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Medicine Letures- Cell > Fate of Newly Synthesised Proteins > Flashcards

Fate of Newly Synthesised Proteins Flashcards

1
Q

What are the two different types of ribosomes for different mRNAs?

A
  • free ribosomes

- bound ribosomes

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2
Q

What is the function of free ribosomes?

A
  • produce soluble proteins for release into cytoplasm

- initial codons in mRNA don’t code for hydrophobic amino acids

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3
Q

What is the function of bound ribosomes?

A
  • produce proteins for secretion from the cell/incorporation into membranes/lysosomes
  • initial codons in mRNA code for a short sequence of hydrophobic amino acids
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4
Q

What do structural motifs determine?

A

They determine protein localisation and their function

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5
Q

What is the function of the ER signal sequence and where is it found?

A
  • determines which ribosomes translate the mRNA strand

- found on the N-terminal and is hydrophobic

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6
Q

Where are internal hydrophobic sequences found?

A

Usually found on mRNA strands that code for the transmembrane regions of integral membrane proteins

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7
Q

What is the function of nuclear localisation sequences and which structure are the proteins that have this destined for?

A
  • code for basic amino acids

- destined for the nucleus

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8
Q

What are mitochondrial targeting signals and where do these proteins go?

A
  • alternating pattern of hydrophobic and basic amino acids

- destined for the mitochondria

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9
Q

Which organelles are involved in protein synthesis and what happens at each of these?

A
  • mRNA leaves the nucleus
  • translation occurs on a ribosome on RER (for proteins that will be secreted/part of membrane)
  • translated strand goes to golgi apparatus (packaged into vesicles)
  • vesicles fuse with plasma membrane and empty contents into extracellular fluid (secretion)
  • can also add a new membrane to surface of cell (including some hydrophobic proteins)
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10
Q

What are the functions of the RER?

A
  • synthesis of proteins for subsequent packaging and secretion from cell/insertion into intracellular structures
  • involved in initial steps of glycosylation
  • site of disulphide bond formation
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11
Q

What are the functions of the SER?

A
  • modification of newly synthesised proteins, addition of carbs/phosphate/lipid groups
  • responsible for detoxification in liver
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12
Q

What is the process of the synthesis of secretory and membrane proteins?

A
  1. ER signal sequence is recognised by ‘signal recognition particle’ as peptide is translated from mRNA on a cytosilic ribosome
  2. SRP directs ribosome to ER membrane which binds to it
  3. Translation continues with growing peptide chain into ER while signal sequence remains bound to channel
  4. Signal peptide is cleaved from protein and is either
    - deposited in lumen (secretory protein) or,
    - arrests in ER (membrane protein)
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13
Q

Which enzyme catalyses the formation of disulfide bonds, where is it found and what is this an example of?

A
  • enzyme= protein disulfide isomerase (PDI)
  • found inside ER
  • is an example of a folding chaperone
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14
Q

What is the relationship between polypeptide chains and chaperones?

A
  • polypeptide chains interact with chaperone

- promotes folding of proteins into correct conformation (shape)

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15
Q

What happens to incorrectly folded proteins

A

They’re degraded

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16
Q

When does misfolding of proteins occur?

A
  • when exposed to high temperatures
  • causes increased expression of certain chaperones
  • called heat shock proteins
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17
Q

What happens when the degradative process is overwhelmed?

A

The protein aggregates causing cell death

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18
Q

How are unwanted proteins marked?

A

Marked by tagging lysine residues with a ubiquitin polymer

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19
Q

What are ubiquitin ligases used for?

A

They’re used to attach the ubiquitin polymers to proteins

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20
Q

Which molecules recognise ubiquitin?

A

Proteasomes

21
Q

Where does degradation occur and how?

A
  • in the hollow core of proteasomes

- each amino acid is cleaved off

22
Q

Which enzymes do proteasomes destroy?

A

Only destroy internal cell proteins

23
Q

How are proteins transported to the golgi?

A

Portions of the ER are pinched off and they form transport vesicles

24
Q

How is the transportation of the protein to the golgi regulated?

A

Regulated by specific coat proteins found on vesicle surfaces and GTPases

25
Q

What does the fusion of vesicles and golgi depend on?

A

Depends on snares- a protein family

26
Q

What are the 2 fates of a protein?

A

Either secreted or form part of the membrane

27
Q

Why do secreted proteins have to leave the cell?

A
  • as they don’t contain a membrane spanning region

- made up of exon 1 and 2, not 3

28
Q

What happens to membrane proteins?

A
  • made up of exon 3, has membrane spanning region
  • protein locks onto vesicle membrane
  • vesicle becomes a part of the membrane
29
Q

What does the different types of proteins made from a single mRNA transcript depend on?

A

Depends on how exons are spliced and what structural motifs they have

30
Q

What is the structure of the golgi apparatus?

A
  • set of flattened bags
  • cis face= surface closer to nucleus, where vesicles leave the golgi
  • trans face= surface closer to plasma membrane, where vesicles leave the golgi
31
Q

What are the 5 functions of the golgi?

A
  1. Further modification of newly synthesised proteins
  2. Synthesis of glycolipids
  3. Proteolytic conversion of pro-forms of proteins to mature forms
  4. Packaging of proteins destined for export into secretory vesicles (leave cell) or granules (inside cell)
  5. Targeting of proteins to lysosomes
32
Q

Glycosylation in the golgi

A
  • secreted and membrane proteins are glycosylated

- depends on recognition of specific sequence patterns on target protein

33
Q

Where is carbohydrate addition initiated?

A

In the ER

34
Q

When does carbohydrate initiation occur?

A

When the protein is being synthesised

35
Q

What are 2 common types of glycoproteins

A

N-linked and O-linked

36
Q

What is the difference between the branches of N-linked and O

A

N-linked have branched chains

O-linked have short chains

37
Q

Examples of N-linked glycoproteins

A
  • serum glycproteins involved in haemostasis

- most membrane proteins

38
Q

Examples of O-linked glycoproteins

A

-mucins and blood group antigens

39
Q

How are most biologically active proteins synthesised?

A

-as longer, precursor molecules

40
Q

What is proteolytic cleavage and where does it occur?

A
  • where specific enzymes cleave precursor proteins into their biologically active form
  • e.g. when proinsulin is cleaved into insulin
  • occurs in golgi
41
Q

What are the 2 pathways for protein secretion? Give examples of each

A

Constitutive:
-e.g. albumin from liver cells
Regulated:
-e.g. insulin from pancreatic beta cells

42
Q

What cells are involved in the constitutive pathway?

A

Common to all cells

43
Q

What is the regulated pathway dependent on?

A
  • ligand signalling

- e.g. insulin release is dependent on Ca2+ signalling

44
Q

What are lysosomal enzyme converted from and to, where does this process begin?

A
  • converted from inactive precursors to active enzymes

- process begins in golgi

45
Q

What does the activation of lysosomes involve?

A

Involves proteolytic removal of part of polypeptide chain

46
Q

5 ways proteins can be modified after synthesis

A
  1. Glycosylation
    - addition of carb groups
  2. Phosphorylation
    - addition of phosphate groups (key way in which proteins are regulated)
  3. Acetylation
    - addition of acetyl groups
  4. Methylation
    - addition of methyl group
  5. Lipoprotein formation
    - addition of lipid groups
47
Q

How are proteins regulated by phosphorylation?

A

-proteins are phosphorylated by kinases to activate them

48
Q

How does the regulation of phosphorylase work?

A
  • phosphorylase (which releases glucose-1-phosphate from glycogen) is usually inactive in cells- phosphorylase b
  • upon exposure to glucagon or adrenaline, phosphorylase kinase enzyme gets activated- phosphorylase b (inactive) to a (active)
  • phosphorylase a can then perform its function
49
Q

How can phosphorylase a be switched off?

A
  • protein phosphatase (activated after exposure to insulin)

- removes the phosphate and converts it back to phosphorylase b

Medicine Letures- Cell (11 decks)

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