the equatorial subunits) allows the properly folded protein to then be released, as the GroES co chaperone dissociates from GroEL.
