Intro to Pharm and Tox Topics   

cGMP/PKG System and Nitric Oxide

The synthesis and catabolism of cGMP is analogous to that of cAMP. Guanylyl cyclases exist in both soluble and membrane-bound forms. There are at least two soluble forms, which are heterogeneous dimers with alpha-1 and beta-1 subunits and containing heme groups. Soluble guanylyl cyclases are activated by NO and Ca2+.

There are at least three membrane-bound guanylyl cyclases, which are activated by atrial natriuretic peptide and other natriuretic peptides and by guanylins. They are homodimers with a ligand-binding domain, a transmembrane domain and a C-terminus cyclase domain.

cGMP works by binding to protein kinases or ion channels. An example of a cGMP regulated channel is the Na+ channel in rod cells outer segments, were cGMP action depolarizes the cell.

cGMP-dependent protein kinases (PKGs) are analog to PKAs. They are dimers, each subunit with a regulatory and a catalytic domain. Four cGMP molecules bind to the regulatory subunits. The catalytic subunits phosphorylate serine or threonine residues.

Phosphodiesterases convert cGMP into GMP. Increased phosphodiesterase activity results in decreased cGMP levels that close channels and hyperpolarize the cell.

cGMP transduction cascades are regulated by:

Nitric oxide increases the activity of soluble guanylyl cyclases, and is produced from arginine by catalysis of nitric oxide synthase (NOS):

    NO     +
            arginine                                                    citrulline

NOS is activated by Ca2+. There are three forms of NOS: endothelial, neuronal and inducible

Nitric oxide passes through membranes readily and affects cells be non-receptor mechanisms. It is short lived and hard to measure. Conversion of NO to peroxinitrite turns it into a longer acting reactive oxygen species. NO donors like sodium nitroprusside are used to manipulate a system and determine if NO is involved.

Continue to "Lipases and IP3" or take a quiz: [Q1] [Q2] [Q3] [Q4].

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

1- What is guanylyl cyclase?

2- Describe the structure of soluble guanylyl cyclase.

3- How are soluble guanylyl cyclases activated?

4- Describe the structure of membrane-bound guanylyl cyclases.

5- How are membrane-bound guanylyl cyclases activated?

6- How does cGMP act?

7- Give an example of an ion channel regulated by cGMP?

8- Describe the structure of PKGs.

9- How are PKGs activated?

10- What is the activity of PKG?

11- What is the cellular response to high phosphodiesterase levels?

12- List four regulators of the cGMP/PKG transduction cascade.

13- What is the effect of nitric oxide on the cGMP/PKG transduction cascade?

14- How is nitric oxide produced in the cell?

15- How is nitric oxide synthase activated?

16- List the 3 forms of nitrous oxide synthase

17- How does nitric oxide acts on cell?

18- How can nitric oxide have a longer duration of action?

19- How is nitric oxide measured in the cell?

Continue scrolling to answers below (after sponsor).

Hey! DON'T PEEK!!! Finish the questions fist!

Answers:

1- What is guanylyl cyclase?
Protein that generates cGMP from GTP, exists in both soluble and membrane-bound forms.

2- Describe the structure of soluble guanylyl cyclase.
There are at least two forms, both heterodimers, with alpha-1 and beta-1 subunits and containing a heme group.

3- How are soluble guanylyl cyclases activated?
By nitrous oxide and calcium.

4- Describe the structure of membrane-bound guanylyl cyclases.
There are at least three forms, all homodimers with a ligand binding domain, a transmembrane domain and a C-terminal cyclase domain.

5- How are membrane-bound guanylyl cyclases activated?
By atrial natriuretic peptides and other natriuretic peptides and guanylins.

6- How does cGMP act?
By binding protein kinases (PKGs) or ion channels.

7- Give an example of an ion channel regulated by cGMP?
The Na+ channel in rod outer segments, depolarizes rod cells.

8- Describe the structure of PKGs
Dimers with two regulatory and two catalytic subunits.

9- How are PKGs activated?
When 4 cGMP molecules bind to the regulatory subunit, the catalytic subunits are activated.

10- What is the activity of PKG?
Phosphorylates serine and threonine residues.

11- What is the cellular response to high phosphodiesterase levels?
Decreased cGMP levels that close channels and hyperpolarize the cell.

12- List four regulators of the cGMP/PKG transduction cascade
nitric oxide
PKG inhibitors
phosphodiesterase inhibitors
stable analogs of cGMP

13- What is the effect of nitric oxide on the cGMP/PKG transduction cascade?
increases the activity of soluble guanylyl cyclase.

14- How is nitric oxide produced in the cell?
From arginine by catalysis of nitric oxide synthase, yielding citrulline.

15- How is nitric oxide synthase activated?
By Ca2+.

16- List the 3 forms of nitrous oxide synthase
endothelial
neuronal
inducible

17- How does nitric oxide acts on cell?
It passes through membranes readily and affets cells by non-receptor mechanisms. It is short lived.

18- How can nitric oxide have a longer duration of action?
Conversion into peroxinitrite turns it into a longer acting reactive oxygen species.

19- How is nitric oxide measured in the cell?
It is short lived and hard to measure. NO donors like sodium nitroprusside are used to manipulate a system and determine if NO is involved.

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