Cell signalling 2

Question 1

What are the 3 main classes of cell surface signalling receptor?

Answer 1

> ion-channel-coupled receptors
G-protein-coupled receptors
Enzyme-coupled receptors

Question 2

What do ion-channel-coupled receptors do?

Answer 2

Membrane depolarisation

Question 3

What do G-protein-coupled receptors do?

Answer 3

Raise levels of second messenger molecules e.g. cAMP, Ca2+

Question 4

What do enzyme-coupled receptors do?

Answer 4

Signals activate enzyme activity integral to the receptor

- activate own intrinsic enzymatic activity

Question 5

What do changes in protein conformation allow?

Answer 5

Switching between active & inactive states in a reversible manner

Question 6

How are proteins turned on & off via protein phosphorylation?

Answer 6

On: Protein kinase + ATP
(Adds Pi)

Off: Protein phosphatase
(Removes Pi)

Question 7

How are proteins turned on & off via GTP-binding proteins?

Answer 7

On: GDP removed & GTP binds

Off: GTP hydrolysis
Removes Pi

Question 8

What are the features of G protein coupled receptors (GPCR)?

Answer 8

> 7-pass transmembrane receptors
Have extracellular, transmembrane & intracellular domains
wide range of signals

Question 9

Describe the structure of GPCR

Answer 9

3 subunits: alpha, beta & gamma

Alpha subunits = GTPase enzymes
= GTP -> GDP + Pi

Question 10

What happens in the resting state of GPCR?

Answer 10

Trimeric G protein tethered to inner leaflet of plasma membrane

Alpha subunit associated w/ GDP molecule

Question 11

Describe GPCR signalling

Answer 11

1. Signal binds
   - -> changes receptor conformation = allows G protein to bind to receptor
2. Alters G protein conformation
   - -> promotes release of GDP = replaced by GTP
3. Further conformational change in alpha subunit
   - -> activated & dissociates from beta & gamma complex & receptor

Question 12

How is cardiac pacing regulated by the vagus nerve?

Answer 12

1. AcH binds to receptor
   - > GTP activates alpha subunit
   - > leaves activated beta-gamma complex
2. activate beta-gamma causes K+ channel to open
   = depolarises plasma membrane
3. G-protein inactivation via GTP hydrolysis removes Pi
   - > alpha binds to beta-gamma complex
   - -> K+ channel closes

Question 13

How is GPCR signalling propagated by the active alpha/GTP subunit?

Answer 13

Alpha/GTP subunit associates w/ specific target enzymes

Target enzyme remains active while bound to subunit
= propagates the signal

Question 14

How are GPCR signals self-inactivated?

Answer 14

Alpha subunit interacts with target enzyme
-> activates GTPase activity of subunit
-> subunit hydrolyses GTP to GDP
= restores inactive alpha subunit conformation
& causes release from target enzyme
= restores resting state (unless signal is still present)

Question 15

What do enzymes activated by alpha/GTP subunits do?

Give examples

Answer 15

Raise the levels of second messenger molecules

Gs G proteins raise cAMP levels

Gq G proteins raise Ca2+ ion levels

Question 16

Describe how alphaGs increases cAMP levels

Answer 16

1. AlphaGs binds to adenylyl cyclase

2. Catalyses conversion of ATP -> cAMP via the removal of 2 Pi

Question 17

How is cAMP signalling reversed?

Answer 17

cAMP broken down by a phosphodiesterase

= linear 5’-AMP

Question 18

What do high levels of cAMP activate?

What does this result in the phosphorylation of?

Answer 18

A-kinase

cytoplasmic enzymes
–> changes their activity directly

TFs
–> alters gene expression

Question 19

Describe how alphaGq increases Ca2+ levels

Answer 19

1. Activated alpha subunit binds to Phospholipase C
2. Converts Inositol phospholipid –> IP3
3. IP3 binds to calcium ion channel
4. Ion channel opens -> Ca2+ diffuses out of ER into cytoplasm

Question 20

What do high levels of cytoplasmic Ca2+ cause?

Answer 20

Ca2+ binds to calmodulin

• > changes conformation
• > allows complex to interact w/ specific target proteins
• > alters target protein conformation + activity

Target proteins mainly = CaM-kinases (Ca2+/calmodulin modulated kinases)