Signaling: Receptors

Question 1

Structure of G-protein coupled receptor

Answer 1

7 helical transmembrane domain with connecting loops, intracellular C terminus and extracellular N terminus.
Forms barrel structure in membrane with ligand binding pocket extracellular and associated G-protein tetramer intracellular.

Question 2

G-protein trimer components

Answer 2

Galpha, Beta gamma, and GDP

Question 3

How are G-proteins activated?

Answer 3

Ligand binding to GPCR causes conformation change that catalyzes GDP dissociation from G-alpha. High cytosolic GTP concentration causes GTP association. GTP binding causes G-alpha and Beta-gamma subunits to dissociate from GPCR but remain at membrane due to lipid modifications.

Question 4

G-protein inactivation

Answer 4

Endogenous GTPase activity will cleave GTP into GDP and cause re-association of G-protein trimer with GPCR.
GAPs accelerate this process.

Question 5

GAP

Answer 5

GTPase activating protein

Question 6

GEF

Answer 6

Guanine Nucleotide Exchange Factor

Question 7

Two G-protein associated second messengers

Answer 7

Gs protein activates Adenylyl cyclase which produces cAMP

Gq protein activates phospholipase C which cleaves PIP2 into DAG and IP3 second messengers

Question 8

Function of second messengers?

Answer 8

Molecules produced/released following extracellular signals that propagate signal through cytoplasm or to nucleus to cause cellular effect.

Question 9

Receptor Desensitization

Answer 9

Prolonged ligand exposure leads to G-Receptor Kinase activation which phosphorylates GPCRs and inhibits their binding of G-proteins. Phosphorylation recruits beta-arrestin, a scaffold adapter, which also inhibits G-protein binding.
GPCRs can be internalized and then recycled to PM or degraded in lysosome.

Question 10

Beta Arrestin

Answer 10

Binds GPCR following phosphorylation by GRK. Able to produce signaling through ERK or JNK MAPK pathways

Question 11

Drugs that affect G-protein cascades (2)

Answer 11

Beta blockers: Beta1-adrenergic R antagonists that block binding of NE and prevent Gs activation of AC for cAMP production. This reduces cardiac contraction

Phosphodiesterase inhibitors: prevent cAMP degradation and increase signal longevity. These can be general: caffeine or theophyline. Or specific: Milinirone for PDE3 or Rolipram for PDE4

Question 12

Beta1 Adrenergic Rs

Answer 12

NE binding
Gs subunit, AC activation, Ca enters, increased heart rate
Antagonists: Beta blockers

Question 13

Alpha1 Adrenergic Rs

Answer 13

NE, Epi, Phenylephrine binding
Gq subunit, PLC activating
Smooth muscle contraction
Antagonists: Prazosin

Question 14

M2 Muscarinic Rs

Answer 14

ACh binding
Gi subunit, AC inhibited
Reduced heart contraction
Beta gamme unit and GIRK channel
Antagonist: Atropine

Question 15

Beta2 Adrenergic Rs

Answer 15

Epinephrine, Isoproterenol, Albuterol binding
Gs subunit, AC activation
Bronchodilation, smooth muscle relaxation

Question 16

M3 Muscarinic Rs

Answer 16

ACh Binding
Gq subunit, PLC activating
Bronchoconstriction
Antagonists: Ipratroprium (long acting)