Biosignaling 1

Question 1

definition of a signal

Answer 1

non-covalent interaction between ligand and receptor

Question 2

intrinsic effect

Answer 2

physiological effect

Question 3

receptor definition

Answer 3

membrane-bound or soluble protein/protein complex which exerts an intrinsic effect after the binding of a ligand

Question 4

antagonist vs agonist

Answer 4

agonist - ligand or structural analog which binds to receptor and produces effects of the natural ligand
antagonist - structural analog that binds and produces no signal and blocks effects of agonists

Question 5

6 features of signal transduction

Answer 5

specificity
amplification
modularity
desensitization/adaptation
integration
localization

Question 6

specificity of signal transduction

Answer 6

complementarity, non-covalent bonding, tissue specific receptor, tissue specific target receptor
ex. thyrotropin releasing hormone TRH only has receptors in the pituitary

Question 7

amplification of signal transduction

Answer 7

enzymes activate other enzymes, and the number of affected molecule grows exponentially

Question 8

modularity of signal transduction

Answer 8

multiple domains or motifs give different options to what response will be induced by binding receptor
some will not be catalytic with the receptor

Question 9

desensitization/adaptation of signal transduction

Answer 9

response to signal transduction acts as negative feedback to reduce response
receptor fatigue with continued stimulation
ex. smell, skin sensation, bright light after dark

Question 10

integration of signal transduction

Answer 10

when 2 signals have opposite effects and regulatory outcome results from integrated input of both (crosstalk)
balance of signals, net signal wins

Question 11

localization of signal transduction

Answer 11

enzyme to degrade intracellular message is located near receptor to prevent diffusion of message, keeping it local in response

Question 12

GPCR overview

Answer 12

stands for Guanosine-protein coupled receptor
conformational change of receptor triggers association of G protein
beta-andrenergic receptor is an example
50% of drugs target GPCR, largest class of signaling receptors are GPCR

Question 13

G protein structure

Answer 13

3 subunits
Gs (stimulating protein): Gsy, GsB, and Gsa
Gsy and GsB are anchored subunits
1. Gsa contains GDP, association with epinephrine-receptor complex causes displacement of GDP and binding of GTP, activating Gsa
2. GTP-bound Gsa then dissociates from other subunits and activates with adenylyl cyclase
3. adenylyl cyclase converts ATP to cAMP second messenger (pyrophosphate released), to activate PKA
4. intrinsic GTPase activity in Gsa hydrolyzes GTP to GDP, thus ending signal transduction (reassociates with other subunits)

Question 14

Protein kinase A structure

Answer 14

2 catalytic subunits connected to an anchoring protein keeping it inactive
1. AKAP - A Kinase Anchoring protein, holds two dimer domains with regulatory subunits
2. On the regulatory subunits are the catalytic subunits
3. 2 cAMP binds to each of 2 regulatory subunit, causing dimerization
4. this makes substrate binding cleft of of catalytic subunit active and available

Question 15

cellular proteins regulated by PKA-cAMP

Answer 15

glycogen synthesis - glycogen synthase
glycogen breakdown - phosphorylase b kinase (a and b subunits)
glycolysis - pyruvate kinase (rat liver)
conversion pyruvate to acetyl coA - pyruvate dehydrogenase complex L
Triacyl mobilization and FA oxidation - hormone-sensitive lipase
glycolysis/gluconeogenesis - phosphofructokinase 2

Question 16

example of signal amplification with glycogen breakdown

Answer 16

1. GPCR cAMP PKA pathway cascades to activate phosphorylase b kinase
2. which activates glycogen phosphorylase a,
3. which catalyzes breakdown of glycogen to glucose-1-phosphate and then glucose released
   each step increases molecules activated by 10x

Question 17

Steps of cAMP GPCR termination

Answer 17

1. cyclic nucleotide phosphodiesterase converts cAMP to AMP, terminating cascade pathway
2. modulators of GTPase activity GAP (GTPase activator protein) and RGS (regulators of G protein signaling) strengthen hydrolyzing GTP to GDP, inactivating G protein

Question 18

Desensitization of beta-andrenergic receptor mechanism

Answer 18

1. Once Gsa has dissociated, GsBy subunits recruits B-andrenergic receptor kinase (Bark)
2. Bark phosphorylates serine residue on terminal end of receptor
3. B arrestin binds to these phosphorylated serine residues, block reassociation of G protein
4. B-arrestin-receptor (Barr) complex is endocytosized from cell membrane
5. When Barr disassociates, receptor is dephosphorylated and returns to plasma membrane

Question 19

For beta-andrenergic pathway, stimulators and inhibiting pathways

Answer 19

epinephrine (b-andrenergic) as ligand stimulates the response pathway
prostaglandins E1, E2, (PGE1, PGE2) produce inhibitory pathway, alpha-andrenergic
net response is how cells will respond

Question 20

localization/nucleation using AKAP5

Answer 20

A Kinase Anchoring Protein, anchors process to the cell membrane
scaffolding protein binds PKA, b-andrenergic receptor, 2 phosphatases and adenylyl cyclase to facilitate signal transduction pathway

Question 21

4 classes of G protein alpha subunit

Answer 21

Gsa - stimulates adenylyl cyclase
Gsi - inhibits adenylyl cyclase
Gqa - stimulates phospholipase C
G12a/G13a - regulate actin cytoskeleton

Question 22

GPCR pathway using IP3 and Ca2+

Answer 22

1. hormone binds receptor, conformational change occurs
2. Gqa binds receptor, displacing GDP for GTP
3. GTP bound Gqa associates with phospholipase C, activating it
4. Phospholipase C cleaves PIP2 –> DAG and IP3
5. DAG moves laterally to protein kinase C, binding (cell membrane)
6. IP3 binds Ca2+ ion channel on ER, releasing Ca2+ into cytosol
7. Ca2+ binds to protein kinase C with DAG and they activate it
8. Protein kinase C phosphorylates proteins producing cellular response

Question 23

signals that act through IP3/Ca2+ pathways

Answer 23

acetylcholine, angiogenin, glutamate, GRH (Gonadotropin-releasing hormone), Histamine 1, light, oxytocin, serotonin, TRH (thyrotropin-releasing hormone), vasopressin