III. Signal transduction and cell cycle | 41. cAMP signaling pathway; regulation of gene expression by cAMP

Question 1

I. Basics
1. What is Cyclic adenosine monophosphate (cAMP)?

Answer 1

Cyclic adenosine monophosphate (cAMP) is a 2nd messenger, used for intracellular signal transduction, such as: transferring the effects of hormones like glucagon and adrenaline into cells, which cannot pass through the plasma membrane.

Question 2

I. Basics
2. What is cAMP synthesized?

Answer 2

cAMP is synthesized from ATP by adenylate cyclase located on the inner side of the plasma membrane.

Question 3

I. Basics
3. What is the location of adenylate cyclase?

Answer 3

It is located on the inner side of the plasma membrane.

Question 4

I. Basics
4. How does the process of cAMP synthesis occur?

Answer 4

• A ligand will bind on a receptor, which is a G-protein coupled receptor (GPCR), will cause conformation change.
• The change will activate the stimulatory G-protein (Gs) by the replacement of GDP with a GTP
• Gs can then activate adenylate cyclase, which will catalyze the production of cAMP from ATP.

Question 5

II. Protein kinase A
1. How does cAMP work?

Answer 5

cAMP works by mainly activating protein kinase A, which mediates most of the cellular effects of cAMP.

Question 6

II. Protein kinase A
2. How does Activation of PKA work?

Answer 6

• In the inactive state, PKA consists of a complex of two regulatory subunits and two catalytic subunits – with the regulatory subunits blocking the catalytic centers of the catalytic subunits
• The binding of cAMP molecules to the regulatory subunits alters the conformation of the complex, causing the dissociation between the regulatory and catalytic subunits
• The released catalytic subunits are then able to phosphorylate target proteins.

Question 7

II. Protein kinase A
3. What are the effects of PKA?

Answer 7

• In the cytosol, PKA can alter metabolic routes and other signaling pathways by phosphorylating proteins, thereby switch them ON/OFF.
• Some examples:
  +) Phosphorylation of glycogen synthase and pyruvate kinase will lead to inactivation
  +) Phosphorylation of phosphorylase kinase will lead to activation. The activated phosphorylase kinase will go on and phosphorylate glycogen phosphorylase, leading to its activation

Question 8

II. Protein kinase A
4. What are the examples for the effects of PKA?

Answer 8

1. Phosphorylation of glycogen synthase and pyruvate kinase will lead to inactivation
2. Phosphorylation of phosphorylase kinase will lead to activation.
   - The activated phosphorylase kinase will go on and phosphorylate glycogen phosphorylase, leading to its activation

Question 9

II. Protein kinase A - EUKARYOTES
5B. How can PKA initiate transcription in EUKARYOTES?

Answer 9

• After PKA phosphorylates and activates the CREB protein, it will bind to a DNA sequence called CRE
  (cAMP response element)
• CREB protein will then recruit CBP (CREB-binding protein), which has histone acetyltransferase activity = makes DNA more accessible
• When CREB protein is bound to CRE together with CBP, general TFs and RNA polymerase II will be recruited, to initiate transcription of downstream genes

Question 10

II. Protein kinase A - EUKARYOTES
5B. How does PKA work in PROKARYOTES?

Answer 10

• cAMP, for example, is involved in the positive regulation of the lac operon.
• In an environment with a low glucose concentration, cAMP accumulates and binds to the allosteric site on CRP (cAMP receptor protein), a transcription activator protein.
• The protein assumes its active shape and binds to a specific site upstream of the lac promoter, making it easier for RNA polymerase to bind to the adjacent promoter to start transcription of the lac operon, increasing the rate of lac operon transcription.
• With a high glucose concentration, the cAMP concentration decreases, and the CRP disengages from the lac operon.