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MGD > Protein Processing > Flashcards

Protein Processing Flashcards

0
Q

Give two examples of proteins secreted by constitutive secretion.

A

Collagen and serum albumin

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1
Q

What is constitutive secretion?

A

A continuous process

Proteins are packaged in vesicles and released continuously by exocytosis

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2
Q

Give features of regulated secretion

A

Proteins released in response to a signal eg hormone

Proteins packaged into vesicles but not released until signal is received

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3
Q

Give an example of a protein secreted by regulated secretion

A

Insulin

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4
Q

Give the nine steps of the protein synthesis pathway

A
  1. Free ribosome initiates protein synthesis from mRNA molecule
  2. Hydrophobic N-terminal signal sequence is produced
  3. Signal sequence of newly formed protein is recognised and bound to by the signal recognition particle (SRP)
  4. Protein synthesis stops.
  5. GTP-bound SRP directs the ribosome synthesising the protein to SRP receptors on cytosolic face of the ER
  6. SRP dissociates
  7. Protein synthesis continues and the newly formed polypeptide is fed into the ER via a pore in the membrane (peptide translocating complex)
  8. The signal sequence is removed by a signal peptidase once the entire protein has been synthesised
  9. The ribosome dissociates and is recycled
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5
Q

List the three protein modifications that can occur in the ER and the enzymes needed to do them

A

Signal cleavage - signal peptidase

Disulphide bond formation - protein disulphide isomerase

N-linked glycosylation - oligosaccharide-protein transferase

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6
Q

Give protein modifications that can occur in the Golgi.

A

O-linked glycosylation - glycosyl transferase

Trimming and modification N-linked oligosaccharides

Further proteolytic processing

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7
Q

Describe N-linked glycosylation

A

The oligosaccharide is built up on a Dolichol phosphate carrier molecule sitting in the membrane
The oligosaccharide is then transferred onto the amide group of asparagine

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8
Q

What is a Dilochol Phosphate carrier molecule?

A

A long chain hydrocarbon molecule that inserts into the membrane with its phosphate group protruding
Involved in N linked glycosylation

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9
Q

What is O-linked glycosylation?

A

Modification of hydroxyl groups on serine and threonine.

Glycosyl transferase builds up a sugar chain from nucleotide sugar substrates.

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10
Q

Which enzymes carry out proteolytic processing?

A

Endoproteases which are sequence specific

Exoproteases - amino peptidase and carboxypeptidase

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11
Q

What is proteolytic removal of the N-terminal known as and where does it occur?

A

Pre-segment removal

Occurs in the ER

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12
Q

Where does the removal of the pro-segment occur?

A

Golgi apparatus

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13
Q

What happens first, removal of the pre- or pro-segment?

A

Pre

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14
Q

Why is glycosylation of proteins important?

A

Correct protein folding
Protein stability
Facilitates interaction with other molecules

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15
Q

What is glycosylation?

A

The attachment of carbohydrate groups to proteins via glycosidic linkages

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16
Q

What is a proprotein?

A

An inactive precursor of a protein

17
Q

What is a preproprotein?

A

A protein containing both a signal peptide and propeptide

18
Q

Why are disulphide bonds added to some proteins?

A

Improves stability - especially for extracellular proteins because conditions outside of the cell may be harsher

19
Q

How is mature insulin formed?

A

Preproinsulin is synthesised - an inactive, single polypeptide

Signal sequence is removed by signal peptidase and 3 disulphide bonds are formed - pro insulin

Proteases recognise pairs of basic amino acid residues and cut it into three peptides - A, B and C.

Mature insulin is composed of A and B held together by disulphide bonds.

C peptide is secreted with insulin.

20
Q

What is a basic unit of collagen called?

A

Tropocollagen

21
Q

What are the three main amino acids in collagen?

A

Glycine, proline and hydroxyproline

22
Q

How are the polypeptides in collagen arranged?

A

Three polypeptides

Left-handed triple helix

23
Q

What properties does the arrangement of polypeptides in collagen lead to?

A

Non-extensible/compressible

High tensile strength

24
Q

How is collagen produced while within the cell?

A

Tropocollagen produced as preprocollagen.

  • Pre marks protein for secretion
  • Pro prevent collagen from forming fibres within the cell

Pro collagen is secreted by exocytosis

25
Q

How is collagen formed once pro collagen is outside of the cell?

A

Procollagen peptidases cleave N and C terminal peptides (the pro-segment)
Tropocollagen form covalent cross links
Aldehyde residues are produced from lysine via the enzyme lysyl oxidase. They can form cross links

26
Q

What does lysyl oxidase require to work?

A

Vitamin B6 and Cu 2+ ions

27
Q

Describe nuclear targeting

A

Proteins that need to enter the nucleus contain a nuclear localisation sequence which is contained in the primary sequence.

Importin alpha and beta bind to the NLS in the cytosol.

Complex translocates to the nucleus - energy dependant process.

Nuclear protein is released within the nucleus and Importins bind to a small GTPase protein (Ran)

Importins are exported from the nucleus and can be recycled

Ran is transported back to nucleus following hydrolysis of GTP.

28
Q

Are mitochondrial proteins folded or unfolded?

A

Unfolded

29
Q

What type of sequence do proteins destined for the mitochondrial matrix contain?

A

Amphipathic N-terminal signal sequence (10-80 amino acids)

30
Q

Which two stages of mitochondrial targeting requires energy in the form of ATP?

A

Transport of the protein across the inner mitochondrial membrane by TIM proteins.
Folding the protein into its native conformation once inside the matrix - helped by chaperones.

31
Q

In mitochondrial targeting, what does the protein MSF do?

A

It is a chaperone which stabilises the unfolded protein in the cytosol.

32
Q

In mitochondrial targeting, what does the protein TOM do?

A

Recognises the signal sequence on the protein and forms a protein channel to allow it through the outer membrane of the mitochondria.

33
Q

In mitochondrial targeting, what does the protein TIM do?

A

Transports the protein across the inner membrane of the mitochondrion.

34
Q

In mitochondrial processing, what does the protein MPP do?

A

Cleaves the signal sequence on the protein being transported in the matrix of the mitochondrion.

35
Q

What type of enzymes are targeted to lysosomes?

A

Lysosomal hydrolases

36
Q

Describe lysosomal targeting

A

A signal patch on the lysosomal hydrolase attracts M6P - binds to N-linked oligosaccharides on the protein.
This occurs in the Golgi and involves two enzymes.

M6P groups on the protein are recognised by receptors on the trans side of the Golgi and vesicles are pinched off for transport for transport to the lysosome.

Once at the lysosome, the acid pH causes the protein and M6P to dissociated.

Phosphate group is removed from the M6P to ensure that the protein doesn’t return to the Golgi with it.

37
Q

What are the two enzymes needed to bind M6P to the lysosomal hydrolase? Which one can be deficient?

A

N-acetylglucosamine phosphotransferase - deficient

N-acetylglucosamine phosphoglycosidase

38
Q

What is I-Cell disease?

A

A genetic defect in N-acetylglucosamine phosphotransferase.
Lack of M6P addition to lysosomal targeted proteins.
Lysosomal hydrolases are mis targeted for secretion.
They can have a high concentration in urine and blood.

39
Q

How are proteins returned to the ER from the Golgi?

A

ER resident proteins have the sequence Lys-Asp-Gly-Leu (KDEL) near the C-terminus.
If these proteins are transported to the Golgi, they will interact with KDEL receptors, a process that is enhanced by the low pH there.
ER proteins bound to the receptors are transported in vesicles.
ER proteins dissociate from the receptors in the neutral pH within the ER.
KDEL receptor is transported back to the Golgi.

40
Q

What does the triple helix structure of collagen allow?

A

High tensile strength
Non-extensible
Non-compressible

41
Q

Why does a lack of vitamin C lead to scurvy?

A

The enzyme prolyl hydroxylase allows hydrogen bonds to stabilise the triple helix
Prolyl hydroxylase requires vitamin C and iron ions to do this
Scurvy is due to weak tropocollagen triple helices.

MGD (10 decks)

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