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Question 1

what are the three mechanisms of small molecule signaling?

Answer 1

1. independent of plasma membrane proteins
2. through ion channels
3. downstream of G-protein-coupled receptors

Question 2

how do small, hydrophobic molecules move into cells?

Answer 2

• diffuse passively into cells
• do not require a transmembrane ion channel or a receptor for signal perception

Question 3

give five examples of small, hydrophobic molecules that can passively diffuse into cells

Answer 3

O2, CO2, N2, NO, steroid hormones

Question 4

what receptors do small, hydrophobic molecules bind to?

Answer 4

they are bound by intracellular receptors

Question 5

describe how small, hydrophobic molecules cause changes in cells

Answer 5

1. transported by carrier proteins to help them move through the hydrophilic extracellular space
2. released from the carrier and diffuse into the target cell
3. often bound by nuclear receptor superfamily proteins within the cell

Question 6

what two subtypes does the nuclear receptor superfamily include?

Answer 6

includes both nuclear receptors and cytoplasmic receptors that then translocate to the nucleus

Question 7

describe the structure of the nuclear receptor superfamily

Answer 7

• N-terminal transcription-activating domain activates transcription of target genes
• middle DNA-binding domain binds promoters of specific target genes
• C-terminal ligand-binding domain binds to small hydrophobic signaling molecules

Question 8

what are ‘orphan’ nuclear receptors?

Answer 8

bind unknown ligands

Question 9

explain how nuclear superfamily proteins act as both receptors and effectors

Answer 9

1. ligand binding causes a conformational change in the protein
2. DNA-binding domain is now free to bind promoters of target genes
3. inhibitory proteins are released and coactivator proteins are recruited

Question 10

use nitric oxide (NO) as an example of a small, hydrophobic signal molecule

Answer 10

• NO is fast and locally-acting due to instability (5-10s)
• NO can diffuse out of endothelial cell and activate signaling in smooth muscle cell
• guanylyl cyclase is both the receptor and the effector

Question 11

NO is made from

Answer 11

arginine

Question 12

an —— gradient exists across the plasma membrane

Answer 12

electrochemical

Question 13

Na+ electrochemical gradient

Answer 13

extracellular concentration > cytoplasmic concentration

Question 14

K+ electrochemical gradient

Answer 14

cytoplasmic concentration > extracellular concentration

Question 15

ion-channel-coupled receptors

Answer 15

• gated by a signaling molecule
• closed until a signal is received
• once open, channels mediate passive transport: ions will flow down their electrochemical gradient

Question 16

state 4 ways in which ion channels may be gated

Answer 16

• voltage-gated
• ligand-gated (extracellular ligand)
• ligand-gated (intracellular ligand)
• mechanically gated

Question 17

describe a resting synapse

Answer 17

1. synaptic vesicles of neurotransmitters are waiting near the plasma membrane of the pre-synaptic cell
2. gated ion channels in the target cell are closed

Question 18

describe an active chemical synapse

Answer 18

1. a nerve impulse causes neurotransmitter release by vesicle fusion to the plasma membrane
2. neurotransmitters in the extracellular space open the ligand-gated ion channels in the target cell
3. ions move into the target cell down their concentration gradient, triggering responses in the target cell

Question 19

what is the overall function of G-protein-coupled receptors (GPCRs)?

Answer 19

to mediate diverse signals

Question 20

how many GPCRs are there in humans?

Answer 20

over 800

Question 21

what is the pharmaceutical importance of GPCRs?

Answer 21

almost half of pharmaceuticals target these pathways

Question 22

how are GPCRs activated?

Answer 22

by small molecules, light, proteins, and more

Question 23

describe the structure of GPCRs

Answer 23

• 7-transmembrane domain proteins
• heterotrimeric large G-protein complex has 3 subunits: Gα, Gβ, Gγ

Question 24

describe what happens when a GPCR is activated

Answer 24

1. the activated GPCR acts as a GEF to exchange GDP for GTP on a Gα subunit. GTP-bound Gα is active
2. once activated GTP-Gα typically dissociates from Gβγ (or sometimes they stay stuck together; it depends on the G-protein). both Gα and Gβγ can activate different targets
3. over time, GTPase activity of Gα hydrolyses GTP to GDP. regulator of G-protein signaling (RGS) can also act as a GAP to promote GTP hydrolysis by Gα
4. GTP-bound Gα will re-associate with Gβγ

Question 25

how do different G proteins differ?

Answer 25

- activate different signaling pathways - some rely more on Gα, some more on Gβγ, some on all three subunits - some signal through adenylyl cyclase to make cAMP; some activate phospholipase enzymes to signal through lipids and Ca2+ channels

Question 26

what is cAMP?

Answer 26

a small molecular secondary messenger

Question 27

how is cAMP synthesised?

Answer 27

from ATP by adenylyl cyclase

Question 28

how is cAMP broken down?

Answer 28

it is broken down to AMP by cAMP phosphodiesterase

Question 29

describe how cellular concentrations of cAMP change

Answer 29

cellular concentrations of cAMP are usually very low but large amounts are made in response to signals

Question 30

how does GPCR activation lead to an increase in cAMP concentrations?

Answer 30

- GPCR activates the heterotrimeric G-protein complex Gs - GTP-bound Gα from Gs activates adenylyl cyclase - adenylyl cyclase converts ATP to cAMP

Question 31

inactive Protein Kinase A (PKA)

Answer 31

4 subunits: 2 regulatory subunits, 2 catalytic subunits

Question 32

active PKA

Answer 32

- 2 molecules of cAMP bind each regulatory subunit - catalytic subunits are released and are now active kinases

Question 33

what impact does cAMP have on PKA?

Answer 33

it activates it

Question 34

descrive Gs signaling

Answer 34

1. Ligand binds and activates the GPCR 2. Activated GPCR acts as a GEF to exchange GDP for GTP on the G⍺ subunit of Gs 3. GTP-G⍺ activates adenylyl cyclase 4. Adenylyl cyclase coverts ATP to cAMP 5. cAMP binds regulatory subunits of PKA to release the catalytic domains from inhibition 6. Activated PKA moves to the nucleus 7. Activated PKA phosphorylates CREB 8. CREB-binding protein + phosphorylated CREB bind cAMP-responsive element (CRE) to activate transcription of target genes

Question 35

what mediates the sense of smell for humans?

Answer 35

GPCRs and cAMP: - humans have ~350 different GPCRs that each recognise a different odourant

Question 36

what physiological adaptation enables precise odour coding?

Answer 36

each olfactory neuron expresses many copies of only one GPCR type per neuron in the cilia

Question 37

how do olfactory GPCRs signal?

Answer 37

1. odorant recognition activates the GPCR, which activates Golf 2. Golf triggers cAMP production 3. cAMP opens cAMP-gated cation channels 4. cation influx triggers an action potential

Question 38

why is carbon monoxide so lethal?

Answer 38

we cannot smell it as we do not have the GPCR that binds to it

Question 39

describe Gq signaling

Answer 39

1. signal activates GPCR 2. GPCR makes GTP-Gα on Gq, releasing Gβγ 3. GTP-Gα and Gβγ activate PLCβ 4. PLCβ cleaves PI(4,5)P2 to DAG and IP3 5. IP3 diffuses in the cytoplasm to open Ca2+ channels in the ER, causing Ca2+ to move into the cytoplasm. DAG diffuses in the membrane 6. DAG and Ca2+ activate PKC (coincidence detector) 7. PKC phosphorylates downstream targets

Question 40

what do DAG and IP3 have in common?

Answer 40

they are small molecule secondary messengers