Physiology 3140A Lecture Notes - Lecture 21: Adenylyl Cyclase, Guanosine Monophosphate, Globular Protein
2 May 2018
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Cell Physiology Lecture 21
Cyclic GMP and nitric oxide in cell-cell signalling
November 8 2017
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
- Amount of cGMP in cell: balance between synthesis by guanylate cyclase and degradation by cGMP-
specific phosphodiesterase
- cGMP formed in the cells from GTP, made by guanylate cyclase (enzyme similar to adenylate cyclase
[starting with ATP molecule])
o Analogous to cAMP
o Have 3 high energy phosphate groups
o Enzyme: guanylate cyclase (analogous to adenylate cyclase)
- Two phosphate groups get cleaved off
- Guanylate cyclase forms a cyclical ring – ring structure formed, with one remaining phosphate group
o HIGH ENERY FORM – a lot of biological activity in terms of signaling
o Forms cGMP
- cGMP specific phosphodiesterase DOES NOT use cAMP as the substrate
o THEY ARE SPECIFIC
o Breaks the ring structure open and generates 5’ GMP
o No longer has any signaling function – inactivation of the molecule
Guanylate cyclase
- Two distinct forms of guanylate cyclase in cells:
o Cytosolic form
▪ Globular protein in the cytoplasm
o Membrane-associated form
▪ Anchored at the membrane
o The forms are VERY different than other proteins!
o These are two different structures of protein, therefore the two versions do not interconvert
o Two different gene products
find more resources at oneclass.com
find more resources at oneclass.com
- Unlike some other enzymes (e.g. protein kinase C):
o PKC may be present in a cytoplasm, and the cell information & becomes transformed and the
protein moves to the PM & can anchor at the PM
▪ It is in the PM form that it is active
▪ The cytosolic and membrane associated form are the SAME protein, they can
interconvert
▪ Translocate back and forth between the plasma membrane and the cytoplasm
o Cytosolic and membrane-bound forms of guanylate cyclase do not interconvert by
translocation of enzyme between cytosolic and membrane compartments of cell
- Different than adeynlate cyclase:
o Big family of proteins but most are in the plasma membrane
o Duplicated molecule
o Lots of transmembrane domains, allowing it to be inserted into the PM
o (6 TM + catalytic domain) x2
o Acts to amplify biological signals. Have one receptor ligand complex, G protein involved, but
when it activates the effector adenylate cyclase, getting amplification of the signal due to
multiple catalytic domains
Two forms of guanylate cyclase represent different gene products
- These 2 forms of guanylate cyclase distinct forms of the protein, both of which convert GTP to cGMP
o Note: the two forms of guanylate cyclase are two different proteins, two different gene
products. It is not the same gene product that translocates from the cytoplasm to the
membrane (like protein kinase C)
CYTOSOLIC PROTEIN:
- There are two critical domains:
o Cyclase catalytic domain
▪ To be able to recognize the GTP+ GTP has to be able to bind to it
▪ Has to have the enzymatic activity to convert GTP to cGMP
o Heme containing domain
▪ Can bind gases!
▪ Serves as a ligand binding domain
▪ It has a ligand, or substance that can bind to it; it is a receptor
• Some heme domains bind CO, CO2, O2
▪ This specific heme domain has affinity to bind nitric oxide (NO)
▪ Has iron in it, component of heme
- Something binds to the heme domain (NO), causing a conformational change allowing the cyclase
catalytic domain to function
- If NO (nitric oxide) is present & binds to it, it can activate the cyclase catalytic domain of the protein
- It can have its activity modified by nitric oxide;
o NO is a ligand that can be synthesized in one cell and released from that cell (diffused) to
another cell and find cytosolic guanylate cyclase & change its activity
find more resources at oneclass.com
find more resources at oneclass.com
MEMBRANE BOUND:
- Have a single-pass transmembrane domain
- Outside of the cell, you have the ligand binding domain
o Ligands are mostly peptides
- On the inside, there is cyclase catalytic domain:
o It is the same as the one in the cytosolic protein
o Function = bind GTP & convert to cGMP
- Ligand approaches cell & is recognized by the receptor
- Ligand – receptor complex, causes a conformation rearrangement upon binding of ligand to receptor
o Information transmitted through the protein
o Cyclase domain becomes activated
- The cyclase domain is not constitutively active. Need the peptide ligand binding at the outside
surface of the cell to turn on the cyclase domain
o Not constantly getting GTP binding and converting to cGMP
- REMEMBER TYROSINE KINASE RECEPTORS:
o Single transmembrane proteins that have ligand binding domain on the outside of the cell &
have enzymatic catalytic domain on the inside of the cell
Cytosolic guanylate cyclase
- Looking at the role of vascular endothelium in regulating cGMP and guanylate cyclase & how this
plays a critical role in the diameter of blood vessels
o Blood vessels will either constrict or dilate depending on the signals that are getting to them
o cGMP and guanylate are involved in some aspects of these processes
REGULATION OF CGMP FORMATION IN VASCULAR ENDOTHELIUM:
- Several hormones transiently increase cGMP levels in vascular smooth muscle and cause
relaxation (vasodilation)
o Number of hormones that circulate in the blood. If they meet their receptors, they can
initiate signalling cascade causing the blood vessels to dilate (diameter of blood vessel
increases)
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Cyclic gmp and nitric oxide in cell-cell signalling. By analogy with camp, cyclic-gmp (cgmp) is formed in many cell types from gtp by a catalytic reaction with guanylate cyclase. Amount of cgmp in cell: balance between synthesis by guanylate cyclase and degradation by cgmp- specific phosphodiesterase cgmp formed in the cells from gtp, made by guanylate cyclase (enzyme similar to adenylate cyclase. [starting with atp molecule]: analogous to camp, have 3 high energy phosphate groups, enzyme: guanylate cyclase (analogous to adenylate cyclase) Unlike some other enzymes (e. g. protein kinase c): pkc may be present in a cytoplasm, and the cell information & becomes transformed and the protein moves to the pm & can anchor at the pm. Have one receptor ligand complex, g protein involved, but when it activates the effector adenylate cyclase, getting amplification of the signal due to multiple catalytic domains. Two forms of guanylate cyclase represent different gene products.
