Question 1

Which of the following post-translational modifications (PTMs) is most likely to occur on a serine residue and is critical for regulating protein activity in cell signaling pathways?

Accepted Answer

Phosphorylation

Question 2

A researcher mutates all lysine residues in a protein to arginine. Which PTM is most directly affected by this mutation, and what is a likely consequence for the protein?

Accepted Answer

Ubiquitylation; the protein will be resistant to proteasomal degradation

Question 3

Which of the following best describes the chemical reaction of phosphorylation?

Accepted Answer

Addition of a phosphate group to a hydroxyl group, typically on serine, threonine, or tyrosine

Question 4

A kinase is mutated so that its ATP-binding pocket is disrupted. What is the most likely effect on its enzymatic activity?

Accepted Answer

It will be unable to transfer phosphate groups to substrates

Question 5

During ubiquitylation, which enzyme is responsible for transferring ubiquitin from the E2 enzyme to the substrate protein?

Accepted Answer

E3 ubiquitin ligase

Question 6

Which lysine residue on ubiquitin is most commonly used to form polyubiquitin chains that target proteins for proteasomal degradation?

Accepted Answer

$$K48$$

Question 7

The proteasome degrades proteins in a processive manner. Which of the following features enables this processivity?

Accepted Answer

Multiple protease active sites within the proteasome core

Question 8

Which of the following best describes the two stages of proteasomal regulation for substrate degradation?

Accepted Answer

Recognition by ubiquitin and commitment via ATP hydrolysis and unfolding

Question 9

A cell is treated with a drug that inhibits cyclin-dependent kinases (Cdks). Which phase of the cell cycle is most likely to be affected first?

Accepted Answer

G1/S transition

Question 10

Which of the following mechanisms can inactivate Cdks during the cell cycle?

Accepted Answer

Phosphorylation by inhibitory kinases

Question 11

The Anaphase Promoting Complex/Cyclosome (APC/C) is activated by which of the following co-activators to trigger the metaphase-anaphase transition?

Accepted Answer

Cdc20

Question 12

A ribosome profiling experiment is performed to study translation. Which of the following best describes the principle of this technique?

Accepted Answer

Sequencing mRNA fragments protected by ribosomes to determine translation rates

Question 13

Which of the following statements about tRNA is correct?

Accepted Answer

tRNAs are transcribed by RNA polymerase III and charged with amino acids by aminoacyl-tRNA synthetases

Question 14

A protein is synthesized with an N-terminal ER signal sequence. Which of the following best describes its targeting pathway?

Accepted Answer

SRP binds the signal sequence, pauses translation, and directs the ribosome to the ER membrane

Question 15

A membrane protein has a single internal start-transfer sequence and no stop-transfer sequence. What is the predicted orientation of this protein in the ER membrane?

Accepted Answer

N-terminus in the cytosol, C-terminus in the ER lumen

Question 16

Which of the following is an example of a ligand-gated ion channel discussed in class?

Accepted Answer

NMDAR (N-methyl-D-aspartate receptor)

Question 17

Which of the following best defines a second messenger in cell signaling?

Accepted Answer

A small intracellular molecule that relays signals from receptors to target molecules

Question 18

A mutation causes a GPCR to be constitutively active. Which of the following is a likely cellular consequence?

Accepted Answer

Continuous activation of downstream signaling pathways, even in the absence of ligand

Question 19

In a negative feedback loop in cell signaling, which of the following best describes the system's behavior?

Accepted Answer

The output of the pathway inhibits its own activity, stabilizing the response

Question 20

A patient has a mutation in the gene encoding the SRP receptor. Which of the following cellular processes is most likely to be disrupted?

Accepted Answer

Targeting of newly synthesized proteins to the ER

Unit 2 Study Guide: Post-Translational Modifications, Cell Cycle, Translation, Protein Targeting, and Cell Signaling

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