Exam 4 Protein Metabolism

Question 1

What is the first amino acid in eukaryotic translation initiation?

Answer 1

Methionine (Met), carried by initiator tRNA (tRNAi).

Question 2

How does prokaryotic translation initiation differ?

Answer 2

UsesN-formylmethionine (fMet)instead of regular methionine.

Question 3

What is the wobble hypothesis?

Answer 3

Nontraditional base-pairing between the 5′ nucleotide (1st nucleotide) of the anticodon and the 3′-nucleotide (last nucleotide) of the codon.

Question 4

How does wobble pairing reduce the number of tRNAs needed?

Answer 4

One tRNA can recognize multiple codons (e.g., inosine (I) in tRNA pairs with U, C, or A).

Question 5

What are the ribosomal subunits in prokaryotes?

Answer 5

30S (small)+50S (large)=70S ribosome.

Question 6

What are the ribosomal subunits in eukaryotes?

Answer 6

40S (small)+60S (large)=80S ribosome.

Question 7

Name the three tRNA binding sites on ribosomes.

Answer 7

A site(aminoacyl),P site(peptidyl),E site(exit).

Question 8

What modified bases are found in tRNA (besides A, C, G, U)?

Answer 8

Inosine (I), pseudouridine (Ψ), dihydrouridine (D), methylated bases (e.g., m⁷G).

Question 9

If the codon isUUC, what is the anticodon?

Answer 9

AAG(orGAAif wobble pairing allows inosine at the 5′ end).

Question 10

What enzyme attaches amino acids to tRNA? Is it specific?

Answer 10

Aminoacyl-tRNA synthetase(specific to each amino acid and tRNA).

Question 11

What bond links amino acids to tRNA?

Answer 11

Ester bondbetween the amino acid’s carboxyl group and tRNA’s 3′-OH.

Question 12

What are the two steps of tRNA “charging”?

Answer 12

1. Amino acid + ATP → aminoacyl-AMP + PPi. 2. Aminoacyl-AMP + tRNA → aminoacyl-tRNA + AMP.

Question 13

In eukaryotes, all proteins start with which amino acid?

Answer 13

Methionine (Met)at theN-terminus.

Question 14

What is required for bacterial initiation complex formation?

Answer 14

mRNA, fMet-tRNA, 30S/50S subunits, initiation factors (IF-1, IF-2, IF-3), GTP.

Question 15

What delivers aminoacyl-tRNA to the ribosome during elongation? Elongation factors?

Answer 15

EF-G (bacteria) or eEF-2 (eukaryotes); EF-Tu-GTP(bacteria) /eEF1α-GTP(eukaryotes).

Question 16

What is a polysome?

Answer 16

Multiple ribosomes translating the same mRNA simultaneously.

Question 17

How many NTPs are hydrolyzed per amino acid added?

Answer 17

4(2 ATP for tRNA charging + 2 GTP for elongation/translocation).

Question 18

Translate this mRNA:5′-AUGUUUCAAUAA-3′.

Answer 18

Met-Phe-Gln-Stop(AUG = Met, UUU = Phe, CAA = Gln, UAA = Stop).

Question 19

What happens if the mRNA lacks a stop codon?

Answer 19

Ribosome stalls;NSD (non stop decay)rescues by tagging the protein for degradation.

Question 20

What are the ribosome binding sites for tRNA during elongation?

Answer 20

A site(incoming tRNA),P site(growing peptide),E site(exiting tRNA).

Question 21

True or False: Ribosome assembly on mRNA requires ATP.

Answer 21

False(uses GTP for initiation/elongation).

Question 22

True or False: tRNA charging requires aminoacyl-AMP formation.

Answer 22

True(intermediate step before tRNA attachment).

Question 23

What drives translocation of the ribosome?

Answer 23

EF-G-GTPhydrolysis (bacteria) /eEF2-GTP(eukaryotes).

Question 24

True or False: Translation termination requires GTP hydrolysis.

Answer 24

False(uses release factors, but no NTP).

Question 25

Order these steps: Peptide bond formation, tRNA release, 50S binding, aminoacyl-tRNA binding.

Answer 25

1. 50S binds 30S initiation complex. 2. Aminoacyl-tRNA binds A site. 3. Peptide bond forms. 4. tRNA released.

Question 26

What NTP is hydrolyzed during peptide translocation?

Answer 26

GTP (by EF-G/eEF2).

Question 27

True or False: Eukaryotic initiation involves 5′-3′ mRNA looping.

Answer 27

True (5′ cap and 3′ poly-A tail interact via initiation factors).

Question 28

True or False: Prokaryotes use Met (not fMet) for initiation.

Answer 28

False (prokaryotes use fMet; eukaryotes use Met).

Question 29

True or False: Prokaryotic transcription/translation are coupled.

Answer 29

True (no nucleus allows simultaneous processes).

Question 30

What are the three steps of bacterial elongation?

Answer 30

Aminoacyl-tRNA binding: EF-Tu-GTP delivers correct charged tRNA to A site, matching the mRNA codon. Peptide bond formation: Peptidyl transferase (23S rRNA in 50S subunit) catalyzes bond formation b/w P-site peptide & A-site amino acid. Translocation: EF-G-GTP hydrolyzes GTP to shift ribosome, moving the tRNA-peptide to P site & ejecting the deacylated tRNA from the E site.

Question 31

Name four posttranslational modifications.

Answer 31

Proteolytic cleavage,  phosphorylation,  glycosylation,  ubiquitination.

Question 32

What is ubiquitin’s role?

Answer 32

Tags proteins for degradation by the  proteasome in the cytosol.

Question 33

What enzyme degrades ubiquitin-tagged proteins?

Answer 33

Proteasome (ATP-dependent protease complex).

Question 34

How does phosphorylation regulate proteins?

Answer 34

Kinases add phosphates to Ser/Thr/Tyr, altering function (e.g., cell signaling).

Question 35

Where does glycosylation occur? Function?

Answer 35

ER/Golgi (adds sugars to proteins for cell recognition/secretion).

Question 36

What is the function of chaperone proteins?

Answer 36

Assist in proper protein folding and prevent aggregation.

Question 37

What is a signal peptide?

Answer 37

Short N-terminal sequence targeting proteins to the ER for secretion.

Question 38

What is alternative splicing?

Answer 38

Produces different proteins from the same gene by including/excluding exons.

Question 39

What is the role of SRP (signal recognition particle)?

Answer 39

Pauses translation and directs ribosomes to the ER membrane.