Physiology 3140A Lecture Notes - Lecture 3: Signal Transduction, Gtpase, Protein Kinase
22 May 2018
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Physiology 3140
Lecture 3
- Ligand has to bind to receptor – whatever is in bw has to lead to effector activation and then the
increase in second messenger
- There are 2 important things that can happen:
o Phosphorylation/dephosphorylation events that act as molecular switches to turn on
and turn off things (this can happen as a cascade)
▪ Signal in →transduction activation of kinase → phosphorylation of another
protein and activates is
• In this scenario, phosphorylation is usually activation
▪ This is turned off by a phosphatase
o Exchange of GTP and GDP
▪ High energy phosphate guanine molecules that exchange back and forth to
cause a conformational change to begin a transduction of information
▪ Once ligand is bound to receptor, this signaling takes place and takes an inactive
process and makes it active
▪ G protein linked and enzyme linked receptors transmit through cytoplasm with
lots of targets at both cytoplasmic and nuclear level
- These are very highly regulated processes – these modulatory event determine the duration and
magnitude of signal
- Ligand binds to receptor (alpha subunit of G protein)
o GDP dissociates and GTP binds
o This leads to an activated alpha subunit
o This can allow it to interact with other things
o You are going to get a conformational change in the alpha subunit but at the same time
you have a GEF (manipulates and changes the GDP for GTP)
o In the absence of GEF, you wouldn’t get exchange of GDP and GTP
o GEF is a protein or a protein domain that activates a monomeric GTPase (alpha subunit
can be considered to be a GTPase bc its an enzyme)
▪ Some of the receptors actually have a GEF as a domain of its protein so that
when the ligand binds, the receptor undergoes a conformational rearrangement
which makes the affinity for the G protein higher – so now the alpha subunit can
leave
▪ And when this happens, the GEF activity facilitates conformational
rearrangement of the alpha subunit so the affinity for binding GTP increases and
GDP decreases
▪ Or in separate cases, GEF is a completely separate protein that does this
o And it the GEF that changes the shape so that GDP has a low binding affinity and the
binding affinity for GTP increases
o You have to get a conformational change in a protein, which causes some to lower the
affinity for one thing and enhances the affinity for another
▪ In this case the alpha subunit and GEF interact that the shape causes the affinity
for binding GDP decreases while the affinity for binding GTP increases
▪ This is the thing that has to happen
o Somehow this situation has to be turned off bc you have a ligand bound to the receptor
and its telling the cell some info that is going to lead to a change in its behaviour but bc
there is going to be more signals coming at some point that has to be turned off
▪ The phosphate group has to be actively taken off the GTP so it regenerates GDP
▪ This is an enzymatic catalysis that takes the phosphate off
▪ This is bc the alpha subunit has GTPase activity which is regulated by the GAP
protein
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Protein Phosphorylation
- one of the most common strategies cells have for mediating change within the cell
- ubiquitous (Common bw all cells)
- reversible covalent modification
o covalent bc the phosphate group that gets added on is added on through an ether
oxygen
o the reversal has to take place through another enzymatic process
- kinases are constitutively active (always active)
o this means that it is expressing its activity
- when a protein is not constitutively active (in this case, its an enzyme)→sometime has to be
done to it to activate it (ex: by phosphorylation or they might bind something else – Ca2+ - there
may be a Ca2+ binding protein and when Ca2+ binds to them, they then become active)
- usually the kinase needs to be activated (its not usually constitutively active) – it has to undergo
a conformational change when its activated which allows it to be able to recognize its substrates
(otherwise, it has a low binding affinity for its substrates)
Example Phosphorylation Cascade
- Ligand binds to receptor at cell surface, there is an intermediary (ions or G proteins), leading to
an inactive kinase being activation
o Kinases can turn on kinases which turn on other kinases…
- As you go through the cascade, you are changing the specificity for the outcome
- Whatever the cellular response is that changes cell behavior (activation of metabolic process, or
changes in cell shape/division/proliferation), the initial step is not capable of mediating it
- There must be a cascade of event to eventually get to the protein that must be
activated/inactivated to get to the end result
- There is not a direct relationship (in most cases) between the ligand binding to the receptor,
initiating the original thing, and getting to the cellular response
- The cell must go through many steps and a lot of work to get to the final result
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find more resources at oneclass.com
