W2 - Receptors (Ch 3)

Question 1

Describe the types of intracellular communication.

Answer 1

• contact-dependent: via membrane-bound signal mol.
• paracrine: mol. released by one type of cell, act on another type
• synaptic: neurotransmitters released to transmit electrical signals
• endocrine: hormones released into blood, affect distant cells
• autocrine: mol. released act on same cell/cells of same type
• gap junctions: intracell. mol. less than 1200 Da, electrical signals

Question 2

Which 4 features characterize a receptor?

Answer 2

• high affinity to a special ligand (or substrate analogue) ∽ 10^-9 mM
• can be saturated
• reversible binding
• evoke a biological response

Question 3

What is K_d?

Formula.

Answer 3

defines concentration range in which receptor works best
→ concentration at which 50% of [L] bound to receptor

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

• [R] = receptor concentration
• [L] = ligand concentration
• [RL] = receptor-ligand complex concentration

_{concept similar to Km <em>(cf. biochem)</em>}

Question 4

What is EC 50?

What is the difference to K_d?

Answer 4

efficient concentration that evokes 50% of the biological response

CAN be = K_d, but doesn’t have to (cf. potency, efficacy)

Question 5

What is the difference btw potency and efficacy?

Answer 5

• potency: amount of ligand required to produce an effect of given intensity
  → the ↓ [L], the ↑ potency
• efficacy: ability of ligand-receptor complex to produce a maximum biological response
  → the ↑ response, the ↑ efficacy

Question 6

What is the difference btw agonist and antagonist?

Answer 6

• agonist → biological response
• antagonist → NO biological response

_{<u>DO NOT confuse pharmacological/muscular antagonists: </u><br></br><strong>- pharmacol. ant.</strong> inhibit agonist <strong>w/o causing an own effect</strong><br></br>- muscular ant. cause opposite movement}

Question 7

What is basal activity?

Answer 7

activity of receptor even in the absence of a ligand

Question 8

Explain different types of agonists and antagonists.

Answer 8

dependent on efficacy

• full, partial agonists: evokes biological response
• antagonist (= neutral)
• inverse agonist: inhibits biological response to a level below basal activity

Question 9

Briefly explain the function of a pharmacological agonist in case of diabetes insipidus.

Answer 9

diabetes insipidus = lack of ADH (= vasopressin)
→ no reabsorption
⇒ very high V, very low Osm urine excreted (- 20l/d)

⇒ vasopressin-agonist desmopressin administered in form of nasal drops
_{(would be degraded in intestines if administered per os)}

Question 10

Why is desmopressin instead of vasopressin administered in case of diabetes insipidus?

Answer 10

• vasopressin binds to V1 and V2 receptor
• desmopressin only binds to V2 → no add. vasoconstriction that would incr. blood pressure

Question 11

What are 2 general classes of receptors?

What are their substrates?

Answer 11

• plasma membrane receptors: ligands that cannot cross plasma membrane
• nuclear receptors: small hydrophobic mol. (steroid hormones, thyroid hormones, retinoids, vit D)

Question 12

What are 2 types of signal transduction pathways of plasma membrane receptors?

Answer 12

intracell. signals = molecular switches
active - inactive form

• signaling by phosphorylation: phosphorylation = activation (ATP → ADP + P, P transferred to substrate)
• signaling by GTP-binding protein: binding of GTP to effector = activation

reverse effects (dephosphorylation, hydrolysis of GTP to GDP) cause inactivation

Question 13

What is adaptation?

Other terms.

Answer 13

also: desensitization

reversible reduction (= internalization)/inactivation of receptors due to prolonged exposure to ligand, associated w/ β-arrestin

Question 14

What are types of plasma membrane receptors?

Answer 14

• G protein-coupled receptors (GPCRs)
• ion channel receptors
• receptors w/ enzyme activity
• enzyme activity-linked receptors

Question 15

Explain the structure of G proteins.

Answer 15

• 7 transmembrane domains
• heterotrimeric: α, β, γ-complex in resting state
• in resting state GDP bound to α-subunit

Question 16

Explain the function of G proteins.

Answer 16

Question 17

What are GEFs and GAPs?

Answer 17

• GEFs: facilitate exchange of GDP to GTP at α-subunit of G protein (activate G proteins)
• GAPs: enhance hydrolysis of GTP to GDP + P (inactivate G proteins)

Question 18

List some important G protein families.

Answer 18

• G_i/0
• G_q/11
• G_12/13
• G_s
• G_t

Question 19

List effects of the G_i/0 protein family.

Answer 19

• inhibition of adenyl cyclase
• activation of K⁺ channels → hyperpolarization
• inactivation of Ca²⁺ channels → no Ca²⁺ influx
• phospholipase A₂ activation

Question 20

Explain the cyclic AMP system.

Which family of G proteins stimulate, resp. inhibit it?

Answer 20

1. a) G_S stimulate
   b) G_i/0 inhibit adenyl cyclase
2. adenyl cyclase: ATP → cAMP (= second messenger)
3. activates PKA
4. a) phosphorylates proteins → direct effect
   b) enters nucleus, activates transcription factor CREB → indirect effect

Question 21

Which enzyme degrades cAMP?

To … ?

Example for an inhibitor.

Answer 21

cAMP diphosphoesterase:
cAMP → 5’ AMP

caffein = inhibitor

Question 22

Which receptors stimulate G_S proteins?

Answer 22

β-adrenergic receptors, ACTH receptors (adrenocorticoreceptors)

Question 23

Which receptors stimulate G_i/0 proteins?

Answer 23

α₂-adrenergic receptors, M₂, M₄ ACh receptors, opiate (μ, δ, κ) receptors

Question 24

Where do phospholipase A₁, A₂, C, and D cleave phospholipids?

Answer 24

• phospholipase A₁: cuts ester bond at C1
• phospholipase A₂: cuts ester bond at C2
• phospholipase C: cuts just before phosphate group
• phospholipase D: cuts just after phosphate group

Question 25

Which G proteins activate phospholipase A₂. Explain its function.

Answer 25

activated by G_i/0 **releases arachidonic acid** from plasma membrane to be metabolised

Question 26

List 3 metabolic pathways of arachidonic acid and their products + function.

Answer 26

_**​COX** produce_ * **thromboxanes:** platet aggregation, vasoconstriction * **prostaglandins:** platelet aggregation, bronchoconstriction, induce inflammation * **prostacyclins:** inhibit platelet aggregation, vasodilation _**5-lipoxygenase** produces_ * **leukotrienes:** allergic/inflammatory response (e.g. asthma, rheumatoid arthritis) **_epoxygenase_** _produces_ * **HETE, EET:** incr. Ca²⁺ release from ER, stimulate cell proliferation

Question 27

What are inhibitors of COX 2? Function?

Answer 27

* *non-steroidal antiinflammatory drugs**, e. g. aspirin, ibuprofen

Question 28

Explain the function of G_t protein transducin.

Answer 28

activates phosphodiesterase **in rod cells of the eye** **in response to light** to convert cGMP to GMP → closes cGMP activated cation channels ⇒ altered membrane voltage

Question 29

Which receptors stimulate G_q/11 proteins?

Answer 29

**α₁-adrenergic** receptors, **M₁, ₃, ₅ ACh** receptors, **angiotensin II** receptors

Question 30

Which G proteins activate phospholipase Cβ. Explain its function.

Answer 30

activated by G_q/11 converts **PIP2 → InsP3** (IP3) **+ DAG** * InsP3 → binds to ER → opens Ca²⁺ channels * DAG → activates PKC → phosphorylates proteins

Question 31

What are ways for Ca²⁺ to enter the cell other than binding of InsP3 to the ER membrane?

Answer 31

* via **voltage sensitive-Ca²⁺ channels** * via other **ligand gated-Ca²⁺ channels**

Question 32

What are store-operated Ca²⁺ channels? Relate the concentration of Ca²⁺ in ECF, cytoplasm and ER.

Answer 32

located in ER membrane, open **if Ca²⁺ content in ER too low** | (in ECF and ER 10^-3M, cytoplasm 10^-7M)

Question 33

Explain the role of Ca²⁺ as second messenger. Give examples.

Answer 33

1. binds to **calmodulin CaM** 2. activates **CaM kinases,** e.g. * cAMP phosphodiesterase *(cf. cAMP degr.)* * myosin light-chain kinase → sm. mm. contraction

Question 34

Which receptors stimulate G_12/13 proteins? What is their function?

Answer 34

**thrombin** receptors, **angiontensin II** receptors ⇒ activate Rho

Question 35

Classify small G proteins. Function?

Answer 35

​also regulated by GAPs, GEFs

Question 36

Classify adrenergic receptors according to: * subtypes * affinity * G protein * signaling pathway and name one example.

Answer 36

NE = norepinephrine E = epinephrine

Question 37

Explain the function of ligand gated-ion channel. Example.

Answer 37

**signal molecule binds to channel** → opens e.g. acetylcholine binds to nicotinic ACh receptors at neuromuscular junction → Na⁺/K⁺ channels open

Question 38

What do all excitatory, and all inhibitory ligand gated-ion channels have in common? List examples.

Answer 38

* **excitatory:** ion channels specific to cations * **inhibitory:** ion channels specific to anions

Question 39

What are the 2 general types of acetylcholine receptors?

Answer 39

* **nicotinic acetylcholine receptors** (nAChRs) = ligand gated-ion channels * **muscarinic acetylcholine receptors** (mAChRs) = G protein-coupled receptors

Question 40

What types of nicotinic acetylcholine receptors nAChRs are there? List their specific inhibitors.

Answer 40

* **muscle type:** mediates skeletal mm. movement, inhibited by curare * **neural type:** in symp./parasymp. ganglia, inhibited by ganglial blockers

Question 41

What types of muscarinic acetylcholine receptors mAChRs are there? List their common inhibitor. To which G proteins are they coupled? What is their action?

Answer 41

_5 subtypes, M₁ - M₅_ - all inhibited by **atropine** * **M_{1, 3, 5}** coupled to G_q/11 → Ca²⁺, InsP3, PKC * **M_{2, 4}** coupled to G_i/0 → cAMP ↓, K⁺ ↑, Ca²⁺ ↓

Question 42

What are the 3 types of receptors w/ enzyme actitivity? Examples for each.

Answer 42

* **receptor guanylyl cyclases:** ANP receptor * **receptor threonine/serine kinases:** TGF-β receptor * **receptor tyrosine kinases:** NGF receptor, insulin receptor

Question 43

What are the types of receptor guanylyl cyclases? What is their common function?

Answer 43

* membrane bound receptor guanylyl cyclases * soluble receptor guanylyl cylclases ⇒ **convert GTP to cGMP**

Question 44

Explain the function of the receptor that binds ANP. What type of catalytic receptor is it?

Answer 44

_GC-A_ = **membrane bound-receptor guanylyl cyclase** * *structure:* ligand receptor, kinase-like domain that binds ATP, catalytic domain * ligand = e.g atrial natriuaretic peptide ANP * **converts GTP to cGMP** **​**→ here: inhibited Na⁺/H₂0 reabsorption by collecting duct

Question 45

Explain the function of soluble receptor guanylyl cyclases referring to an example.

Answer 45

* *structure:* only catalytic domain w/ α- and β-subunit * activated by intracellular NO * **converts GTP to cGMP** → here: intracellular ↑Ca²⁺ binds to CaM → activates NO synthase to produce NO from Arg → activates soluble GC → cGMP relaxes smooth mm. _{nitrogylcerin increases NO production → former treatment for angina pectoris}

Question 46

Explain the function of the TGF-β receptor. What kind of catalytic receptor is it?

Answer 46

**threonine/serine kinase** * *structure:* type I, type II subunits, multimeric * type I activates downstream effectors TGF-β binds to type II → phosphorylates type I → activates R-Smad → forms dimer w/ Co-Smad → translocates into nucleus to act. transcription factors

Question 47

Explain the general structure of receptor tyrosine kinases w/r/t their function. Examples.

Answer 47

* *extracellular domain:* cysteine-rich, iG-like, fibronectin type-III-like * *transmembrane domain*: α-helix * *cytosolic domain:* **tyrosine kinase**, can have tyrosine insert regions (= interrupted) **w/ SH₂ binding domain** ALWAYS: dimeric → **autophosphorylation** → _different classes:_ many GF receptors, insulin receptors, etc.

Question 48

Name some proteins that can directly bind to the SH₂ domain of activated receptor tyrosine kinases. Function?

Answer 48

* **PI 3 kinase →** phosphorylates lipids, causes inhibition of apoptosis *(cf. own card)* * **GAPs** → enhance hydrolysis of GTP, inactivate Ras * **PLCγ** → same function as PLCβ: PIP2 → IP3 + DAG * **SOS** → Ras-GEF, part of Ras-MAP kinase signaling pathway *(cf. own card)*

Question 49

Explain the function of receptor tyrosine kinases in the Ras-MAP-kinase signaling pathway.

Answer 49

1. **Grb2** recognizes SH₂ domain of phosphorylated tyrosine on the activated receptor 2. recruits **SOS** by means of two SH₃ domains 3. SOS (= Ras-GEF) stimulates inactive Ras to replace its bound **GDP by GTP** 4. activates Ras to relay the **signal to MAP kinases** *(cf. own card)*

Question 50

Explain the MAP kinase cascade triggered by Ras. What is the final consequence?

Answer 50

1. Ras activates MAP kinase kinase kinase **Raf** 2. Raf activates MAP kinase kinase **Mek** 3. Mek activates MAP kinase **Erk** 4. Erk enters nucleus to **activate transcription factors**

Question 51

Explain the pathway of PI 3-kinase activated by receptor tyrosine kinases.

Answer 51

1. **extracellular survival signal** activates RTK, recruits + activates PI 3-kinase 2. PI 3-kinase produces **PI(3,4,5)P₃** (= docking site for 2 serine/threonine kinases) w/ PH domains 3. **PDK1 and Akt** bind to PI(3,4,5)P₃ on plasma membrane 4. **phosphorylation + activation of Akt** by PDK1 and mTOR → dissociates from plasma membrane 5. **Bad** is phosphorylated by Akt → releases **apoptosis-inhibitory proteins** ⇒ promote cell survival _{(phosphorylated Bad binds to a ubiquitous cytosolic protein → keeps Bad out of action)}

Question 52

What are enzyme activity-linked receptors? Examples.

Answer 52

receptors which **don't have intrinsic kinase activity**, but associate enyzmes ⇒ are **dimeric** e.g. receptors for GH, prolaction, cytokines

Question 53

Explain the function of the GH receptor.

Answer 53

1. receptor dimerizes in response to **GH binding** 2. binds **1+ JAK tyrosine kinases** → phosphorylate themselves and the receptor 3. **STAT tyrosine kinases bind** to complex + are phosphorylated 4. STATs dissociate as dimers + translocated to the nucleus → **phosphorylate key transcription factors**

Question 54

Which proteins can be considered oncogenes?

Answer 54

_oncogenes_ = genes that potentially cause cancer if mutated * **GF receptors** * **Ras** * **PI 3-kinase** * **Grb2**

Question 55

Explain the function of intracellular receptors.

Answer 55

hormone either: * binds to **cytoplasmic receptor** that enters nucleus _after_ binding (e.g. cortisol, aldosterone) * binds to **nuclear receptor** that is already in connection w/ DNA (e.g. thyroid hormones, steroids, vit D₃, retinoic acid) ⇒ regulate transcription in 2 steps, early/late response

Question 56

What are the 2 ways of signal termination of G proteins?

Answer 56

_ligand disassociates, then_ * α-subunit hydrolyses GTP, reassociates w/ βγ-subunit - **facilitated by RGS proteins** * phosphorylation of C terminal by receptor (GPCR) kinase and subsequent binding of **β-arrestin →**promotes**receptor internalisation** → G protein cannot access receptor anymore = **desensitization**

Question 57

Explain the process of receptor internalisation.

Answer 57

1. **arrestin** connects receptor to internalisation protein clathrin 2. **coat assembly** of clathrin around receptor 3. **bud formation** 4. **vesicle formation**, dynamin cleaves connection of receptor to membrane 5. **uncoating**, vesicle loses clathrin coat

Question 58

Explain the action of insulin upon GLUT 4 receptors.

Answer 58

insulin binding to insulin receptor causes to **relocalization of GLUT 4 receptors to plasma membrane** to boost glucose uptake