Protein (wk 4)

Protein (wk 4)

1. Protein denaturation and folding

   • native prot. has conformational flexibility for function
     • balance between folded and transiently unfolded states
   • Proteostasis
     • 1. synthesis by ribosome 2. folding and refolding 3. sequestration and degradation
       • as proteins are synthesized in their ribosomes, they must fold into their native conformation, sometimes this happens spontanouesly, more often with specialized proteins and enzymes called chaperones
       • Proteins that are not properly folded often have exposed hydrophobic surfaces, which forms aggregates
         • aggregates are not inert and causes diseses
           • Parkinson's
           • diabetes
           • Alzheimer's
         • cells have elaborate pathways for recyclin or degradation of irreversibly misfolded proteins
     • enzymes and specialized proteins

2. Loss of Structure results in Loss of Function DENATURATION

   • A loss of three­dimensional structure sufficient to cause loss of function
     • not necessarily complete unfolding
   • Causes
     • Heat
       • gradual increase in temperature-> conformation remains intact until an abrupt change loss of structure and function
         • shows that protein folding is a cooperative process
           •  loss of structure in one part of the protein destabilizes other parts  
       • heat stable proteins only differ slightly from homologous proteins derived from bacteria such as E. coli. How those small differences contribute to structural stability in heat is little understood
     • extremes of pH
       • alter the net charge of protein
         • cause electrostatic repulsion and disruption of H bonds
     • miscible organic solvents
       • alcohol of acetone
     • solutes such as urea or guanidine hydrochloride

3. Amino acid sequence determines teritiary structure

   • Anfinsen
     • purify ribonuclease A in urea and a reducing reagent
       • renaturation- all the disulfide bonds are re-formed in the same positions
   • denaturation of some proteins is reversible

4. Polypeptides fold rapidly in with a stepwise process

   • Levinthal's paradox
     • folding of polypeptide is not a completely random, trial and error process
       • short cuts exist