4. Protein folding, misfiling and degradation Flashcards
sign of protein misfolding
hydrophobic patches exposed on protein surface
spontaneous refolding
- protein synthesised from N-terminus to C-terminus
- N-terminal region starts to fold before C terminal region is synthesised
chaperones
proteins that help guide protein folding along productive pathways by permitting misfolded proteins to return to a proper pathway
what do chaperones recognise?
exposed hydrophobic patches
condition for chaperone upregulation
when misfolded proteins accumulate
chaperone functions (5)
- fold newly made proteins into functional conformations
- refiled misfolded or unfolded proteins into functional conformations
- disassemble potentially toxic aggregates
- assemble and dismantle large multiprotein complexes
- mediate transformations between inactive/active forms of proteins
how do chaperones work?
through ATP-dependent cycles, blocking exposed hydrophobic patches
2 major classes of chaperones
- molecular chaperones
- chaperonins
molecular chaperones overview
- operate as single molecules
- i.e. Hsp70: Heat shock protein
- bind to exposed hydrophobic residues of nascent proteins
- protect from aggregation until properly folded
- cycle of client protein binding and conformational change associated with ATP binding and hydrolysis
Hsp70 function
help newly-synthesised proteins follow correct folding pathways
molecular chaperone cycle (Hsp70)
- substrate binding site available on chaperone: unfolded protein binds to it
- ATP bound to nucleotide-binding domain leads to hydrolysis of ATP, producing ADP
- ADP bound: substrate binding site blocked –> misfolding blocked
- ADP exchanged to ATP: open binding site
- protein released and folds correctly
chaperonins overview
- form enclosed multisubunit refolding chamber made of inward-facing protein binding subunits
- i.e. Hsp60
- undergo concerted ATP-binding/hydrolysis and conformational change
2 types of chaperonins
- group I chaperonins
- group II chaperonins
group I chaperonins features
- 2 rings (7 subunits) + co-chaperone lid (7 subunits)
- each ring is a folding chamber where an unfolded protein enters
- found in bacteria, mitochondria
group II chaperonins features
- no separate lids: lid function is ATP dependent
- 2 rings (8/9 homomeric/heteromeric subunits)
- found in eukaryotic cytoplasm
group I chaperonin process (Hsp60)
- misfolded protein binds in chamber –> hydrolysis of ATP to ADP removes bottom cap (GroES)
- addition of ATP: GroES cap binds to top, sealing the protein
- addition of ATP: GroES cap binds to bottom –> top GroES cap falls off
- protein released properly
what happens to irretrievably misfolded proteins?
they are destroyed by proteolytic cleavage into small fragments
what shows chaperones are essential for life?
highly preserved in amino acid sequence, very ancient
E1
ubquitin-activating enzyme
E2
ubiquitin-conjugating enzyme
E3
ubiquitin ligase
E1 function
activate ubiquitin using energy of ATP, and transfer activated ubiquitin to E2
E2 function
transfers the activated ubiquitin to the target protein
E3 function
- necessary for E2 to work: instructs E2 which proteins to ubiquitinate
- recognises misfolded/damaged proteins
