Protein Degradation In Eukaryotes

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Protein Degradation In Eukaryotes

• Joyleen Collier
• 6/12/02

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Reasons for Protein Degradation

• To recognise and eliminate unassembled proteins.
• Dispose of damaged and misfolded proteins.
• Regulation of cellular metabolism, used to
  increase and decrease the number of enzyme
  molecules and regulatory substances.

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General Principles

• Proteolysis
• The degradation of a protein, usually by
  hydrolysis at one or more of its peptide bonds.

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Mechanisms Of Protein Degradation

• Lysosomal mechanism
• Degrade extra cellular proteins taken in by
  endocytosis.
• ATP- dependent cytosolically-based mechanism.
• Involves ATP, the protein is labeled with
  Ubiquitin.

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Ubiquitin Mediated Pathway
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• Proteins are tagged for destruction in
  proteolytic complexes called Proteasomes by
  attachment of Ubiquitin.
• Best mechanism of intracellular protein
  degradation.

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Ubiquitin

• Abundant in all Eukaryotes.
• Slowest rate of evolutionary divergence.
• Small protein, 76 residues, adopts a stable
  compact globular conformation with 4 strands of
  b- sheet and a single a- helix.
• Exits either free or covalently linked to
  proteins.

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Proteasome

• Large complex protein in the cytosol that is
  responsible for degrading proteins that have been
  marked for destruction by ubiquitination or other
  means.

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• The Proteasome contains-
• Core Particle
• Contains 2 copies of each of 14 different
  polypeptides.
• Regulatory Particle
• One at each end of the core particle, each
  made of 14 different proteins same of the
  subunits have sites that recognise Ubiquitin.

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Process

• Ubiqiutin is added to a Lysine residue of target
  protein by conjugating enzymes. A series of
  additional Ubiquitin molecules is added.
• A multiubiquitin chain is formed.

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• This chain is recognised by a receptor protein in
  the proteasome. Such chains of Ubiquitin
  molecules must be attached to the Protein for it
  to be degraded.
• The complex binds to sites on the regulatory
  particle which recognises ubiquitin.
• It is unfolded by ATPases using ATP.
• Unfolded protein is translocated into central
  cavity of core particle.

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• Active sites on inner surface break specific
  peptide bonds of the chain.
• It produces a set of peptides about 8 amino acids
  long.
• These leave the core, by an unknown mechanism.
• Ubiquitin is released for further use.

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Lysosomal Mechanism

• Lysosomes are compartments inside the cell,
  roughly spherical and bound by a single membrane.
• They contain proteases known as CATHEPSINS.
• These hydrolytic enzymes degrade proteins and
  other substances taken in by endocytosis.
• Lysosomes have low internal pH, hydrolases prefer
  acidic medium.

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• Lysosomes involved in uptake and digestion of
  exogenous proteins.
• Proteins taken up from extracellular medium by
  endocytosis.
• A region of the plasma membrane of a cell
  invaginates to form a closed vesicle surrounded
  by cytoplasm.

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References

• Creighton,T.E Proteins Structures and molecular
  properties, 2nd edition, New York W.H.Freeman,
  1993.
• Alberts,B et.al Molecular biology of the cell,
  3rd edition, New York Garland Publishing, 1994.
• http//www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb2/
  part1/protease.htm
• http//users.rcn.com/jkimball.ma.ultranet/BiologyP
  ages/P/Proteasome.html
• http//www.neuro.wustl.edu/neuromuscular/mother/de
  grade.htmubiquitinbinding