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Biochemistry > Exam #4: Protein Translation & Post-Translational Processing > Flashcards

Exam #4: Protein Translation & Post-Translational Processing Flashcards

1
Q

Eukaryotic Ribosomes

A
  • Human Cytosolic vs. Mitochondrial
  • 80S Assembled Size
  • 60S Large Subunit w/ 50 Proteins, 25S, 5.8S & 5S RNA
  • 40S Small Subunit w/ 34 Proteins. 18S RNA
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2
Q

Prokaryotic Ribosomes

A
  • 70S Assembled Size (smaller)
  • 50S Large Subunit w/ 34 proteins, 23S & 5S RNA
  • 30S Small Subunit w/ 21 proteins, 16 S RNA
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3
Q

Differences between Prokaryotic & Eukaryotic Ribosomes

A
  • Different assembled size

- Different size, # proteins, & RNA composition of small & large subunits

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4
Q

How are ribosomes assembled/ translation initiated?

A
  • Small ribosomal subunit loaded with mRNA & tRNA
  • The loaded small subunit binds the large subunit (concluding initiation)
  • This process is guided by initiation factors
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5
Q

Outline Elongation

A

1) Initiator methionine-tRNA binds P site
2) Second aminoacyl-tRNA is placed into the A site, which is EF-1-GTP dependent (Elongation Factor-1)
3) Peptidyl bond is formed
4) Ribosome moves down one codon, which is EF-2-GTP dependent, and empties the A site
5) Uncharged tRNA leaves via the E site

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6
Q

How is translation terminated?

A

1) One of three STOP codons enters A site
2) Eukaryotic Release Factor (eRF)- GTP pairs w/ STOP codon
3) GTP is hydrolyzed & peptide is released from P site
4) Ribosome separates into subunits

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7
Q

How do chaperons assist in protein folding?

A
  • Chaperones are proteins that associate with partly folded proteins
  • Guide the folding process by binding hydrophobic regions
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8
Q

Where does exported protein synthesis occur?

A
  • Synthesized on ER
  • Trafficked through Golgi in vesicles
  • Final destination
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9
Q

Unfolded Protein Response

A

Accumulation of unfolded proteins in the ER induces the unfolded protein response that:

  • Inhibits protein translation
  • Induces chaperone production
  • Consider apoptosis if the amount of unfolded protein is in excess of repair
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10
Q

Glycosyltransferases

A

Transfer sugar form an activated sugar nucleotide to an acceptor substrate

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11
Q

What types of proteins are glycosylated and why?

A
  • Cell surface & exported proteins are glycosylated
  • B/c glycosylation increases solubility, stability, & size
  • B/c glyocsylation puts a carbohydrate on the protein surface that is used as a recognition site
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12
Q

N-Linked Glycosylation

A
  • Oligosaccharide added to amino group of asparagine

1) Synthesis of universal oligosaccharide on dolichol phosphate
2) Transfer of universal oligosaccharide to nascent polypeptide in ER
3) Modification of universal oligosaccharide in Golgi apparatus to produce high mannose and complex type

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13
Q

O-Linked Glycosylation

A
  • Occurs only on fully folded protein in the Golgi apparatus
  • Glycosyltransferases transfer N-acetyl-galatosamine to serine/ theronine
  • Further transfer of sugars enlarges the carbohydrate
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14
Q

Differences between N-Linked & O-Linked Glycosylation

A

1) N-linked starts in ER & continues in Golgi; O-linked Golgi only
2) N-linked= asparagine, O-linked= serine/theronine
3) N-linked= universal first, then adjust; O-linked= add one carbohydrate at a time

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15
Q

Congenital Disorders of Glycosylation

A

Affect N-linked Glycosylation that impairs extracellular enzymatic functioning

  • CDG-I= Defective synthesis of universal oligosaccharide
  • CDG-II= Defective trimming of universal oligosaccharide
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16
Q

Post-translational Modificiation of Amino Acids

A

1) Proline hydroxylation (collagen)
2) Lysine acetylation (histones)
3) Thiol-group in cysteine converted into aldehyde to from C-alpha-formylglycine (lysosomal sulfatases)

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17
Q

4 Ways Hydrophobic Molecules are Added to Proteins

A

1) Myristoylation- addition of myrisitic acid to N-terminal
2) Palmitoylation- addition of palmitic acid to cysteine
3) Prenylation- addition of isoprenoids to cysteine near C-terminus
4) GPI anchor

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18
Q

Cystic Fibrosis

A
  • Caused by a deletion of one gene from CFTR1
  • Deletion interferes with protein folding & glycosylation
  • Consequently, protein is degraded in cytoplasm instead of trafficking to plasma membrane
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19
Q

I-Cell Disease

A
  • Transfer of phosphate to mannose is impaired
  • Cannot generate mannose 6-phosphate for lysosomal degaradation
  • Proteins accumulate in lysosome
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20
Q

Protein Import into Mitochondria

A

1) Mitochondrial proteins synthesized with N-terminal presequence
2) Chaperones stabilize in unfolded form
3) Presequence interacts with receptor in outer mitochondrial matrix
4) Complex of TOMs & TIMs provides a channel for the preprotein to enter the mitochondria
5) Presequence is cleaved by matrix proteases

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21
Q

Lysosomal Degradation

A
  • Nonspecific degradation of extracellular & intracellular proteins
  • High mannose glycoproteins are phosphorylated at mannose residues
  • Mannose 6-Phosphate targets vesicles to lysosome
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22
Q

Proteasome Degradation

A
  • Specific degradation of cytoplasmic proteins

- Signaled by poly-ubiquitination

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23
Q

Ubiquitin & Protein Degradation Steps

A

1) Activation= E1
2) Conjugation= E2
3) Ligation= E3

-Poly-ubiquitinated proteins are degraded

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24
Q

What factors determine protein half-life?

A

1) Conformation i.e. improperly folded are degraded
2) N-terminal residue
3) Other sequence elements e.g. PEST that shortens half life

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25
Q

Ricin

A
  • Bioterrorism agent extracted from Castor beans
  • Mechanism: glycosidase that removes adenine bases from rRNA in the large ribosomal subunit
  • Part of the Ribosome Inactivating Proteins (RIPs)
26
Q

5’ Cap

A

5’ methylguanosine cap

27
Q

Start Codon

A

AUG, codes for methionine

28
Q

Stop Codon

A
  • UAG
  • UAA
  • UGA
29
Q

Poly A Tail

A

100-200 Adenine bases on 3’ end of mRNA

30
Q

5’ UTR

A

Region just downstream of 5’ cap (before the START codon) that is not translated

31
Q

3’ UTR

A

Region downstream of STOP codon that is not translated

32
Q

Open Reading Frame

A

Region from START to STOP codons

33
Q

Monocistronic mRNA

A
  • Eukaryotic

- One open reading frame per mRNA

34
Q

Polycistronic mRNA

A
  • Prokaryotic

- Multiple open reading frames per mRNA

35
Q

“Wobble”

A
  • Inosine in tRNA anticodon can pair with 3 mRNA bases
  • A, C, & U
  • why some tRNAs can pair with multiple mRNA codons
36
Q

Aminoacyl-tRNA Synthetases

A
  • Links tRNA to respective amino acid

- Requires ATP

37
Q

Describe the role of initiation factors in translation.

A

1) eIF2a is activated by GTP binding
2) eIF2a-GTP binds initiator methionine-tRNA, forming the ternary complex
3) Ternary complex binds the small ribosomal subunit (40S)
4) mRNA binds forming the pre-initiation complex
5) pre-initiation complex binds the large ribosomal subunit (60S)
6) GTP is hydrolyzed and eIF2a-GDP is released

38
Q

Generally, what happens during elongation?

A

Ribosome covalently links amino acids delivered by tRNA

39
Q

When does elongation begin?

A

After initiator methionine-tRNA binds the P site

40
Q

How do many common antibiotics function?

A

Interfering w/ the prokaryotic ribosome

41
Q

Streptomycin

A
  • Binds small ribosomal subunit

- Inhibits initiation & causes mistranslation

42
Q

Neomycin & Gentamicin

A
  • Bind ribosomes

- Causes mistranslation of codons

43
Q

Tetracycline

A
  • Blocks A Site

- Prevents tRNA binding

44
Q

Chloramphenicol

A
  • Prevents peptidyl bond formation
45
Q

What is the difference between an antibiotic & a toxin?

A
  • Antibiotic interferes w/ prokaryotic ribosomes

- Toxin interferes w/ eukaryotic ribosomes

46
Q

Diptheria Toxin

A

Inactivates EF-2 (elongation factor 2) by ADP ribosylation

47
Q

At what level is gene expression regulated?

A

Translation

48
Q

2 mechanisms of translation regulation

A

1) Prevent recognition of the START codon by protein binding to 5’ UTR
2) Regulating initiation factors e.g. phosphorylation of eIF-2 makes it inactive

49
Q

Heat Shock Proteins

A
  • Chaperone proteins damaged in the course of heat & stress

- Mutations lead to protein misfolding disorders including Charcot Marie Tooth Disease

50
Q

Charcot Marie Tooth Disease

A
  • Protein misfolding disorder

- Caused by mutations in HSP genes

51
Q

Outline the Synthesis of Exported (expored from cell) Proteins

A

1) Hydrophobic signal sequence emerges from ribosome
2) The Signal Recognition Peptide (SRP) binds the signal sequence and moves the complex to the ER
3) SRP binds a docking protein on ER that transfers ribosome to translocon
4) SRP dissociates & emerging peptide is threaded into ER
5) Signal peptidase cleaves the signal in the ER

52
Q

Where does protein glycosylation occur?

A
  • Starts in the ER

- Continues in the Golgi Apparatus

53
Q

What 3 things are glycosyltrasnferases specific for?

A

1) Sugar donor
2) Acceptor molecule
3) Type of bond formed

54
Q

Why is O-linked Glycosylation important?

A

Blood group antigens

55
Q

O-antigen

A

Lacks GalNAc or Gal in H-antigen

56
Q

A-antigen

A

GalNAc on H-antigen

57
Q

B-antigen

A

Gal on H-antigen

58
Q

Modifications that typically take place at the termini of newly synthesized proteins

A

1) Trimming at the N-terminus to replace methionine
2) Proteins tethered to external side of plasma membrane by C-terminal GPI anchor
3) Addition of hydrophobic moieties

59
Q

Deafness-Dystonia Syndrome

A

Rare mitochondrial disorder caused by a TIM mutation that impairs cellular energy production by preventing the formation of a fully functional mitochondrion

60
Q

N-terminal residues associated with proteins degraded quickly

A

Lysine & Arginine

61
Q

N-terminal residues associated with stable proteins

A

Methionine & Serine

62
Q

CGD Symptoms

A
  • protein losing enteropathy (pathology of the intestine)
  • hypoglycemia
  • hypotonia

Biochemistry (17 decks)

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