Physiology 3140A Lecture Notes - Lecture 9: Adenylyl Cyclase, Vascular Smooth Muscle, Smooth Muscle Tissue
22 May 2018
School
Department
Course
Professor
Physiology 3120
Dr. Rylett
Cyclic GMP and Nitric Oxide in Cell-Cell Signaling
Cyclic GMP (cGMP)
- By analogy with cAMP, cyclic-GMP (cGMP) is formed in many cell types from GTP by a catalytic
reaction with guanylate cyclase
o Similar to adenylate cyclase bc they start with ATP molecule and have 3 high energy
phosphate groups
o Guanylate cyclase ~ adenylate cyclase
- Guanylate cyclase
o 2 of the phosphate groups gets cleaved off
o guanylate cyclase forms a cyclic ring structure with one phosphate group left (this is in a
very high energy form)
o this also has a lot of biological activity in terms of signaling
- Amount of cGMP in cell: balance between synthesis by guanylate cyclase and degradation by
cGMP-specific phosphodiesterase
o cAMP phosphodiesterase recognizes cAMP
o cGMP phosphodiesterase recognizes cGMP and breaks that ring structure to create
5GMP
- so cGMP is very biologically active (especially in terms of cell signaling)
- the GTP does not act as a cell signaling molecule
- 5GMP also does not have any signaling molecule
adenylate cyclase
- has 6 transmembrane domains that help it insert itself into the PM
- another catalytic domain
- acts to amplify biological signals: might have one receptor-ligand complex, a G protein involved
but when it activates the effector adenylate cyclase, you are getting amplification of the signal bc
there is multiple catalytic domain
- the SAME protein interconverts bw the cytosolic form and being at the membrane (translocates
bw each form)
- but guanylate cyclase is different
Guanylate Cyclase
Can exist in 2 distinct forms of guanylate cyclase in cells:
- cytosolic form
- membrane-associated form
Both the membrane associated and cytosolic form are unlike some other enzymes (eg. PKC)
- May be present in the cytoplasm, then the cell receives some type of information and it becomes
transformed and moves to the PM to anchor there
find more resources at oneclass.com
find more resources at oneclass.com
- Guanylate cyclic cytosolic and membrane-bound forms are 2 completely SEPARATE gene
products and they do NOT interconvert by translocation of enzyme between cytosolic and
membrane compartments of cell
o The cytosolic form is a globular protein which exists in the cytoplasm
o The membrane associated form is anchored at the membrane
Two forms of guanylate cyclase represent different gene products
- cytosolic and membrane-bound forms of guanylate cyclase do not interconvert by translocation
of enzyme between cytosolic and membrane compartments of cell
- CYTOSOLIC FORM has some critical domains
o cyclase catalytic domain
▪ its an enzyme so it must have a catalytic domain to be able to recognize the GTP
(the GTP has to be able to bind to it) and it has to also be able to have that
enzymatic activity to be able to then convert it to cGMP
o it also has a heme containing domain
▪ it has iron in it
▪ it can bind gases (ex: O2, CO2, CO)
▪ serves as a ligand binding domain
▪ you it is sort of like a receptor
▪ this heme domain has a ligand/substance that can bind to it
▪ this particular heme domain has affinity for binding nitric oxide
▪ binds calcium
- MEMBRANE BOUDND FORM:
o single transmembrane proteins that have a ligand binding domain on outside of cell and
enzymatic catalytic domain on inside of cell (similar to the structure of RTK)
o the ligands for this are mostly peptides
o the cyclase catalytic domain is same as the one you see in the cytosolic protein (function
is to bind GTP and convert it to cGMP)
o like any other receptor-ligand complex, you get the ligand approaching the cell, being
recognized by the receptor and then causing some sort of conformational
rearrangement, information transmitted through the protein and when that happens,
the cyclase domain becomes activated
o this is an example of when the cyclase domain is NOT constitutively active – you have to
have the peptide ligand binding at the outside surface of the cell to then turn on the
cyclase domain – so you are not constantly getting GTP binding and converting
o same thing with the cytosolic form of the guanylate cyclase – receptor ligand
arrangement – where you have this heme containing domain that NO can bind to→this
causes a conformational rearrangement, allowing the catalytic domain to function
o SOO, if NO is present and binds to it, it can activate the catalytic domain of that protein
find more resources at oneclass.com
find more resources at oneclass.com
Cytosolic guanylate cyclase
Regulation of cGMP Formation in vascular endothelium:
This is an overview:
- Blood vessels will either constrict/dilate depending on the signals getting to them
- several hormones can cause a transiently increase cGMP levels in vascular smooth muscle and
cause relaxation (vasodilation)
o so hormones circulate in the blood, and if they reach their receptor, they can initiate a
signalling cascade of events that can cause the blood vessels to dilate →blood flow is
regulated by this process
- 2 pieces of information made it hard to find mechanism of hormone action:
o relaxation required vascular endothelial cells be intact - muscle without endothelium no
longer sensitive to hormones (though still sensitive to cGMP analogues)
▪ if you have a blood vessel with the smooth muscle but no endothelium, then
hormones no longer mediate an effect to change the blood vessel diameter,
eventhough they are still sensitive to cAMP analogues
o hormone effects on cGMP could not be demonstrated in cell homogenates (lysates) even
when endothelial cells were present
▪ ex: if you take a blood vessel and you homogonize it (disrupting the structural
components)
▪ if you had hormones there, it would still bind to its receptor but it will no longer
regulate cGMP
- Shows: hormones act on endothelial cells by release of secondary transmitter / substance that
activates cGMP synthesis in muscle cells - no conventional transmitter could be identified
Now lets look at it point by point:
- several hormones transiently increase cGMP levels in vascular smooth muscle and cause
relaxation (vasodilation)
o depending on how much the smooth muscle cells are contracted or not contracted, the
blood vessel will have a diff diameter
o blood and smooth muscles are not in contact with each other bc there is a thin line of
endothelial cells that line the blood vessel wall
o hormones will be traveling around in the blood and they will be looking for their
receptors
o the message to smooth muscle cells whether you need to be more constricted or more
relaxed is not a direct affect of the hormone binding to the receptors on the smooth
muscle cells
o the endothelial cell barrier is in the way of that so the receptors are located on the
endothelium
o hormones bind to receptors and when effect is relayed an you get dilation, that is
coupled to an increase in cGMP
o so hormones are travelling along the blood vessel, recognizing their receptors on the
endothelial cell surface and that is coupled with a change in relaxation of these smooth
muscle cells and is also coupled with an increase in cGMP
o SO WHENEVER THERE IS HIGH LEVELS OF CGMP, THERE IS VASODILATION
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Cyclic gmp and nitric oxide in cell-cell signaling. Amount of cgmp in cell: balance between synthesis by guanylate cyclase and degradation by cgmp-specific phosphodiesterase: camp phosphodiesterase recognizes camp, cgmp phosphodiesterase recognizes cgmp and breaks that ring structure to create. 5(cid:495)gmp so cgmp is very biologically active (especially in terms of cell signaling) the gtp does not act as a cell signaling molecule. 5(cid:495)gmp also does not have any signaling molecule adenylate cyclase. Can exist in 2 distinct forms of guanylate cyclase in cells: cytosolic form membrane-associated form. Both the membrane associated and cytosolic form are unlike some other enzymes (eg. pkc) May be present in the cytoplasm, then the cell receives some type of information and it becomes transformed and moves to the pm to anchor there. Two forms of guanylate cyclase represent different gene products cytosolic and membrane-bound forms of guanylate cyclase do not interconvert by translocation of enzyme between cytosolic and membrane compartments of cell.
