cell bio module 2

• Molecular chaperones


  Monomeric proteins that function by binding to hydrophobic amino acid residues on a polypeptide and preventing the protein from forming incorrect folds
• Molecular chaperones


  • They are ubiquitous, seen in all organisms and all subcellular compartments where protein folding occurs
  • They are not specific to a subset of proteins, but can assist many different proteins with distinct structures and functions
• Molecular chaperones


  • Hsp (heat shock) proteins
  • BiP in the endoplasmic reticulum
  • DnaK in bacterial cells
• How Hsp70 molecular chaperone functions

  1. Hsp70 binds to hydrophobic patches on unfolded proteins

  2. ATP hydrolysis changes the conformation of Hsp70, allowing the target protein to fold properly

  3. ADP is released, new ATP binds, and the folded protein is released
• Chaperonins


  Large macromolecular complexes containing many different proteins that form a chamber or barrel for an unfolded protein to move into and fold in isolation
• Chaperonins


  • TRiC in the eukaryotic cytosol
  • GroEL in bacteria and chloroplast organelles
  • HSP60 in mitochondria
• How chaperonin (GroEL) function works

  1. GroEL has two chambers that work in an alternating fashion

  2. Unfolded protein binds to GroEL, GroES cap binds to close the chamber

  3. Conformational change enlarges the chamber, allowing the protein to fold in isolation

  4. ATP hydrolysis allows the GroES cap to come off, releasing the folded protein
• GroEL chaperonin


  • It is made up of 7 Hsp60 subunits, each binding to 1 ATP molecule
  • The individual conformational changes of the Hsp60 subunits lead to the overall chaperonin function
• If proteins cannot be folded, aggregates can form which can be detrimental to the cell
• Protein degradation


  If proteins cannot be folded, the final option is for the cell to remove these proteins via degradation in the proteasome
• Hsp60


  Binds to a single ATP molecule
• 7 ATP molecules are required for GroEL chamber activity at any one time
• GroEL chamber activity


  1. Each of the seven Hsp60 subunits has to change shape

  2. GroEL chamber is in the open (or tight conformation)

  3. GroEL is closed by the GroES lid (so it is in the relaxed conformation)
• It is the individual behavior of the Hsp60 subunits that lead to overall chaperonin function
• Chaperonin function


  Eight TRiC subunits move in the same way as the Hsp60 subunits, in order to coordinate the chaperonin function
• Occasionally proteins don't fold
• Unfolded proteins become a risk to a cell as they lack appropriate functions and are likely to aggregate and form complexes that are harmful to the cell
• Cells have a mechanism for removing proteins from the cell though degradation
• Proteins that are degraded

  • Misfolded proteins
  • Denatured proteins
  • Proteins present at a high concentration
  • Proteins endocytosed into the cell
  • Proteins cyclically made and degraded during the cell cycle
• Protein degradation


  1. Protein is tagged by the covalent attachment of ubiquitin

  2. Ubiquitin tag is recognized by the proteasome

  3. Target protein is cleaved into short peptide sequences
• Ubiquitinylation/ubiquitination


  Covalent attachment of ubiquitin to proteins targeted for degradation
• Ubiquitinylation process


  1. E1 ubiquitin activating enzyme recognizes free ubiquitin and picks it up

  2. E2 ubiquitin conjugating enzyme facilitates the attachment of ubiquitin to the target protein

  3. E3 ubiquitin ligase recognizes the specific target for degradation and attaches ubiquitin to it

  4. Multiple ubiquitins are subsequently added in a process called polyubiquitinylation
• Proteasome


  • Wall created by a series of identical subunits to create a hollow interior barrel, with caps at the ends enclosing the proteasome contents
  • Contains proteolytic enzymes that will breakdown any protein found within this core
• Proteasomal degradation


  1. Polyubiquitinated protein is recognized by the cap of the proteasome

  2. Target protein is unfolded as it enters the narrow opening of the cap

  3. Ubiquitins are removed prior to entry into the proteasome

  4. Target protein is cleaved by the non-specific proteolytic activity of the proteasome complex

  5. Small peptides leave at the other end of the proteasome
• Neurodegenerative disease: spinocerebellar ataxia

  • Mutation in the ataxin 1 gene creates a misfolded Ataxin protein
  • Misfolded protein is still tagged with ubiquitin and carried to the proteasome, but it cannot be unfolded
  • Ataxin protein builds up in the cell forming lethal aggregates
  • Misfolded proteins can prevent proteasome function by saturating the entry sites and preventing the appropriate degradation of many other proteins in the cell
• Cells are capable of synthesizing proteins through translation
• Protein folding may require assistance within the busy environment of the cell, which is the role played by chaperones and chaperonins
• Proteins that need to be removed from a cell can be removed due to the process of ubiquitinylation and the activity of the proteasome