deck_637867

Question 1

What do cells require in order to respond to extracellular signalling molecules?

Answer 1

Receptors

Question 2

What are three types of receptors?

Answer 2

• Ligand-gated ion channel (nicotinic acetylcholine)- Receptors with intrinsic enzymatic activity (insulin receptors)- G protein coupled receptors (muscarinic acetylcholine receptors)

Question 3

What is the function of g protein coupled receptors?

Answer 3

• Muscle contraction* Stimulus-secretion coupling* Catabolic and anabolic processes* Light, taste and smell perception

Question 4

What is the structure of a g protein coupled receptor?

Answer 4

• Heterotrimeric alpha, beta and gamma subunits* Beta and gamma bind together and function as a single subunit

Question 5

How do g protein coupled receptors work?

Answer 5

1. The alpha subunit has a guanine nucleotide binding site which binds GTP and slowly hydrolyses it to GDP2. Receptor activation leads to GDP being released from the alpha-subunit and GTP binding in its place3. Binding of GTP decreases the affinity of the alpha subunit for the receptor and the beta-gamma subunit4. Both the alpha and the beta-gamma subunits are released and can interact with effectors5. Effector interaction is terminated by the intrinsic GTPase activity of the alpha subunit > GTP hydrolysed to GDP6. Affinity of the alpha sub-unity for the beta-gamma subunit increases7. The alpha-beta-gamma subunit is reformed and awaits reactivation by an agonist-activated receptor to re-initiate the cycle

Question 6

What effect does the pertussis toxin have on g-protein coupled receptors?

Answer 6

• Contains an enzyme which chemically modifies and inactivates Gi type proteins – thus uncoupling the receptor-effector linkage

Question 7

What effect does the cholera toxin have on g-protein coupled receptors?

Answer 7

• Contains a similar enzyme activity which modifies Gs type proteins* Leads to irreversible activation – heterotrimeric unit is not reformed

Question 8

What is adenylyl cyclase?

Answer 8

• An integral plasma protein which can either be activated by Gs or inhibited by Gi via the activation of different receptors

Question 9

Give an example of a situation in which adenylyl cyclase is activated?

Answer 9

By noradrenaline at beta-adrenoreceptors

Question 10

Give an example of when adenylyl cyclase receptors are inhibited?

Answer 10

At alpha 2-adrenoreceptors

Question 11

What does adenylyl cyclase do in cells?

Answer 11

1. The enzyme hydrolyses cellular ATP to generate cAMP2. cAMP then interacts with protein kinase A3. PKA then phosphorylates a variety of other proteins within the cells to affect (either positively or negatively) their activities

Question 12

What does increased cAMP cause in cells?

Answer 12

• Increased glycogenolysis and gluconeogenesis in the liver* Increased lipolysis in adipose tissue* Relaxation of a variety of types of smooth muscle* Postive inotropic and chronotropic effects in the heart

Question 13

What is phospholipase C?

Answer 13

• The effector enzyme in the signalling pathway involving the hydrolysis of PIP2 to InsP3 and DAG