Lecture 22 - Cell Signalling & Transduction II Flashcards
what are the 3 kinds of receptors?
- ligand-gated channels
- plasma membrane receptors
- nuclear receptors
what are the 2 kinds of plasma membrane receptors?
- those linked to G proteins
- those linked to protein kinases
members of the GPCR superfamily, the largest superfamily in the human genome, have ____ _____________ ____ ____________-
7 transmembrane alpha-helices
what does variation of G-protein linked receptors amino acid sequences lead to?
- the extracellular portion of each receptor has a unique messenger binding site
- the cytosolic portion allows the receptor to interact with only certain types of G proteins
what are some examples of signalling molecules?
- proteins
- peptides
- amino acid derivatives
- fatty acids
- photons
- olfactory molecules
three extracellular loops act as the ________ _________ ____________ and these structures can vary among different GPCRs
ligand binding pocket
three intracellular loops provide ________ ________ for signalling proteins, including the ___________ ______ ______________-
binding sites, heterotrimeric G-protein
what is the job of the receptor and how does it do it?
transfer an extracellular signal across the membrane, does this by altering the confornatio
what is the job of the receptor and how does it do it?
transfer an extracellular signal across the membrane, does this by altering the conformation
def: non covalent interactions within the transmembrane alpha-helices stabilizes the inactive structure and bury the G-protein binding sites
inactive state
def: ligand binding disrupts transmembrane interactions, rotating these alpha-helices, causing the cytoplasmic loops to “unmask” G-protein binding sites
active state
what are the 2 classes of GTP proteins?
- small monomeric G proteins
- large heterotrimeric G proteins
where are heterotrimeric G proteins held and how?
in the plasma membrane by the covalent attachment of lipid chains
what are the 3 parts of the heterotrimeric G protein?
alpha, beta, gamma subunits
which part of the heterotrimeric G protein associates with the GDP/GTP molecule?
the G-alpha subunis, this portion binds and hydrolyzes GTP/GDP
what happens when a ligand binds to a GPCR?
the receptor undergoes conformation change that unmasks the binding site allowing G protein association
what does the binding of the G-protein to the GPCR do?
induces conformational change in G-alpha, causing the release of GDP and binding of GTP
what happens to G-alpha after it is GTP bound?
G-alpha has low affinity for G-beta/G-gamma which leads to dissociation of the trimeric complex, G-alpha can now freely activate the effector protein
what does activation of the effector lead to?
production of a second messenger which activates more intracellular signalling proteins
what will eventually happen to the G-alpha protein associated with the effector?
- the G-alpha subunit will eventually hydrolyze the GTP, into GDP + Pi,
- once hydrolyzed to GDP = SWITCH OFF
- G-alpha conformational change, reducing its affinity for effector, and increasing affinity for G-beta/G-gamma
what does the G-beta/G-gamma subunit do?
- can also transduce signals
- they can associate with K+ channels and open the gate
what does the cell do to prevent overstimulation of GPCRs?
the cytoplasmic of GPCRs are phosphorylated by G-protein coupled receptor kinases (GRKs)
what does phosphorylation of GPCRs do?
- increase their affinity for Arrestins, which compete with the heterotrimeric G proteins for binding
def: cells stop responding to the stimulus despite the continued presence of stimulus on its outer surface
desensitization
what are the 3 alternative outcomes of termination?
- receptor can resume signalling within the endosome
- receptor can be traffic to the lysosome for degradation
- the receptor can be returned to the cell surface where the phosphorylation sites must be removed
def: if a receptor returns to the cell surface, they can resume signalling
re-sensitization
one of the most important and widespread signalling events, regulated by different GPCRs, involves the release/formation of ___________ ____________
second messengers
def: formed from cytosolic ATP by adenyl cyclase, an enzyme is anchored in the plasma membrane
cyclic AMP (cAMP)
adenyl cyclase is inactive until bound by an active _________, subunit of specific
G-alpha
def: a stimulatory G-alpha protein, activated by binding of a ligand to a specific GPCR
G-alpha S
adenyl cyclase =
phosphodiesterase =
adenyl cyclase = generates cAMP
phosphodiesterase = degrades cAMP
how does phosphodiesterase function work within the cell?
it is constantly being produced but when an effector is activated and generating secondary signals, they outnumber the phosphodiesterase.
what is the main target of cAMP
Protein Kinase A, where it acts as an allosteric activator, and regulates the separation of the regulatory and catalytic subunits
def: phosphorylates a variety of proteins on serine or threonine residues, using ATP as the source of phosphate
PKA
how does cAMP activate PKA’s?
it binds to their regulatory subunits, causing them to change conformation
then the catalytic subunits detach and are now activated and can phosphorylate target proteins in the cell
cAMP stimulates the ____________ of the enzyme system responsible for synthesis of ___________, as PKA phosphorylates glycogen synthase to inactivate it
inactivation, synthesis of glycogen
PKA also activates ____________ ______________, which activates ___________ _________, which catalyzes the cleavage of glycogen into glucose 1-phosphate, leading to glucose release
phosphorylase kinase, glycogen phophatase
how does PKA choose what proteins to activate?
- different cells express different PKA substrates
- AKAPS (A Kinase-Anchoring Proteins) acts as signalling hubs, sequestering PKA to specific locations within the cell)
def: provide scaffolding for coordinating protein-protein interactions by sequestering PKA to specific cellular locations
AKAPs
when cAMP levels rise, _____ is activated
PKA
which substrates are the first ones to become phosphorylated?
the ones present in close proximity
what does the activation of enzyme phospholipase C trigger?
the cleavage of the relatively uncommon membrane phospholipid PIP2
what does PIP2 generate?
IP3 and DAG, function as second messengers in a variety of cellular events
lipids cleaved from the membrane are used as _________ __________
second messengers
briefly describe the IP3 and DAG second messenger process?
- ligand binds to membrane receptor
- activates specific G protein, G-alpha-q
- G-alpha-q actives a phospholipase which cuts PIP2 into IP3 and DAG
- IP3 diffuses through the cytosol and binds a ligand gated calcium channel, the IP3 receptor channel
how are cytoplasmic Ca2+ concentrations maintained at low levels?
- Ca2+ ions channels are closed
- Na+/Ca2+ antiporters and calcium ATPases in the plasma membrane and SER actively transport Ca2+ out of the cytosol
how can signalling increase cytosolic [Ca2+]?
- voltage gated Ca2+ channels opened by nerve impulses
- IP3 receptor channels can open upon IP3
- Ryanodine receptor channels are sensitive to calcium (calcium induced release)
how do Calmodulin and Ca2+ ions interact?
Calmodulin binds to 2 Ca2+ ions, changes its conformation and the increases its affinity for a variety of effectors
Goes from S shaped to U shaped
