Nitric oxide and cyclic GMP Flashcards
Explain Nitric Oxide ?
- Produced in response to a variety of agents including acetylcholine, bradykinin, thrombin, Ca2+, excitatory amino acids, etc
- Gas which diffuses rapidly through membranes allowing it to act as intracellular and extracellular messenger
- Very reactive free radical which reacts with species containing unpaired electrons such as oxygen, superoxide, metal ions: half life of less than 10s: metabolised to nitrate and nitrite (inert)
Structure of Nitric Oxide Synthase ?
- NO synthase - 2 distinct forms – constitutive and inducible
- Constitutive
-Endothelial NOS (eNOS) - found in endothelial cells and neurons - Neuronal NOS (nNOS) - found in neurons in CNS and skeletal muscle: Ca2+/calmodulin dependent: Produces pmoles of NO over short term (secs)
- Inducible
iNOS - involved in the immune system where it is induced to combat infections
Nitric Oxide is an example of paracrine signalling
- Highly diffusible gas with very short half-life
- Signals to neighbouring cells (paracrine); non-hormonal
Outline of pathway from NO downstream ?
- Nitric oxide can diffuse from a neighbouring cell, or be synthesised in the cell by NO synthase in response to e.g. Ca2+ downstream of hormonal signalling
- The NO produced activates a soluble receptor (i.e. not membrane-bound) called soluble guanylate cyclase
- This makes a second messenger called 5’3’cGMP (analogous to 3’5’cAMP)
- cGMP activates its own effector molecule, a kinase called protein kinase G (PKG)
- PKG phosphorylates downstream target proteins to achieve the outcome in the cell
Explain 3’5’cyclic GMP ? e.g. what it’s made and broken down by ?
- Analogous to 3’5’ cAMP, but with a guanine nucleotide
- Made by guanylate cyclase; broken down by cGMP phosphodiesterase
- Intracellular concentration 10-100x less than that of cAMP
How are Guanylate Cyclase Receptors similar to adenylate cyclase ?
Like adenylate cyclase, guanylate cyclase has two catalytic domains that must be brought together for activity
However, Guanylate Cyclase Receptors unlike AC functions as ?
A dimer
- There are soluble (cytoplasmic) and particulate (membrane-bound) versions
Solubility of Guanylate Cyclase Receptors ?
Soluble has no ligand binding site but does have haem domain
Explain what group, what it’s linked to, activated by ?
- No hormone binding site however has haem prosthetic group in the regulatory domains which interact with free radicals causing conformational change and activation
- Linked to changes in intracellular Ca2+
- Activated by nitroprusside and nitroglycerin which can liberate nitric oxide within tissues
Nitric oxide (NO) and other free radicals are major regulators of ?
Metabolism and cell function
Explain cGMP-stimulated Protein Kinase (PKG) ?
- Exists as an antiparallel dimer of a single polypeptide chain
- Catalytic site inhibited by N-terminal pseudosubstrate domain
- Two cGMP binding sites per monomer: Binding cGMP causes release of pseudosubstrate from active site and activation of protein phosphorylation
- Phosphorylates Arg-Arg-X-Ser motifs and therefore has similar substrates as cAMP-dependent protein kinase
What can cGMP also activate?
cGMP also:
- Activates cGMP-activated ions channels increasing ion fluxes
- Activates cGMP-regulated phosphodiesterase
Explain NO in action ?
- A compound (nitrophorin) in the saliva of the kissing bug releases NO in the bloodstream after it bites a human host. This leads to vasodilation and an extended feeding period for the insect
- (Note that histamine will bind to nitrophorin, inhibiting the anti-inflammatory response)
What can activation of PKG by cGMP lead to ?
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
What is Nitroglycerin used for?
Nitroglycerin – used to relieve the chest pains experienced in angina where there is compromised blood flow in the heart
