Cell communication Part 2
What was the first gas known to act as a biological messenger?
Nitric oxide (NO)
Why can nitric oxide easily pass between cell membranes?
Because it is small and lipid soluble
What are some functions of nitric oxide in the body?
It acts as a neurotransmitter, vasodilator and bactericide
How does NO cause smooth muscle relaxation in blood vessels?
It diffuses into smooth muscle cells and activates guanylyl cyclase
What enzyme catalyzes the production of NO in the body?
Nitric oxide synthase
What is the role of guanylyl cyclase in NO signalling?
It acts as a receptor for NO and produces cGMP
How does cGMP contribute to smooth muscle relaxation?
It causes the smooth muscle to relax and increases blood flow?
What overall effect does NO have on blood vessels?
It causes vasodilation by relaxing smooth muscle in artery walls
What is a signal transduction pathway?
A sequence of molecular interactions triggered by a signal molecule binding to a receptor
What happens during phosphorylation?
A protein kinase transfers a phosphate group from ATP to a protein
What is dephosphorylation?
The removal of a phosphase group from a protein by a protein phosphatase
What are second messengers?
Small non protein molecules or ions that diffuse through a cell to transmit a signal
How is cAMP produced in a cell?
Adenylyl cyclase coverts ATP into cAMP in response to an extracellular signal
What is the role of cAMP jn signal transduction?
cAMP activates protein kinase A, which then modifies target proteins to produce a cellular response.
Outline the cAMP signalling pathway
1. signal binds to a G protein linked receptor
2. G protein activated and stimulates adenylyl cyclase
3. cAMP levels rise inside cell
4. cAMP activates protein kinase A
5. Protein kinase A modifies target proteins
Why are second messengers like cAMP important?
They amplify and spread the signal throughout the cell
Why can calcium ions act as second messengers?
Because cytosolic calcium levels are normally much lower than outside the cell, so small changes in calcium ion concentration causes large relative changes
Which molecules help release calcium into the cytosol during signal transduction?
Inositol triophosphate and diacylglycerol
What are the four aspects of signal regulation?
Amplification of signal
Specificity of response
Overall efficiency
Termination of signal
How does adrenaline trigger glycogen breakdown?
It binds to G protein linked receptors, raises cAMP levels, activates protein kinase A which phosphorylates enzymes that break down glycogen
What role does protein kinase A play in glycogen breakdown?
It phosphorylates and activates enzymes that break down glycogen
How do scaffolding proteins enhance signal transduction?
By organising multiple proteins in a pathway close together to increase efficiency
Why is signal termination important?
To stop the response once it is no longer needed
Why can the same signal have different effects in different cells?
Because different cells have different collections of proteins, including receptors and signalling molecules
What is meant by pathway branching and crosstalk?
Pathway branching occurs when a signal activates multiple downstream pathways, and cross talk occurs when different signalling pathways interact
What are scaffolding proteins?
Large relay proteins that attach other relay proteins involved in the same signalling pathway
Why is signal termination important in cell signalling?
To prevent uncontrolled or continuous signalling once it is no longer needed
What mechanisms contribute to signal termination?
Ligand concentration fall, leaving fewer receptors bound
Unbound receptors revert to inactive site
Protein phosphatases reverse phoshorylation
Second messengers are degraded or sequestered