Question

Cell Biology Tutorial: GPCRs

Answer 1

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Answer 2

\({\bf{acetylchloline~receptors}}\) are found in cardiac muscle and slow the rate of contraction. Ligand binding activates a potassium ion channel that releases K+ into the exoplasm,hyperpolarizing the membrane. In the active form, the G-alpha-i complex is bound to the receptor like so:

Answer 3

the G-alpha i subunit then dissociates, a GDP is exchanged for GTP as usual, and the G-beta subunit binds to the channel, opening the channel

Answer 4

\({\bf{rhodopsin}}\) is a light-sensitive GPCR that is used by rod cells in the eye. It is linked to the G-protein transducin. When rhodopsin absorbs a photon, the retinal moiety is converted from the cis isomeric form to the trans isomeric form, changing the conformation of the opsin protein, allowing rhodopsin to bind to the G-alpha-t subunit of transducin, exchanging GTP for GDP. In the dark, trans-retinal is converted to cis-retinal.

In the inactive state the membrane is depolarized, increasing the secretion of neurotransmitters. cGMP activates a series of nonselective ion channels (that admit sodium and calcium ions) which maintains the depolarized state.

In the active state, light causes a reduction in the cGMP concentration, causing these cation channels to close. cGMP phosphodiesterase is responsible for hydrolyzing cGMP which drops the concentration. G proteins do not act directly on the ion channels.

Answer 5

The G alpha t - GTP complex is localized to the cytoplasmic membrane and when rhodopsin is activated, move along the membrane and bind to the inhibitory gamma subunits of phosphodiesterase (PDE). The active alpha-beta dimer is released, converting cGMP to GMP. In the dark, the concentration of cGMP is increased by guanylyl cyclase.

To become active, rhodopsin is phosphorylated by phodopsin kinase. Arrestin binds to rhodopsin after it is phosphorylated, preventing G-alpha-GTP from interacting with the active complex, and stopping activation of PDE.

Answer 6

\({\bf{acetylyl~cyclase}}\) is an effector protein that synthesizes cAMP from ATMP. cAMP activates a protein kinase. Ligand binding to G-alpha-s activates adenylyl cyclase, ligand binding to G-alpha-i receptors inhibits the enzyme. Otherwise, the mechanism works very similarly to the GPCRs studied above.

Some stimulatory hormones: epinephrine, glucagon, ACTH
Some inhibitory hormones: PGE, adenosine

Answer 7

\({\bf{cAMP}}\) activates protein kinase A. Inactive PKA has two regulatory R subunits and two catalytic subunits. Each R subunit has a pseudosubstrate domain which binds to active sites but is not phosphorylated, which means it can inhibit the activitiy of actual catalytic subunits. Each R subunit has two cAMP binding sites, CNB-A, CNB-B.When cAMP binds to the cAMP binding site, it induces a change in the R subunit making it no longer able to bind and inhibit the catalytic domain, activating its kinase activity.

The first binding of cAMP lowers the dissocition constant of the second cAMP.

Answer 8

\({\bf{glycogen~metabolism}}\)

The removal of glucose from the glycogen polymer is a phosphorolysis reaction catalyzed by glycogen phosphorylase (GP). Activated PKA inhibits glycogen synthesis (by phosphylating GS) and stimulates glycogen degradation (by phosphorylating GPK which activates glycogen degradation.

Phosphoprotein phosphatase (PP) counteracts the effects of GS, GPK, and GP. When cAMP is high, PKA activates IP which keeps PP inactive. In its active form, PP removes the phosphate residues added by PKA/GPK, having the opposite effect of PKA.

Answer 9

\({\bf{CREB}}\)

PKA induces the expression of enzymes in gluconeogensis. The cAMP response element (CRE) binds the phosphorylated transcription factor CREB. phosphorylated CREB binds to CRE-targets and a co-activator CBP.P300 linking CREB to RNA polymerase II which stimulates gene transcription.

Answer 10

\({\bf{IP3/DAG}}\)

IP3 and DAG are two second messengers generated from phospholipase C

IP3 channels, upon binding, release calcium ions temporarily into the cytosol.

DAG activates protein kinase C, which then phosphorylates transcription factors or regulates glycogen metabolism. DAg activates PKC which inhibits glycogen synthesis, while IP3 increases the concentration of calcium ions, which increase the rate of glycogen degradation.

Answer 11

\({\bf{smooth~muscle~and~the~NO~pathway}}\)

Endothelial cells respond to acetylcholine by releasing NO which triggers relaxation. Calcim ions bind to calmodulin, leading to an increase NO formation, which diffuses to local cells and increases their dilation. This effect is mediated by cGMP. cGMP prevents the inhibition of the PKG kinase, which in turn activates a signal pathway that dilates the blood vessel.

Answer 12

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Answer 13

The source of this information is \({\scriptstyle{Molecular~Cell~Biology~Eigth~Edition}}\)
\({\scriptstyle{Lodish,~et.al.}}\)