Question 1

Which of the following best describes the role of a second messenger in a signal transduction pathway?

Accepted Answer

A small molecule produced in response to a signal that binds to other specific receptors to propagate the signal.

Question 2

Which of the following is a characteristic feature of G-protein coupled receptors (GPCRs)?

Accepted Answer

They are heptahelical (7TM) integral membrane proteins that activate heterotrimeric G-proteins upon ligand binding.

Question 3

In the β-adrenergic signaling pathway, what is the immediate effect of epinephrine binding to its receptor on the cell surface?

Accepted Answer

Activation of a heterotrimeric G-protein by exchanging GDP for GTP on the Gα subunit.

Question 4

Which of the following equations correctly represents the relationship between the concentration of ligand-bound receptor $$[RL]$$, free ligand $$[L]$$, and the dissociation constant $$K_d$$?

Accepted Answer

$$[RL] = \frac{B_{max} \cdot [L]}{K_d + [L]}$$

Question 5

A mutation causes a Gα subunit to lose its intrinsic GTPase activity. What is the most likely cellular consequence?

Accepted Answer

The G-protein will remain active longer, leading to prolonged activation of downstream effectors.

Question 6

Which of the following best explains why the same ligand can produce different cellular responses in different tissues?

Accepted Answer

Different tissues express different receptors or receptor subtypes for the same ligand.

Question 7

Which of the following is a direct result of cAMP binding to the regulatory subunits of Protein Kinase A (PKA)?

Accepted Answer

PKA catalytic subunits are released and become active, phosphorylating downstream targets.

Question 8

Which of the following signaling molecules is most likely to bind to an intracellular (nuclear) receptor rather than a cell surface receptor?

Accepted Answer

Estrogen

Question 9

In the phosphoinositide cascade, which enzyme cleaves PIP$$_2 to $$generate IP$$_3$$ and DAG?

Accepted Answer

Phospholipase C

Question 10

Which of the following best describes the mechanism by which β-blockers reduce heart rate?

Accepted Answer

They block β-adrenergic receptors, preventing epinephrine from activating the signaling pathway.

Question 11

Which of the following is NOT a typical cellular response to signal transduction?

Accepted Answer

Direct synthesis of ATP from glucose

Question 12

Which of the following statements about receptor tyrosine kinases (RTKs) is correct?

Accepted Answer

They autophosphorylate on tyrosine residues upon ligand binding, initiating downstream signaling.

Question 13

Which of the following best explains the concept of signal amplification in a signaling pathway?

Accepted Answer

A single ligand-receptor interaction can activate multiple downstream molecules, resulting in a large cellular response.

Question 14

Which of the following is the correct sequence of events in the β-adrenergic signaling pathway leading to glycogen breakdown?

Accepted Answer

Epinephrine binds β-AR → Gs activation → Adenylyl cyclase activation → cAMP increase → PKA activation → Phosphorylase kinase activation → Glycogen phosphorylase activation

Question 15

Which of the following best describes the role of GAPs (GTPase-activating proteins) in G-protein signaling?

Accepted Answer

They stimulate the hydrolysis of GTP to GDP, inactivating the G-protein.

Question 16

Which of the following is a direct effect of IP$$_3$$ generated in the phosphoinositide cascade?

Accepted Answer

Release of Ca$$^{2+}$$ from the endoplasmic reticulum

Question 17

Which of the following best explains why steroid hormones can bind to intracellular receptors?

Accepted Answer

They are hydrophobic and can diffuse through the plasma membrane.

Question 18

Which of the following is a correct statement about the inactivation of the β-adrenergic signaling pathway?

Accepted Answer

G$$_s$$ hydrolyzes GTP to GDP, causing it to detach from adenylyl cyclase and deactivate cAMP production.

Question 19

Which of the following best describes the function of rhodopsin in vision?

Accepted Answer

It is a light-activated GPCR that, upon photon absorption, activates a G-protein leading to changes in cGMP levels.

Biochemistry I: Signaling and Receptors – Structured Study Notes

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