Lecture 23 Flashcards
What toxin does Vibrio cholerae secrete?
enterotoxin
Structure of enterotoxin and their jobs
B subunit which consists of five protein chains, contribute to receptor binding to ganglioside receptors on the surface of host target cells.
A subunit that enters the target cell where it is active, catalyzing an arginine side chain of Gs-alpha.
Mechanism of cholera enterotoxin
- B subunit binds to ganglioside receptor on the cell surface.
2 A subunit detach from B subunit and enters cytoplasm via receptor-mediated endocytosis. - Toxin activates Gs protein causing Gs-alpha to dissociate from G protein, then toxin ribosylates the alpha subunit of Gs(G-protein), which makes Gs-alpha sub unit no longer hydrolyse GTP, making it permanently active.
- Permanently active Gs-alpha subunit activate adenylyl cyclase which produce cAMP, which activate PKA and increasing intracellular calcium level.
- This stimulate chlorine secretion through Cl- channel protein through apical membrane to lumen, which water follow (a form of diarrhea).
Which toxin does Bordetella pertussis (Whooping cough) secrete?
Exotoxin
Which area of body does Pertussis infect?
Lining of respiratory tract specifically cilia of respiratory epithelium
Structure of exotoxin
hexamer (S1,2,3,4,5) TWO S4 subunit
S1 = Binds the toxin to cell membrane
the rest is for biological activity
Mechanism of exotoxin
Toxin ribosylates Cystein of Gi-alpha subunit (Gprotein), which leads to permanent inhibition of cilia function, leading to paralysis of cilia and necrosis of cilia.
Even though both enterotoxin and exotoxin targets G protein what is different?
exotoxin targets Gi-alpha (inhibitory)
Enterotoxin targets Gs-alpha (stimulatory)
Other low signalling molecule can cross membrane and modulate directly cytoplasmic signal transducing procceses. What is an example?
Nitric oxide can enter the membrane, bind to cytoplasmic receptor, which activate guanylyl cyclase (membrane bound), turning GTP to cGMP. cGMP activates PKG which results in vasodilation (smooth muscle relaxation thus increasing blood supply to myocardium)
Role of cGMP
Smooth muscle relaxation, platelet aggregation, regulating blood flow and blood pressure
Role of NO
Vasodilator (Relaxation of smooth muscle) (angina pectoris treatment)
prevention of platelet aggregation
function as neurotransmitter in brain
reduce release of superoxide radicals
reduce LDL oxidation
Reduce smooth myocyte growth in arterial wall
Relationship between viagra and cGMP
Viagra inhibits phosphodiesterase-5 which catabolises cGMP in vascular smooth muscle cells. Inhibition increase intracellular cGMP thus leading to relaxation of smooth muscle, allowing the blood to flow to penis.
How does cAMP activate PKA?
4cAMP allosterically binds to Regulatory subunit of PKA, freeing catalytic subunit of PKA (Active)
Job of active PKA
Phosphorylate serine or threonine residues
eg. glycogen synthase (inactivated), phosphorylase kinase (activated)
Inhibitors of phosphodiesterase (2)
caffeine, theophylline (tea)
so keep intracellular cAMP high
Stimulatory effects of caffeine on hepatocytes
- caffeine at high concentration inhibit phosphodiesterase
- This intensifies and prolongs effect of cAMP, maintaining the activating effect of stimulatory hormones eg. glucagon
- This leads to reduction in hepatic glycolytic flux, and increase in glycogenolysis
- Thus there is increased glucose exported from liver.
Hormones that exert their intracellular effects via IP3 second messenger system (5)
oxytocin, vasopressin, angiotensin II, growth hormone, thyroid releasing hormone,
all of which binds alpha1 adrenergic receptor.
Pathway of IP3
Produced from PIP2 by phospholipase C, it stimulates the release of calcium from ER calcium reservoir.
Then, calcium forms a complex with calmodulin, which activates a protein kinase C for phosphorylation of a target enzyme.
Role of DAG
Activates membrane-bound protein kinase C (PKC), which phosphorylate number of target protein, such as channel proteins that control the flow of calcium into and out of the cell.
How is Ca pumped out of cell?
Ca-ATPase and Na/Ca pumps
How does Ca act as secondary messenger?
Ca from various sources (cytoplasm through different channels, ER, mitochondrion) bind with calmodulin forming a complex that acts as secondary messengers.
Example of calcium as a secondary messenger in muscle
- epinephrine is bound to alpha1 adrenergic receptor on muscle.
- It causes Gq alpha subunit to dissociate activating phospholipaseC, that catabolise PIP2 into IP3 and DAG.
- IP3 binds to to receptor on IP3-gated Ca channel on SR, letting Ca out of SR pool.
- Ca bind with calmodulin subunit of phosphorylase kinase b (PKB), activating it without phosphorylation.
- Phosphorylase kinase can then activate glycogen phosphorylase, casuing glycogen degradation.
Role of AMP in muscle
In muscle under extreme conditions of anoxia and depletion of ATP, AMP activates glycogen phosphorylase without it being phosphorylated, leading to glycogen degradation.
effect of cAMP (beta receptor) on smooth muscle
relaxation
