Intro to Pharm and Tox Topics   

cAMP/PKA System

There are stimulatory G-proteins (Gs) and inhibitory G-proteins (Gi). Activation of a Gs leads to cAMP formation when the alpha subunit binds to adenylyl cyclase, a membrane protein that converts ATP to cAMP. GI inhibits adenylyl cyclase.

There are multiple subtypes of adenylyl cyclase which are activated or inhibited by a number of factors. Some isozymes respond to the beta/gamma complex, others respond to protein kinases. Adenylyl cyclase has a short intracellular N-terminus, two hydrophobic stretches of 6 transmembrane helices each, and two intracellular domains (M1 and M2) require for activity.

cAMP can follow one of three possible fates: it may be broken down, can activate protein kinases, or open ion channels. Cytosolic or membrane bound phosphodiesterases convert cyclic nucleotides into mononucleotides, cAMP into AMP. Phosphodiesterases may be activated by Ca2+, phosphorylation or cGMP, and are inhibited by caffeine-like drugs.

cAMP binds to and activates cAMP dependent protein kinases (PKAs), which are dimers of proteins containing one catalytic and one regulatory subunit. Each protein has a binding site for cAMPand another for the protein they phosphorylate. Two cAMP molecules bind to each regulatory subunit to liberate the catalytic subunits:

R2C2   +   4cAMP     R2(cAMP)4   +  2C

The catalytic subunits of PKA can then phosphorylate the cAMP response element binding protein (CREB) which in turn binds to transcription factors and influence genetic expression. PKA phosphorylates serine and threonine in proteins containing the consensus sequence Arg-Arg-X-Ser/Thr-X.

The cAMP/PKA transduction cascade is regulated by:

• Gs-protein and GI-protein coupled receptors
• phosphodiesterases (PDE), which are in turn inhibited by caffeine-like drugs
• forkolin, and activator of adenylyl cyclase
• PKA inhibitors/activators, usually analogs of cAMP

Continue to "cGMP/PKG and Nitric Oxide" or take a quiz: [Q1] [Q2] [Q3] .

Bact to Basics: G-Protein Receptors (Physiology)

Need more practice? Answer the review questions below (after sponsor).

1- What is the function of a Gs-proteins

2- What is the function of GI-proteins?

3- What is adenylyl cyclase?

4- How is adenylyl cyclase activated?

5- Describe the structure of adenylyl cyclase.

6- List 3 fates of cAMP

7- What are phosphodiesterases?

8- How are phosphodiesterases activated?

9- What are PKAs?

10- How do cAMP activate PKAs?

11- What is the activity of active PKA?

12- What is CREB?

13- List 4 regulators of the cAMP/PKA Transduction system

Continue scrolling to answers below (after sponsor).

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Answers:

1- What is the function of a Gs-proteins
Stimulatory G-proteins, their activation leads to cAMP formation when the alpha subunit binds adenylyl cyclase.

2- What is the function of GI-proteins?
Inhibitory G-proteins, their activation leads to inhibition of cAMP formation when the alpha subunit binds to adenylyl cyclase.

3- What is adenylyl cyclase?
A membrane protein that converts ATP to cAMP.

4- How is adenylyl cyclase activated?
Usually by the alpha subunit of a Gs protein, although some isozymes respond to the beta/gamma complex, others to protein kinases.

5- Describe the structure of adenylyl cyclase
Has a short intracellular N-terminus, two hydrophobic stretches of 6 transmembrane helices each, and two intracellular domains, M1 and M2, required for activity.

6- List 3 fates of cAMP
broken down by phosphodiesterases
protein kinase activation
ion channel opening

7- What are phosphodiesterases?
Proteins that convert cyclic nucleotides into mononucleotides, like cAMP to AMP, may be cytosolic or membrane bound.

8- How are phosphodiesterases activated?
By Ca2+, phosphorylation or cGMP.

9- What are PKAs?
cAMP-dependent protein kinases (PKAs) are dimers of proteins containing one catalytic and one regulatory subunit. Each subunit contains binding sites for cAMP and for the protein they phosphorylate.

10- How do cAMP activate PKAs?
Two cAMP molecules bind to each regulatory subunit to liberate the catalytic subunits:
R2C2   +   4cAMP  --->   R2(cAMP)4   +  2C

11- What is the activity of active PKA?
The free catalytic subunits of PKA phosphorylate serines and threonines in proteins with the consensus sequence: Arg-Arg-X-Ser/Thr-X

12- What is CREB?
cAMP response element binding protein (CREB), when phosphorylated by PKA, binds to transcription factors and influence gene expression.

13- List 4 regulators of the cAMP/PKA Transduction system
G-protein coupled receptors, GI and Gs
phosphodiesterases
forkolin, an activator of adenylyl cyclase
PKA inhibitors/activators, usually cAMP analogs

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