10 - Cyclic Nucleotides

Question 1

Example of cyclic nucleotides

Answer 1

• generation and termination of the cAMP.cGMP signal
• the enzymes involves, cyclases and phosphodiesterases, are expressed in numbers isoforms controlled by various input signals
  -nuceloside 5”-triphosphate (+ cyclase as an input signal) -> cyclic nucleoside 3’, 5’- monophosphate (+phosphodiesterase as a input signal) -> nucleoside 5’- monophosphate

Question 2

What makes cyclic AMP?

Answer 2

The enzyme adenylate cyclase

Question 3

What is adenylate cyclase

Answer 3

• adenylate cyclase is a two‐component enzyme system
• it ultimately catalyses the cyclase reaction, but only when it is associated with the hormone‐bound receptor and a regulatory protein called a stimulatory G‐protein (guanylate nucleotide binding protein), which activates adenylate cyclase - the G‐protein is the intermediate between the receptor and the synthesis of cyclic AMP

Question 4

What is the action of G-proteins?

Answer 4

• G‐proteins exist either in an active or an inactive state, depending on the guanylate nucleotide that is bound
• in the inactive state, G‐protein binds to GDP
• in the active state, GTP is bound to the G‐protein. G‐proteins have an intrinsic GTPase activity, which converts bound GTP to GDP
• hydrolysis of GTP by the G‐protein converts the G‐protein back to an inactive state

Question 5

Cycle of G-protein

Answer 5

1. Hormone binds to receptor.
2. The hormone‐bound receptor binds to the G‐protein and causes GDP to be replaced by GTP.
3. GTP‐bound G‐protein interacts with adenylate cyclase.
4. G‐protein hydrolyzes bound GTP to GDP, thereby going back to the ground state

Question 6

Action of cAMP

Answer 6

1. binding of ligand to a specific receptor in the cell membrane
2. activation of G-protein: after he formation of complex, GDP is replaced by GTP
3. activation of enzyme ADENYLATE CYCLASE: activated G-protein either stimulates or inhibits the enzyme adenylate cyclase which is located in plasma membrane
4. formation of cAMP: when adenyl cyclase activated it catalyses the formation of cAMP from cytoplasmic ATP with Mg2+ as cofactor, thus a stimulatory G-protein increase the cAMP level whereas inhibitory G protein decreases the cAMP level
5. action of cAMP: it activates protein kinase A - PKA then phosphorylate other proteins which gives rise to cascade of mechanism that leads to cell response

Question 7

What is the relationship between glycolysis and gluconeogenesis?

Answer 7

Glycolysis is regulated in a reciprocal fashion compared to its corresponding anabolic pathway, gluconeogenesis

Question 8

When does reciprocal regulation occur?

Answer 8

• reciprocal regulation occurs when the same molecule or treatment (phosphorylation, for example) has opposite effects on catabolic and anabolic pathways
• reciprocal regulation is important when anabolic and corresponding catabolic pathways are occurring in the same cellular location

Question 9

What are the three enzymatic points that control of glycolysis occurs at?

Answer 9

1. glucose <=> G(6)P by hexokinase
2. F(6)P <=> F(1), 6BP by phosphofructokinase
3. PEP <=> pyruvate by pyruvate kinase

Question 10

What happens when the cell has high ATP concentration relative to AMP?

Answer 10

• it tells the cell to stop producing ATP via glycolysis and begin generating glucose via gluconeogenesis to store it is glycogen

Question 11

What happens when glycolysis is halted and gluconeogeneisis is activated?

Answer 11

• phosphofructokinase is inhibited by ATP, H+ and citrate
• hexokinase is inhibited by G6-P
• pyruvate kinase is inhibited by ATP, alanine, and phosphorylation
• fructose 1,6-bisphosphatase is activated citrate
• pyruvate carboxylase is activated by acetyl CoA

Question 12

What happens when the cell has a low level of ATP relative to AMP

Answer 12

The cell produces more ATP via glycolysis and stops using ATP to form glucose via gluconeogenesis

Question 13

What happens when glycolysis is activated and gluconeogenesis is inhibited?

Answer 13

• phosphofructokinase is activated AMP and F2,6-BP
• pyruvate kinase is activated by F1,6-BP
• fructose 1,6-bisphosphatase is inhibited by AMP and F2,6-BP
• PEP carboxykinase is inhibited by ADP
• pyruvate carboxylase is inhibited by ADP

Question 14

How is the production of cGMP stimulated?

Answer 14

Nitric Oxide can stimulate production of cGMP by interacting with the haem group of the enzyme souble guanylate cyclase (sGC). This interaction allows sGC to convert GTP into cGMP

Question 15

What is the effect of cGMP?

Answer 15

• once produced cGMP can have a number of effects in cells, but many of those effects are mediated through the activation of protein kinase G (PKG)
• active PKG is ultimately responsible for many of the effects of Nitric Oxide including its effects on blood vessel relaxation (vasodilation)

Question 16

What does activation of PKG by cGMP lead to?

Answer 16

• once produced cGMP can have a number of effects in cells, but many of those effects are mediated through the activation of protein kinase G (PKG)
• active PKG is ultimately responsible for many of the effects of Nitric Oxide including its effects on blood vessel relaxation (vasodilation).
  -activation of PKG by cGMP leads to activation of myosin phosphatase which in turn leads to release of calcium from intracellular stores in smooth muscle cells
• this in turn leads to relaxation of the smooth muscle cells
• in the case of vasodilation the Nitric Oxide is originally produced in the neighbouring endothelial cells before diffusing into the smooth muscle cells where it activates sGC and cGMP

Question 17

Give examples of drugs that block cyclic nucleotide degradation

Answer 17

PDEs