Physiology 3140A Lecture Notes - Lecture 13: Diglyceride, Chalcogen, Cell Membrane
22 May 2018
School
Department
Course
Professor
Physiology 3140
Dr. Rylett
Lecture 13
IP3 Releases intracellular Ca2+ and Diacylglycerol activates PKC
- In the image, we have a GPCR which activates Gq (G protein)
- When the ligand binds to the G protein, its going to induce a conformational rearrangement of
that G protein which allows it to have a high binding affinity for the alpha subunit
- When this happens, we get conversion of GDP to GTP with help from GEF (recall: that GAP will
eventually turn on the GTPase activity of the alpha subunit to eventually hydrolyze it )
- Alpha subunit finds its effector (phospholipase C-beta)
- PIP2 is the substrate for phospholipase C-beta
o Not shown is the conversion of PI to PIP2
- Recall: PIP2 is in relatively low abundance, small % of all the PI in the membrane but it has high
binding affinity to phospholipase C-beta
- When this happens, phospholipase C-beta will hydrolyze the oxygen group and generate a
hydroxyl group on the glycerol backbone, creating diacylglycerol
- DAG is very hydrophobic (it has 2 fatty acids associated with it) so it stays in the PM and is
involved in activation of PKC
- The phosphate head group (IP3) that has been cleaved off is highly charged and highly
hydrophilic so it diffuses away from PM out into the aqueous environment of cytoplasm and is
involved in releasing Ca2+ from either ER or SR
IP3:
- small water-soluble molecule that diffuses rapidly away from plasma membrane into cytosol
looking for places it can bind
- so now it’s a ligand looking for a receptor that it can bind to
- it binds to ligand operated Ca2+ channels that are present on the surface of ER or SR
- thes Ca2+ channels have their ligand binding site facing into the cytoplasm and when IP3 binds
to it, they go into an open conducting state so that Ca2+ can be released
- in addition to these Ca2+ channels binding IP3, they also bind Ca2+
o when IP3 binds, you get a little bit Ca2+ coming out and that Ca2+ will bind to other
sites on the Ca2+ channel
o this leads to positive feedback →leading to an even greateer release of Ca2+
o so the IP3 kind of primes the Ca2+ channel by starting it to open
o SO channels regulated by positive feedback - Ca2+ binds to channels to increase Ca2+
release in sudden, all-or-none manner
two mechanisms terminate the initial calcium response:
- (1) IP3 is rapidly dephosphorylated by phosphatases
o to create IP2 or even back to IP, which are not biologically active
- (2) Ca2+ that enters the cytosol is rapidly pumped back out
o Ca2+ is coming out of the IP3 operated Ca2+ channel but it is going to get pumped back
into the ER by Ca2+ pumps that use metabolic energy
- Ca2+ could also diffuse away and/or bind to other cellular proteins but at some point it has to
be restored back to its resting level (pumped back to ER)
diacylglycerol (DAG) has two potential signaling roles:
- (1) can serve as a substrate to be cleaved by phospholipase A2 to release arachidonic acid
o DAG has 2 fatty acids, if one of those fatty acids happens to be arachidonic acid,
phospholipase A2 can cleave the arachidonic acid off
o So it can be a substrate for production of other biologically active molecules
- (2) activates PK-C that phosphorylates selected proteins in target cells
o PKC is a ser/thr kinase and it has a large number of targets int eh cell that it
phosphorylates but it requires DAG to be able to become fully activated
find more resources at oneclass.com
find more resources at oneclass.com
PKC:
- several isoforms - MOST are calcium-dependent and ALL are phospholipid-dependent (ex: DAG
dependent)
- normally activated during initial rise in cytosolic Ca2+
o the initial rise in cytosolic Ca2+ concentration is due to the IP3 binding to the Ca2+
channels in ER
- once cytosolic Ca2+ concentration increases, Ca2+ can bind to PKC, it undergoes conformational
rearrangement and it translocates from cytoplasm to the membrane to bind phosphatidylserine
(PS) and further activated by DAG
- so there are 2 lipids that are involved: PS and DAG
Mechanism Summary
Activation of PKC also increases genes transcription
- when we talk about PKC, we are talking about a protein/enzyme that is inactive in the cytoplasm
in the resting state
- when the cell receives a signal, a bunch of steps take place, leading to theproduction of IP3 and
DAG
- that PKC that is inactive now becomes transformed and translocates to the membrane to
undergo even further transformation at the membrane where it then can mediate its kinase
function
o note: when we said that there was cytosolic and membrane bound forms of guanylate
cyclase, those are 2 entirely separate gene products – you don’t get that translocation
happening
o THIS IS AN EXAM QUESTION!!
- PKC is the SAME protein in the cytoplasm or at the membrane and one is inactive and the other
is active →the protein translocates from the inactive form in the cytoplasm to the active form in
the membrane
- When PKC is present in the cytoplasm, it is INACTIVE
- So PKC is not constitutively active – it has to undergo some sort of transformation for it to
become activated
- In the image,
o Signaling molecule/ligand binds to GPCR →conformational rearrangement of
receptor→couple Gq with GTP attached to it→Gq looks for and activates its effector
(phospholipase C-beta)→phospholipase C-beta looks for PIP2→production of IP3 and
DAG→DAG stays in the membrane and IP3 moves away
o The very soluble IP3 molecule serves as a ligand for these Ca2+ channels on
ER/SR→Ca2+ moves out (Ca2+also feeds back and causes more Ca2+ to come
out)→PKC can bind Ca2+ through a calmodulin moiety
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Ip3 releases intracellular ca2+ and diacylglycerol activates pkc. In the image, we have a gpcr which activates gq (g protein) When the ligand binds to the g protein, its going to induce a conformational rearrangement of that g protein which allows it to have a high binding affinity for the alpha subunit. When this happens, we get conversion of gdp to gtp with help from gef (recall: that gap will eventually turn on the gtpase activity of the alpha subunit to eventually hydrolyze it ) Alpha subunit finds its effector (phospholipase c-beta) Pip2 is the substrate for phospholipase c-beta: not shown is the conversion of pi to pip2. Recall: pip2 is in relatively low abundance, small % of all the pi in the membrane but it has high binding affinity to phospholipase c-beta. When this happens, phospholipase c-beta will hydrolyze the oxygen group and generate a hydroxyl group on the glycerol backbone, creating diacylglycerol.
