S8) Post-translational Modifications Flashcards

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1
Q

Describe the basis for post-translational modification

A
  • All proteins adopt a unique 3D structure to become active
  • Some proteins need additional processing after translation to adopt this structure
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2
Q

Identify and describe 2 forms of post-translational modifications that can occur

A
  • Proteolytic cleavage – breaking peptide bonds to remove part of the protein
  • Chemical modification – addition of functional groups to amino acid residues
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3
Q

What are the 2 different post-translational destinations for proteins?

A
  • Proteins destined for the cytosol or posttranslational import into organelles
  • Proteins destined for a membrane or secretory pathway via co-translational insertion
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4
Q

Where are proteins destined for the cytosol /posttranslational import into organelles synthesised?

A

Synthesised on free ribosomes

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5
Q

Where are proteins destined for a membrane / secretory pathway via co-translational insertion synthesised?

A

Synthesised on ribosomes bound to rER

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6
Q

What are the 4 requirements for protein sorting?

A
  • A signal (address), intrinsic to the protein
  • A receptor that recognises the signal and which directs it to the correct membrane
  • A translocation machinery
  • Energy to transfer the protein to its new place
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7
Q

Identify 3 organelles which are targeted by proteins

A
  • Peroxisomes
  • Mitochondria
  • Nucleus
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8
Q

Identify 3 types of proteins which are targeted for secretion

A
  • Extracellular proteins
  • Membrane proteins
  • Vesicular proteins (lysosomes, endosomes etc)
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9
Q

Identify the 2 types of secretion from cells

A
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10
Q

What is consitutive secretion?

A

Constitutive secretion is the constant flow of extracellular proteins out of the cell e.g. collagen is a secreted and modified protein

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11
Q

What are the 3 forms of regulated secretion?

A
  • Endocrine – secreting hormones
  • Exocrine – secreting digestive juices
  • Neurocrine – secreting neurotransmitters
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12
Q

In 7 steps, explain how proteins are targeted to the ER/secretory pathway (co-translational transport)

A

⇒ Protein synthesis on bound ribosomes

Co-translational transport of proteins into or across ER membrane

⇒ Budding and fusion of ER-to-Golgi vesicles to form cis-Golgi

⇒ Cisternal progression

Consitutive secretion / Regulated secretion

⇒ Sorting into Lysosomes

Endocytosis

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13
Q

Identify 6 functions of the ER

A
  • Insertion of proteins into membranes
  • Specific proteolytic cleavage
  • Glycosylation
  • Formation of S-S bonds
  • Proper folding of proteins
  • Assembly of multi-subunit proteins
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14
Q

What is N-linked glycosylation and where does it occur?

A
  • N-linked glycosyation is a reaction involving an amino group where sugars are added on an asparagine side chain
  • Occurs in ER
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15
Q

What does glycosylation of proteins do?

A
  • Folds protein correctly
  • Stabilises protein
  • Facilitates interactions with other molecules
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16
Q

What can deficiencies in N-linked glycosylation lead to?

A

Severe inherited human diseases - congenital disorders of glycosylation

17
Q

What is a disulphide bond?

A

A disulphide bond is a linkage derived by the coupling of two thiol groups

18
Q

What happens if there are protein folding problems?

A
  • Protein may be trapped in mis-folded conformation
  • Protein contains mutation resulting in mis-folding
  • Protein may be incorrectly associated with other sub-units
19
Q

Identify 3 ER chaperone proteins that attempt to correct misfolding

A
  • BiP: “Binding Immunoglobulin Protein”
  • Calnexin
  • Calreticulin
20
Q

How do ER chaperone proteins attempt to correct protein misfolding?

A
  • Retain unfolded proteins in the ER
  • Act as sensors to “monitor” extent of protein mis-folding
  • Mediate increased transcription of chaperones
  • Mediate reduction in translation
21
Q

What happens if protein misfolding cannot be corrected?

A
  • Protein may be returned to cytosol for degradation
  • Protein may accumulate to toxic levels in the ER causing disease
22
Q

What is O-linked glycolisation and where does it occur?

A
  • O-linked glycosylation is a process important in proteoglycans, involving the attachment of sugar to -OH group of serine/threonine
  • Occurs in Golgi apparatus
23
Q

Insulin is a protein that undergoes post-translation modifications.

Describe the processing of preproinsulin

A
  • Signal peptide removed on entry into ER
  • Disulphide bonds formed in ER
  • Removal of C chain in Golgi
24
Q

Proteins may be delivered to the plasma membrane via the regulated or secretory constitutive pathway.

Identify 4 proteins delivered via the constitutive secretory pathway

A
  • Immunoglobulins
  • Albumin
  • Prohormones
  • Collagen
25
Q

Proteins may be delivered to the plasma membrane via the constitutive or regulated secretory pathway.

Identify 3 proteins delivered via the regulated secretory pathway

A
  • Insulin
  • Glucagon
  • Trypsin