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MCBG > Post-translational Processing of Proteins > Flashcards

Post-translational Processing of Proteins Flashcards

1
Q

Explain proteolytic cleavage.

A

Breaking peptide bonds to remove part of the protein

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2
Q

Explain chemical modification.

A

Addition of functional groups to amino acid residues.

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3
Q

Why is extracellular post-translation processing of a protein sometimes needed?

A

Because you don’t want the protein to be active within the cell. An example is collagen fibres.

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4
Q

Where are proteins destined that are synthesised on free ribosomes?

A

For the cytosol or post-translational import into organelles.

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5
Q

Where are proteins destined that are synthesised by ribosomes on the rER?

A

For the membranes or secretory pathway via co-translational insertion.

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6
Q

Give 4 required steps for protein sorting.

A

A signal that is intrinsic to the protein.
A receptor that recognises the signal and directs it to the correct membrane.
A translocation machinery like a channel or a vesicle.
Energy to transfer the protein to its new place.

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7
Q

Give three types of proteins that are targeted for secretion and examples of each.

A 
Extracellular proteins (insulin, pepsin)
Membrane proteins (integral, channel)
Vesicular proteins (lysosomes, endosomes)
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8
Q

Give the two types of secretion and briefly outline their function.

A

Constitutive secretion - happens all the time and is a slow continuous secretion.
Regulated secretion - secreted on signal

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9
Q

What do endocrine cells secrete?

A

Hormones

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10
Q

What do exocrine cells secrete?

A

Digestive juices, serous, mucous, sebum.

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11
Q

What do neurocrine cells secrete?

A

Neurotransmitters

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12
Q

What is a lot of rER in a cell an indication of?

A

A lot of secretion of extracellular proteins, membrane proteins or vesicular proteins.

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13
Q

What is a signal sequence?

A

An N-terminal amino acid sequence that is around 5-30 amino acids long. The central region of the signal sequence is hydrophobic and it is able to form an alpha-helix.

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14
Q

What does the ‘Pre’ part of Preproalbumin mean?

A

It refers and defines the signal sequence of the Preproalbumin that is then removed during processing.

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15
Q

Explain what a signal recognition particle (SRP) is.

A

It is a riboprotein complex that recognise and bings to the signal sequence of a protein.

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16
Q

Outline the process of the synthesis of secretory proteins and their translocation across the ER membrane.

A

Ribosome binds to the mRNA and starts to translate it. The signal sequence at the N-terminal is synthesised and a SRP recognise it and binds to the signal sequence. The SRP binds to a SRP receptor on the ER membrane and a channel protein (translocon) on the ER membrane opens, the SRP now breaks off from the signal sequence. The peptide chain starts to move into the ER lumen and the signal sequence gets ‘stuck’ in the translocon due to its hydrophobic nature. A signal peptidase cleaves the signal sequence from the rest of the peptide chain and the rest of the peptide chain is transported into the ER lumen. It is no proalbumin for example instead of preproalbumin.

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17
Q

Outline the process of the synthesis of a type 1 membrane protein at the ER.

A

The same process as with preproalbumin however a peptide chain destined to become a membrane protein has a hydrophobic chain somewhere in the middle of its entire length. The signal sequence is still being cut of but as the peptide chain is being transported into the ER lumen via the translocon it gets stuck due to its hydrophobic sequence called a stop-transfer anchor sequence. This part is then embedded into the ER membrane.

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18
Q

Why is the formation of disulphide bonds important for a lot of proteins?

A

Proteins that are destined to be extracellular can be fragile. The disulphide bonds make the protein more rigid and less prone to damage. The disulphide bonds also folds the protein.

19
Q

What is N-linked glycosylation?

A

Sugars (carbohydrates) are added on an asparagine side chain. It is a reaction involving an amino group.

20
Q

Why is glycosylation of proteins important? (4 examples)

A

Correct protein folding to get correct function.
Protein stability
Facilitates interactions with other molecules. This is especially important for extracellular proteins and membrane proteins.
Deficiencies in N-linked glycosylation lead to severe inherited human diseases such as: Congenital disorders of Glycosylation (CDG) (Defective enzymes)

21
Q

What is the role of protein disulphide isomerase (PDI)?

A

A reduced substrate protein with SH-groups are being oxidised by a oxidised PDI. The oxidised PDI has a disulphide bond which becomes reduced by the hydrogens from the substrate protein. The reduced PDI leaves and the substrate protein becomes oxidised with two S- ions. These S- ions bind together to form a disulphide bond. This fold the proteins.

22
Q

What happens if there are folding problems?

A

Proteins may be trapped in mis-folded conformation.
Proteins contain mutations.
Proteins may associate incorrectly with other sub-units-

23
Q

What is used to correct folding problems?

A

An endoplasmic reticulum chaperone protein. E.g. BiP, Calnexin and Calreticulin.

24
Q

What happens if mis-folding can’t be corrected?

A

Protein may be returned to cytosol for degradation.

25
Q

What happens if the cytosol can’t deal with all the mis-folded proteins that are returning?

A

Proteins might build up to a toxic level in the ER so the ER stops working. This can be caused by a single mutation.

26
Q

Outline the modifications made in the Golgi.

A

Sorting - phosphorylation (cis Golgi network)
Removal of Man (cis cisterna)
Removal of Man and addition of GlcNAc (medial cisterna)
Addition of Gal and NANA (trans cisterna)
Sulfation of tyrosine and carbohydrates (trans Golgi netw)

27
Q

Give an example of a protein that is not mature after modification made in the Golgi.

A

Collagen.

28
Q

What is the basic unit of collagen fibres?

A

Tropocollagen

29
Q

What are the characteristics of Tropocollagen?

A

Long rod-shaped protein
3 polypeptide alpha chains wrapped around each other. (NOT alpha helices!)
Glycine in every third position. (Gly-X-Y)
Triple helix
X and Y are usually proline and hydroxyproline.

30
Q

Where does glycine sit in the triple helix of tropocollagen and explain how it is possible for it to be there.

A

In the middle of the helix. This is due to it being the smallest amino acid. Any other amino acid would make the helix fall apart.

31
Q

Why is the hydroxyproline important in the tropocollagen?

A

Provides hydrogen bonds to stabilise the helix.

32
Q

Briefly outline the process of synthesis and modification of collagen in the ER.

A

The peptide chain enters the ER lumen and signal peptidase cleaves the signal peptide. Prepro alpha chain becomes a Pro alpha chains.
Hydroxylation of proline and lysine occurs to form hydroxyproline and hydroxylysine.
Glycosylation occurs where N-linked sugars are added.
Galactose is added to the hydroxylysine residues.

33
Q

What is needed for the hydroxylation of proline?

A

2-oxoglutarate, prolyl hydroxylase, oxygen, Fe2+ and vitamin C (ascorbate)

34
Q

How does a defiency in vitamin C result in a weak collagen fibre?

A

Proline is not hydroxylated into hydroxproline. Therefore no extra hydrogen bonds are formed in the triple helices of tropocollagen. This means that the tropocollagen becomes weaker.

35
Q

What is scurvy?

A

A deficiency in vitamin C. Weak connective tissue. Can be seen in the gum.

36
Q

Does tropocollagen exist intracellularly? If not, what precursor does?

A

No it doesn’t. The precursor is called Procollagen.

37
Q

What is the main structural difference between tropocollagen and procollagen? Why is this difference important?

A

Procollagen has a sequence of amino acids on each side of its chain (N-terminal and C-terminal) that do not form a triple helix. This makes it look like a candy wrapper. This is important because if the unwound N and C terminals weren’t there tropocollagen would bind to other tropocollagen inside the cell. Collagen fibres would form inside the cell and it would explode.

38
Q

How is pro collagen secretion done?

A

Glucose is added to O-links in the procollagen in the Golgi and it is then transported via a vesicle and transported out of the cell via exocytosis.

39
Q

Where is procollagen converted into tropocollagen?

A

Extracellularly by procollagen peptidases where the N-terminal and C-terminal are cleave off so there is only a triple helix.

40
Q

How do collagen fibres form?

A

By lateral association of tropocollagen by covalent cross-linkage. Lysyl oxidase plays a role here.

41
Q

What is Ehlers-Danlos syndrome?

A

A mutation in collagen type 5 or lysyl oxidase deficiency. This means that interconnective tissue becomes weaker and the junctions between dermis and epidermis get weaker as well. EDS can take many forms, skin, musculoskeletal or cardiovascular depending on what type it is.

42
Q

Why is collagen structurally stable?

A

Disulphide bridges.
Hydrogen bonds.
Covalent bonds.

43
Q

What are the different types of precursors of collagen?

A

Preprocollagen
Procollagen
and the base unit Tropocollagen

MCBG (21 decks)

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