Hsp70: Difference between revisions

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Alexandria Spurgeon, Alexander Berchansky, Michal Harel

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 '''Mechanism of Substrate Binding Domain and ATPase when ATP Binds (Allostery)'''
 Chaperons require energy do to their job. The structure within the chaperon in which ATP hydrolysis occurs in the Nucleotide Binding Domain (NBD) is directly attached to the Substrate Binding Domain (SBD). This is a favorable set up as the energy produced by ATP hydrolysis can be directly coupled with a change in shape of the substrate binding domain that allows for substrate folding/refolding. The interaction between the protein’s function and ATP binding as well as peptide binding is known to be allosteric, or the binding of a molecule to the protein regulates, or transmits a signal, to another area of the protein either enhancing or inhibiting function in that area/domain.
-A <scene name='81/813405/Scene_alex_4/1'>proline switch</scene> has been discovered in position 147 of human Hsp70. A study done to understand this mechanism used E. coli’s DnaK which is a homolog of human Hsp70. Proline 143 is the corresponding residue in DnaK for Proline 147 in the ATPase domain. This proline is universally conserved and undertakes alternate conformations in response to ATP binding and hydrolysis. This proline is directly involved in catalytic residue positioning by facilitating the contact between Lys70 (Lys71 in humans)… and/or Glu171 (Glu175 in humans). Changing the Proline to an Alanine or Glycine residue affected Lysine70’s positioning in the catalytic domain. Furthermore, lack of an extra amide hydrogen in proline, as opposed to Alanine and Glycine’s extra hydrogen on their amide group, seems to be beneficial for interaction with the Glutamine171 residue <ref name="Vogel">Vogel, M., Bukau, B., & Mayer, M. P. (2006). Allosteric Regulation of Hsp70 Chaperones by a Proline Switch. Molecular Cell, 21(3), 359-367. doi:10.1016/j.molcel.2005.12.017</ref>. Both findings show how critical the proline residue is for catalytic domain function because without it the rate of ATP hydrolysis is greatly reduced. The the Lys 70 and Glu171 are positioned ideally for nucleophilic attack by water to hydrolyze the bound ATP. This process then sends a signal to the SBD to open its pocket which finally allows substrate binding <ref name="Vogel">.
+An image of the residues in the NBD contact with the ADP can be viewed <scene name='81/813405/Contact_to_adp/1'>here</scene>.
-An image of the residues in the NBD contact with the ADP can be viewed <scene name='81/813405/Contact_to_adp/1'>here</scene>.
+ATP is not the only means of allosteric control on this enzyme. When a polypeptide binds to the SBD it actually decreases the stabilization between the SBD and NBD domains. The SBD change caused by polypeptide binding is transmitted to the NBD which increases the rate of ATP Hydrolysis in that domain <ref>Young J. C. (2010). Mechanisms of the Hsp70 chaperone system. Biochemistry and cell biology = Biochimie et biologie cellulaire, 88(2), 291-300</ref>.
 The overall picture is this: In the ATP-bound state, the SBD pocket is open and ready for the substrate, a polypeptide, to bind. The binding of a polypeptide makes the ATP-bound state less stable and favor the ADP-bound state promoting hydrolysis of the ATP. This provides energy for the folding of the bound polypeptide. Once the energy is used up and the protein is folded, Hsp70 binds a new ATP. Because of the newly bound ATP, the chaperon will have less affinity for the substrate and release the newly folded protein so it can once again fulfill its role within the cell.