Intracellular Signaling Flashcards
(48 cards)
G-Protein Coupled Receptors (GPCR)
detect photons
(light), hormones, growth factors, drugs, and other endogenous ligands
Monomeric G-proteins
or “small G-proteins”
belong to the
Ras superfamily of small GTPases (>100
proteins).
These proteins are homologous to the
alpha (α) subunit found in heterotrimers.
Heterotrimeric (“large”) G
-proteins
G-protein binds to the receptor
G protein is composed of 3 subunits (α, β, γ) *
the alpha subunit binds either to
guanosine triphosphate (GTP) or
guanosine diphosphate (GDP)
*
G proteins function as molecular
switches. They are “on” when they bind GTP, they are “off” when they bind GDP.
There are several classes of Gα subunits:
Gαs(stimulatory),
Gαi/αo(inhibitory, or
other),
Gαq(→ PLC pathway)
Activation of Heterotrimeric G
-proteins
When a ligand binds to the receptor, a conformational change occurs in the receptor that allows the Gα subunit to
exchange GDP for GTP.
This exchange triggers the dissociation of the Gα subunit from the Gβγ dimer and the
receptor and it activates the subunit(s).
The Gα-GTP monomer and the Gβγ dimer can now modulate the activity ofother intracellular (effector) proteins
Termination of Heterotrimeric G-proteins
The Gα subunit eventually hydrolyzes the attached GTP to GDP through its own enzymatic activity, allowing it to re-associate with Gβγ, forming the “resting” G-protein.
RGS proteins
(Regulators of G protein signaling, also called GAPs - GTPase-Activating Proteins)
accelerate the hydrolysis of GTP to GDP, terminating G-protein activity.
The effector (e.g. adenylyl cyclase)
may also possess GAP activity, which helps deactivate the pathway
Two principal signal transduction pathways of G protein-coupled receptors:
the adenylyl cyclase – cAMP signal pathway (via Gαs
and Gαi),
the Phospholipase C signal pathway (via Gαq)
Adenylate Cyclase and cyclic AMP
Upon binding of the alpha subunit of the G-protein, the
effector protein adenylate cyclase catalyzes
the
conversion of ATP to 3’,5’-cyclic AMP (cAMP)
→ Gαs stimulates, while Gαi
inhibits
cAMP acts as
a second messenger by
interacting with and regulating other
proteins, such as protein kinase A
cAMP is hydrolyzed back to AMP by
phosphodiesterases (PDEs)
G-protein coupled receptors
(Gαq)
activate PLC.
PLC cleaves PIP2
into
inositol -1,4,5-triphosphate (IP3,
soluble) and diacylglycerol
(DAG, membrane bound).
IP3
stimulates the release of
calcium ions from the
endoplasmic reticulum
DAG is an activator of
protein kinase C (PKC).
The ER has two pharmacologically
different Ca2+ stores
- activated via IP3 metabolism (PLC),
- elevation of cytosolic Ca2+ itself (Ryanodine-store)
Calcium stores
Mitochondria act as a
slow Ca2+ buffer
(MCU, mitochondrial calcium uniporter)
Internal calcium regulates:
the gating and kinetics of voltage-gated channels
* the gating and kinetics of ligand-gated channels
* Second messenger activation
* Protein kinases and phosphatases
* Gene expression
Molecular targets of signal transduction
Phosphorylation
addition of phosphate group to a molecule, usually a protein.
Protein Kinases
enzymes that are the effectors of phosphorylation and catalyze the transfer of a phosphate group from ATP to specific amino acids on proteins.
Proteins are phosphorylated predominantly on
Serine, Threonine and Tyrosine residues,
which account for 86, 12 and 2% respectively of the phosphoproteome
→ Serine/Threonine kinases, e.g. PKA, PKC or CaMKII
→ Tyrosine Kinases
Protein Phosphatases
– enzymes that cleave phosphate from target molecule
→ dephosphorylate.
act inopposition to protein kinases
Can be grouped into three main classes based on sequence, structure and catalytic
function.
General mechanisms of protein kinase
activation:
catalytic subunits
regulatory subunits (autoinhibition)
Binding of the second messenger
