GPCRs

Question 1

Components of Trimeric G protein

Answer 1

• complex of 3 subunits (alpha, beta and gamma)

- Alpha subunit - GTPase activity

Question 2

G alpha s Mechanism of Action

Answer 2

• Ligand binds GPCR —> conformational change in receptor which inc affinity for G alpha s —> causes Galpha s to dock on receptor—> docking causes switch from GDP to GTP —> triggers dissociation from receptor and breaks from beta and gamma subunits —> enhances interaction w/downstream effector (adenylyl cyclase)–> inc cAMP
  
  • G alpha s - ATP = more affinity for effector
  • G alpha s -ADP = more affinity for activated receptors and beta/gamma units

Question 3

G alpha q Mechanism of Action

Answer 3

Same as Galpha s but diff effector (PLC-beta)

Question 4

G alpha i Mechanism of Action

Answer 4

Galpha i -bound to GTP INHIBITS adenylyl cyclase (pop of Galpha s)

Question 5

What does activated PLC-Beta do?

Answer 5

• Activated PLC-beta —> hydrolysis of plasma membrane phospholipid —> IP3 (sugar) and diacylglycerol (lipid)
• IP3 —> binds/opens certain Ca++ channels —> inc intracellular Ca++
• Diacylglycerol —> activates protein kinase C

Question 6

Transducin (Gt) Mechanism of Action

Answer 6

Rhodopsin is a GPCR activated by light rather than ligand —> conformational change —> activated Gt —> when GTP-bound Gt activates cGMP phosphodiesterase —> breakdown of gCMP —> close gCMP-gated Na+ channels —> hyper polarization

Question 7

G olf Mechanism of Action

Answer 7

Coupled to olfactory receptors —> when GTP-bound activates adenylylcyclase —> inc cAMP —> open cAMP-gated cation channels —> depolarization (action potential)

Question 8

How are cAMP and PKA related and both restricted?

Answer 8

cAMP diffusion is limited kinetically and specially by phosphodiesterase (breakdown cAMP —> AMP)

cAMP binds R (regulatory) subunit of PKA -> release of activate C (catalytic) subunit —> phosphorylation in cell

• BUT…PKAs can be specially restricted by AKAPs (anchor PKAs by binding R subunits)

Question 9

CREB

Answer 9

Active PKA –> phosphorylation of CREBS which are transcription factors SO alters gene expression

Question 10

CDP Kinase II

Answer 10

Ca++ binds calmodulin —> triggers auto-phosphrylation of CaM Kinase II —> this kinase is in active/phosphorylated state even AFTER intracellular Ca++ levels come down

Question 11

Gbeta/Ggamma Signaling in Heart Muscle

Answer 11

• Once liberated, open K+ channels in heart —> K+ exits heart muscle cell —> harder to depolarize/contract
• Liberated when acetylcholine binds muscarinic receptors in heart muscle cells…SO acetylcholine has an inhibitory effect on heart muscle contraction

Question 12

Cholera Toxin

Answer 12

enzyme that catalyzes transfer of ADP ribose to Galpha s proteins —> can no longer hydrolyze GTP —> remains in active state —> amplified levels of cAMP —> act CFTR —> watery diarrhea

Question 13

Pertussis Toxin

Answer 13

catalyzes ADP-ribosylation of Galpha i —> stabilizes Galpha i in GDP-bound form —> blocks its ability to inhibit adenylyl cyclase -> inc cAMP

Question 14

RGS Proteins

Answer 14

• G alpha s and G alpha q termination
• RGS (regulator of G protein signaling) stimulate the otherwise weak GTPase activity of the alpha subunit
• Leads to hydrolysis —> Galpha s -GTP conformation which has LESS affinity for effector

Question 15

GPCRs in General

Answer 15

• family of receptors; all have 7 transmembrane units and extracellular and cytoplasmic domains
  
  • On plasma membranes
  • Mult GPCRs can bind same G protein —> extra complexity and tissue-specificity
• Can hetero-dimerize
  
  • Ex) Gi and Gq; normally Gi&raquo_space;Gq but inc in Gq in psychosis

Question 16

Beta-Arrestins

Answer 16

• proteins promote internalization of GPCRs via clathrin-coated vesicles
• Once endocytosed…GPCRs can be recycled, degraded OR have distinct signal pathways inside cell (paths that are INDEP of G protein complexes)

Question 17

Biased Ligand

Answer 17

• drugs that can be created to purposely favor the beta-arresting path OR the Galpha path (unique therapy)

Question 18

PARs

Answer 18

type of GPCR

protease cleaves part of the receptor which then acts as the ligand —> activation —> coupled to G proteins whose effectors lead to cell growth

Question 19

3 Ways That Abberant GPCR Regulation Can Lead to Cancer

Answer 19

• 1-overexpression of protease activated receptors (PARs -type of GPCR)
• 2-Mutations that make GCPR constitutively active - TSH receptor
• 3-persistent sim of receptors by agonists made by tumor cells

Question 20

Homologous Desensitization

Heterologous Desensitization

Answer 20

loss of responsiveness after repeated exposure to ligand

when activation of 1 receptor can desensitize another receptor
- Involves second messengers

Question 21

Nicotinic v Beta-Adrenergic Receptor Desensitization

Answer 21

Nicotinic-

• Intrinsic
• Fast (w/in fraction of second)
• Homologous
• Uncouples the binding of ligand to ion channel opening (all w/in the receptor protein)
• REG by must kinases (phosphorylation inc desensitization)

Muscarinic-

• Not intrinsic (happens via kinases/second mess)
• Slower (seconds or minutes)
• Homologous and heterologous
• Uncouples ligand binding from G protein activation
• MEDIATED by must kinases (necessary)

Question 22

Bark

Answer 22

• Beta adrenergic receptor kinase (Bark)
• Bark recognizes ligand-bound receptor and phosphorylates the C terminal —> causes arresting to bind the receptor —> endocytosis

Question 23

Denervation Supersensitivity

Answer 23

• Denervation/nerve injury —> turn on genes for embryo isoforms in addition to still making adult isoforms + OVERALL HIGHER # ACTIVE NUCLEI—> supersensitivity
• Denervation only occurs when loss of activity of nerve; so if you stimulate extracellularly this process will not occur (AKA activity-dependent)