Lecture 27

Question 1

GPCRs

Answer 1

• largest family of cell surface receptors
• can bind to hormones, local mediators and neurotransmitters
• composed of a single polypeptide chain that spans the lipid bilayer 7 times
• ancient, can be found in prokaryotes

Question 2

activation of a GPCR

Answer 2

• binding of an extracellular signal molecule to a GCPR causes it to change conformation causing the associated G-protein to decrease affinity for GDP
• GDP dissociates and is exchanged for GTP
• sometimes g-protein subunits dissociate (this doesn’t always happen) and are switched on
• this in turns activates trimeric G-protein which results in transmission of a signal
• g-protein then interact with target enzymes or ion channels

Question 3

describe the structure of a G-protein

Answer 3

• composed of a, y, b subunits
• a and y are tethered to the plasma membrane by a short lipid tail
• when unstimulated, a is bound to GDP

Question 4

what dictates the length of the response

Answer 4

• the amount of time the g-protein subunits are switched on

Question 5

explain the switching off process

Answer 5

• the a subunit contain a GTPase activity which can hydrolyze GTP to for GDP
• the a subunits reassembles with the by complex and the protein returns to its original, inactive state

Question 6

G-proteins and ion channels

Answer 6

Example: regulation of the heart
- acetylcholine binding to GCPRs of heart pacemaker cells activate G-protein Gi
- then the by subunit binds to K+ ion channels causing it to open
- channel closes when GTP is cleaved and the subunits reassembles with one another

Question 7

the two most common enzyme targets include

Answer 7

• adenylyl cyclase which produces cAMP
• phosphlipase C which produces inositol triphosphate (IP3) and diacylglycerol (DAG)

Question 8

Give examples of second messengers

Answer 8

• cAMP
• IP3
• DAG

Question 9

cAMP signalling pathway

Answer 9

• the g-protein Gs is responsible for the activation of adenylyl cyclase which generates cAMP from ATP, releasing PPi
• cAMP phosphodiesterase converts cAMP into AMP using water

Question 10

what role does caffeine play in cAMP signalling pathway

Answer 10

• it inhibit cAMP phosphodiesterase keeping cAMP levels high

Question 11

cAMP- glycogen breakdown

Answer 11

• cAMP activate protein kinase A (PKA)
• PKA is normally inactivated by regulatory proteins
• when cAMP binds to PKA it releases regulatory proteins
• PKA can then phosphorylate other proteins (like glycogen phosphorylase in skeletal muscle)
• glycogen breakdown is an example of a relatively quick response

Question 12

cAMP slow responses

Answer 12

• cAMP can also cause activation of gene expression - a relatively slow response
• PKA phosphorylates transcriptional regulators which can initiate transcription

Question 13

what is cholera toxin

Answer 13

• a bacterial toxin that modifies the a subunit of the g-protein so that it can no longer hydrolyze GTP therefore the g-protein is always active
• result: in the intestine this can result in a prolonged outflow of Cl- and water (dehydration)

Question 14

Inositol phospholipid pathway

Answer 14

• phospholipase C converts membrane phospholipids into diacylglycerol (DAG) which stays in the membrane and IP3 which is released into the cytosol
• IP3 binds to and opens Ca2+ channels embedded in the ER and free Ca2+ is released into the cytosol
• DAG recruits protein kinase C (PKC)
• activation of PKC requires the binding of Ca2+
• PKC phosphorylates many intracellular protein

Question 15

Ca2+ signalling

Answer 15

• it is important in muscle contraction and secretion
• when Ca2+ channels open, Ca2+ rushes down its electrochemical gradient

Question 16

What is the most common protein Ca2+ binds to

Answer 16

calmodulin

Question 17

Calmodulin

Answer 17

• binds to four Ca2+ ions, inducing a conformational change allowing it to interact with proteins like Ca2+ / calmodulin dependent-protein kinases (CaM-kinases)

Question 18

GPCR signalling- photoreceptors

Answer 18

• light activates GCPR known as rhodopsin
• rhodopsin activates g-protein known as transducin
• transducin activates cyclic GMP phosphodiesterase which reduces cGMP levels by converting it to GMP
• signalling is very fast despite the multiple steps

Question 19

is vision possible in dim light? why?

Answer 19

• yes because photoreceptors can amplify signal so that vision is possible even in dim light

Question 20

What does it mean when we say “photoreceptors are capable of adaption?”

Answer 20

• the adjustment of sensitivity
• allows you to see in bright sunlight
• strong light reduces Ca2+ concentrations which is required by signal amplification enzyme