Lecture 6: GPCRs II

A set of practice questions covering GPCR families, second messenger systems (cAMP, cGMP, IP3/DAG, Calcium), and receptor desensitization mechanisms.

What are the primary effectors of stimulatory G proteins ($G\text{s}ʹ$)?

They stimulate adenylyl cyclase to produce $\text{cAMP}$ and activate receptor kinases.

Which G protein family is responsible for inhibiting adenylyl cyclase and opening potassium channels?

The $\text{Giʹ}$ (inhibitory) family.

What is the specific effector of the $\text{Gqʹ}$ family, and what second messengers does it produce?

It activates $\text{phospholipase CͲ}$ to produce $\text{IP}_3$ and $\text{DAG}$, which releases $\text{Ca}^{2+}$.

Which G protein is activated by rhodopsin in response to light?

$\text{Gtʹ}$ (transducin).

What are second messengers?

Small molecules that carry signals inside cells, regulated by activated G proteins.

Name two examples of small soluble second messengers that diffuse through the cytoplasm.

$\text{cAMP}$ and $\text{cGMP}$.

What membrane-anchored enzyme has 10 isoforms and is activated by $\text{Gʹs}$ but inhibited by $\text{Gʹi}$?

$\text{Adenylate cyclase}$ (also known as adenylyl cyclase).

What is the first step in the $\text{cAMP}$ second messenger system after a ligand binds to a receptor?

The ligand activates the G protein, causing the $ʹ$ subunit to swap $\text{GDP}$ for $\text{GTP}$.

What protein kinase is directly activated by the binding of $\text{cAMP}$?

$\text{Protein kinase A}$ ($\text{PKA}$).

How does the binding of adrenaline to a $\text{GPCR}$ lead to the breakdown of glycogen?

Activated $\text{PKA}$ phosphorylates/activates $\text{phosphorylase kinase}$, which in turn activates $\text{glycogen phosphorylase}$.

List the four mechanisms identified to switch off $\text{GPCR}$ signaling.

(1) Agonist dissociating from the receptor, (2) GTPase activity of $\text{Gʹs}$, (3) $\text{cAMP}$ breakdown by $\text{phosphodiesterase}$, and (4) dephosphorylation of enzymes.

What enzyme converts $\text{GTP}$ to $\text{3', 5'-cyclic guanosine monophosphate}$ ($\text{cGMP}$)?

$\text{Guanylate cyclase}$, which can be receptor-bound or free in the cytoplasm.

What four processes determine the concentration and frequency of second messenger signals?

The rate of production, the rate of diffusion, the rate of removal, and the activity of removing enzymes like phosphodiesterase.

Which G protein family reacts to thrombin and activates $\text{Rho}$?

$\text{G}_{13}ʹ$.

Into what two categories can second messengers generated by receptor-regulated lipases be divided?

(i) Water-soluble molecules that diffuse through cytoplasm (e.g., $\text{IP}_3$) and (ii) hydrophobic molecules that remain in the membrane (e.g., $\text{DAG}$).

What molecule is the phospholipid precursor for $\text{IP}_3$ and $\text{DAG}$?

$\text{Phosphatidylinositol (4, 5)-bisphosphate}$ ($\text{PIP}_2$ or $\text{PtdIns(4,5)P}_2$).

What are the catalytic domains of $\text{Phospholipase C}$?

The X and Y domains.

What kind of kinases are PKCs (protein kinase C)?

$\text{Ser/Thr}$ kinases.

By which two factors are PKCs typically activated?

$\text{DAG}$ (binding at the C1 domain) and $\text{Ca}^{2+}$ (binding at the C2 domain).

How does the phosphorylation of $\text{PLCͲ}$ by PKC affect signaling?

It provides negative feedback for $\text{GPCR}$ signaling, making the signaling 'transient'.

What structural change occurs in PKC when $\text{DAG}$ binds?

It causes the dissociation of the intramolecular pseudosubstrate.

What is the approximate concentration of cytosolic $\text{Ca}^{2+}$ in a resting cell?

Approximately $\text{100 thinspace nM}$.

Where does $\text{IP}_3$ bind to trigger a rise in cytoplasmic calcium?

To ligand-gated $\text{Ca}^{2+}$ channels on the endoplasmic reticulum (ER) membrane.

What is the role of STIM dimers in calcium signaling?

They act as calcium sensors that move and bind to ORAI channels to allow $\text{Ca}^{2+}$ movement from the extracellular space to the ER lumen.

Which two proteins are primarily responsible for the desensitization of $\text{GPCRs}$?

$\text{GRK}$ (receptor kinases) and $Ͳ\text{-arrestin}$.

What are the two ways $Ͳ\text{-arrestin}$ regulates $\text{GPCR}$ signaling?

1) It stops the G-protein from binding; 2) It internalizes the receptor for degradation or recycling.

What is 'Tachyphylaxis' in the context of $\text{GPCR}$ signaling?

The rapid appearance of a decreased response to a drug due to overstimulation of the $\text{GPCR}$, often as a result of receptor desensitization.

What is the role of $\text{Golfʹ}$ and where is it found?

It is found in odorant receptors and functions to activate adenylyl cyclase.

What enzyme breaks down cGMP, and which G protein family regulates it?

$\text{cGMP phosphodiesterase}$, which is activated by the $\text{Gtʹ}$ family.

Why is it important to keep cytosolic $\text{Ca}^{2+}$ low in resting cells?

It allows receptors to regulate the activity of $\text{Ca}^{2+}$ channels to produce transient, signaling-significant rises in $\text{Ca}^{2+}$ concentrations.