3. ECM & MISFOLDED PROTEINS:
Tuesday, July 25, 2023
Pauling & Anfinsen:
Certain key a a at proper positions are essential for folding of proteins into a
functional, unique 3-D conformation.
The process of directing & targeting the folding of intermediate polypeptide to the fully
folded structure is aided, by proteins k/a MOLECULAR CHAPERONES(Chaperonins).
4. Tuesday, July 25, 2023
Protein folding generally occurs via a stepwise process.
Auxiliary proteins (chaperones) assist in protein folding.
Chaperone proteins participate in the folding of over half of all mammalian proteins.
The HSP70 (70-kDa heat shock protein) family of chaperones binds short sequences of
hydrophobic amino acids that emerge while a new polypeptide is being synthesized,
shielding them from solvent.
5. Tuesday, July 25, 2023
Chaperones prevent aggregation, thus providing an opportunity for the
formation of appropriate secondary structural elements and their subsequent
coalescence into a molten globule.
The HSP60 family of chaperones (chaperonins), differs in sequence and
structure from HSP70 and its homologs. HSP60 acts later in the folding process,
often together with an HSP70 chaperone.
The central cavity of the doughnut-shaped HSP60 chaperone provides a
sheltered nonpolar environment in which a polypeptide can fold until all
hydrophobic regions are buried in its interior, thus preempting any tendency
toward aggregation.
6. Tuesday, July 25, 2023
Altered pathologic conformation of proteins may have pathologic consequences.
Prion diseases are protein conformation diseases transmitted by altering the
conformation, and hence the physical properties, of proteins endogenous to the
host. Human prion-related protein (PrP), a glycoprotein encoded on the short
arm of chromosome 20, normally is monomeric and rich in ฮฑ helix.
7. Tuesday, July 25, 2023
Pathologic prion proteins serve as the templates for the conformational
transformation of normal PrP, known as PrPc, into PrPsc.
PrPsc is rich in ฮฒ sheet with many hydrophobic aminoacyl side chains exposed
to solvent. As each new PrPsc molecule is formed, it triggers the production of
yet more pathologic variants in a conformational chain reaction.
Because PrPsc molecules associate strongly with one another through their
exposed hydrophobic regions, the accumulating PrPsc units coalesce to form
insoluble protease-resistant aggregates.
8. Tuesday, July 25, 2023
Glycoproteins & Calnexin Ensure Correct Folding of Proteins in the ER
Calnexin is a chaperone protein in the endoplasmic reticulum membrane;
binding to calnexin prevents a glycoprotein from aggregating. It is a
lectin, recognizing specific carbohydrate sequences in the glycan chain of
the glycoprotein.
Incorrectly folded glycoproteins undergo partial deglycosylation, and are
targeted to undergo transport from the endoplasmic reticulum back to the
cytosol for catabolism.
The glucosyltransferase senses the folding of the glycoprotein and only
reglucosylates misfolded proteins. The soluble endoplasmic reticulum
protein calreticulin performs a similar function to that of calnexin.
9. Aggregated Proteins Are Highly Refractory to Degradation or Repair
Tuesday, July 25, 2023
Modifications to a proteinโs composition or conformation that cause it to
adhere to other protein molecules can lead to the formation of toxic
aggregates, called amyloid.
Such aggregates are a hallmark of several neurodegenerative diseases,
including Parkinson, Alzheimer, Huntington disease, spinocerebellar
ataxias, and the transmissible spongiform encephalopathies.
The toxic effects of these insoluble aggregates are exacerbated by their
persistence, as they are generally refractory to degradation by the
proteases normally responsible for protein turnover.
10. Beta-Thalassemias
Tuesday, July 25, 2023
Thalassemias are caused by genetic defects that impair the synthesis of one of
the polypeptide subunits of hemoglobin.
During the burst of hemoglobin synthesis that occurs during erythrocyte
development, a specific chaperone called ฮฑ-hemoglobinโstabilizing protein
(AHSP) binds to free hemoglobin ฮฑ-subunits awaiting incorporation into the
hemoglobin multimer.
In the absence of this chaperone, free ฮฑ-hemoglobin subunits aggregate, and the
resulting precipitate has cytotoxic effects on the developing erythrocyte.
AHSP has been found to modulate the severity of ฮฒ-thalassemia in human
subjects.
11. Alzheimer Disease
Tuesday, July 25, 2023
Refolding or misfolding of another protein endogenous to human brain tissue,
ฮฒ-amyloid, is a prominent feature of the Alzheimer disease.
The characteristic senile plaques and neurofibrillary bundles contain
aggregates of the protein ฮฒ-amyloid, a 4.3-kDa polypeptide produced by
proteolytic cleavage of a larger protein known as amyloid precursor protein.
In Alzheimer disease patients, levels of ฮฒ-amyloid become elevated, and this
protein undergoes a conformational transformation from a soluble ฮฑ helixโ rich
state to a state rich in ฮฒ sheet and prone to self-aggregation.
Apolipoprotein E has been implicated as a potential mediator of this
conformational transformation.
12. Alzheimer Disease
Tuesday, July 25, 2023
MAO oxidatively deaminates amines including the neurotransmitters
DOPAMINE & SEROTONIN.
PARKINSONISM: Low levels of Dopamine, depression d/t low serotonin.
Deprenyl
Iproniazid
N,N-Dimethyl propargylamine
Suicidal inhibitors of MAO, act by alkylating FAD prosthetic group that also
decreases the toxic formation.
Moclobemide: MAO inhibitor as antidepressant.
SOD, Cu/Zn cytosolar form
13. Tuesday, July 25, 2023
Intracellular degradation of dysfunctional and damaged protein takes place
through:
1. Autophagy
2. UPS
Disruption of autophagy is implicated in several protein aggregating
diseases (Huntington, Alzheimerโs, Parkinsonโs).
UPS is an energy (ATP) dependent pathway of protein degradation.
14. Tuesday, July 25, 2023
If a protein can not be folded into its proper conformation, chaperones
assist in its destruction in the UP Complex.
Failure of ubiquitin proteosome system leads to several pathologic
consequences (neurodegenerative diseases).
15.
16. Ubiquitin
Tuesday, July 25, 2023
Key molecule in protein degradation
Small protein with 76 amino acids
Attachment to ubiquitin to protein to be degraded is called KISS OF DEATH.
Ubiquitin bind to ษ-amino group of lysine of target protein. So, it is a
pseudopeptide, isopeptide or non-ฮฑ-peptide bond.
Minimum 4 ubiquitin molecules must be attached to commit target molecule
to degradation.
N-end rule of ubiquitin binding: It bind to target protein with PEST sequence
in amino-terminal.
17. Proteasome
Tuesday, July 25, 2023
Ubiquitinylated proteins are degraded in proteasomes.
Located in the cytosol.
Large cylindrical structure composed of 50 subunits.
ATP dependant process.
It is a large cylindrical structure. It is composed of:
Four rings with a hollow core containing the protease active sites.
One or two caps or regulatory particles that recognize the
polyubiquinylated substrates.
18. UPS:
Tuesday, July 25, 2023
Ubiquitin attaches to target protein on lysine amino acid.
3 Proteins are required for attachment and degradation of target protein.
1. E1: Ubiquitin activating enzyme
2. E2: Ubiquitin conjugating enzyme
3. E3: Ubiquitin ligase enzyme
