Week 2 Receptors + Intercellular Comm

Question 1

What is the Kd in relation to receptor-binding?

How does it relate to receptor function?

How is it calculated?

Answer 1

• ligand concentration when binding is at 1/2 its maximum
• determines the middle of the range of ligand concentrations in which a receptor functions, and thus defines affinity

K_d = [R]x[L]/[RL]

Question 2

What is the EC50?

What property of ligands does it indicate?

Answer 2

• concentration of ligand when it evokes 50% of its maximum biological response
• indicates potency of a ligand

Question 3

What is potency vs. efficacy?

Answer 3

Potency - a measure of the amount of a substance necessary to evoke a response

Efficacy - a measure of the maximum response achievable from a substance

Question 4

What is an antagonist?

Answer 4

• a ligand which binds to a receptor but does not evoke a biological response

… as opposed to agonists which bind AND evoke

Question 5

What is constitutive activity?

And what kind of special ligands relate to receptors with such activity?

Answer 5

• baseline activity of receptors that are active in the absence of an agonist ligand

neutral antagonists - bind but do not lower const. activity

inverse agonists - bind and lower const. activity from its baseline

Question 6

What is a GEF?

How does it relate to G-proteins?

Answer 6

Guanine Nucleotide Exchange Factor

• G-protein coupled receptors can be considered GEFs because they facilitate GDP release from and GTP binding to a G-protein’s alpha subunit

Question 7

Describe G_s protein activation + its consequences.

Agonists/Receptors?

Effectors?

2nd Messengers?

Kinases?

Biological Responses?

Answer 7

Agonists: catecholamines, ACTH

Effectors: activates adenylyl cyclase, Ca++ channels

2nd Messengers: increase [cAMP], Ca++

Kinases: PKA (cAMP-dependent PK)

Biological Responses: smooth muscle relaxation, secretory cell secretion, lipolysis

Question 8

Describe G_i/o protein activation + its consequences.

Agonists?

Receptors?

Effectors?

2nd Messengers?

Kinases?

Answer 8

Agonists: Norepinephrine, Prostaglandins, etc.

Receptors: M₂, M_{<strong>4</strong>}

Effectors:

1. inactivates adenylyl cyclase
2. activates phosholipase A₂
3. inhibits Ca²⁺ channels
4. activates K⁺ channels

2nd Messengers: decreases cAMP, increases IP3

Kinases: decreases PKA

Question 9

What is the result of phospholipase A₂ activation via G_i-proteins?

Answer 9

• cleavage of the second carbon of a phospholipid from its fatty acid
• this fatty acid is usually arachidonic acid, so…
• results in prostaglandin formation via COX enzymes

Question 10

Describe G_q/G₁₁ activation and activity.

Receptors?

Agonists?

Effectors?

2nd Messengers?

Kinases?

Effects?

Answer 10

Receptors: M_{1, 3 and 5}, Angiotensin II, ⍺-1 adrenergic

Agonists: ACh, Epinephrine

Effectors: Phospholipase C

2nd Messengers: DAG, InsP3

Kinases: Protein Kinase C

Effects: activation of gene transcr. + cell prolif. proteins

Question 11

How do InsP3 and DAG activate a kinase?

Which kinase?

Answer 11

1. InsP3 binds to ligand-gated Ca⁺⁺ channels in the endoplasmic reticulum, releasing calcium into the cytosol
2. Ca++ binds Protein Kinase C, thus binding it to the plasma membrane
3. DAG activates PKC

Question 12

What two ways can a cell change to decrease a biological response to an agonist?

Answer 12

1. Down-regulation - decreasing the number of receptors
2. Desensitization - decreasing responsiveness to agonists

Question 13

Describe how Myosin Light Chain Kinase (MLCK) is activated and its activity.

Answer 13

1. Calmodulin binds 4 Ca⁺⁺ ions when cytolosic calcium concentrations are high.
2. Calmodulin-Ca⁺⁺ Complex activates MLCK
3. MLCK phosphorylates light chains of myosin
4. Phosphorylated myosin light chains interact with actin and smooth muscle contraction occurs

Question 14

What example was given of a calmodulin-dependent kinase (CaM Kinase) other than MLCK?

What is its effect?

Answer 14

Phosphorylase Kinase

• activates phosphorylase
• phosphorylase plays a role in adding glucose to glycogen chains

Question 15

Describe the activation and action of G_12/13.

Ligands?

Effectors?

2nd Messengers?

Kinases?

Answer 15

Ligands: angiotensin II, thrombin

Effectors: Rho-GEF

2nd Messengers: Rho-GTP

Kinases: Rho-kinase

Question 16

Describe mediation of G_⍺t and G_⍺g activity and its effects.

Answer 16

these two G-proteins activate cGMP phosphodiesterase, which…

breaks down cGMP, which…

closes cGMP-gated channels

Question 17

What 2 main kinds of channels are involved in the creation of a calcium signal in a cell?

How is the high ER Ca⁺⁺ concentration maintained (2 ways)?

Answer 17

1. voltage-sensitive Ca channels - in the plasma membrane, take part in depolarization
2. IP₃R Receptors - on ER membrane

ER calcium levels maintained via:

1. Ca-ATPase Pump - cytolosic Ca > ER lumen
2. Store-operated Ca Channels - open on cell membrane when ER Ca is low