Signal-Transduction Pathways
signal transduction pathway: chain of events that convert molecular messages into a physiological response
transduction: conversion of information, the presence or conc. of a signaling molecule, into a downstream message that promotes the physiological response
pathway = signal (hormone, neurotransmitter) → transducer (receptor, ion channel) → effector (2nd messenger)
key steps:
release of primary messenger (signal)
reception of the primary message by a receptor, typically a membrane protein with intracellular and extracellular components
delivery of the message inside the cell by intracellular second messengers
activation of the other molecules that directly alter the physiological response
termination of the signal
specificity:
similar to the “induced fit” model of substrate binding to enzymes
signal molecules bind specifically by complementarily to receptor
agonist: molecule, upon binding, stimulates the downstream signaling
antagonist molecule, upon binding, inhibits downstream signaling
can bind to direct or allosteric sites
Second Messengers
common second messengers (effectors):
cAMP, cGMP
calcium ion
IP3
diacylglycerol (DAG)
consequences of second messengers:
may amplify the signal signifcantly
messengers are often free to diffuse and influence processes throughout the cell
use of common second messengers in multiple signaling pathways creates cross talk
Seven-Transmembrane-Helix (7TM) Receptors
large class of cell-surface receptors
transmit diverse information initiated by hormones, neurotransmitters, odorantes, and light
frequent therapeutic drug target
contain seven helices that span the membrane bilayer
Binding of an Agonist to the β-Adrenergic Receptor→ Stabilization of an Activated Conformation
agonist = ligands that bind to and activate receptors
induction of conformational change in the protein structure
helices 5 and 6 change the most dramatically upon binding
conformational change → activates G protien
G protein stimulates the activity of adenylate cyclase → catalyzes conversion of ATP into cAMP
2nd messenger cAMP carries the signal throughout the cell
in its inactive form, G protein exists as a heterotrimer (alpha, beta, and γ subunits)
alpha subunit = bound to GDP
alpha and γ subunits = anchored to the membrane by covalently attached fatty acids
Gα interacts with surface of receptor
when bound, it opens substantially, which allows the replacement of GDP with GTP
once binded with GTP, Gα dissociates from the βγ dimer and from the receptor
receptor now available to bind other G-protein heterotrimers and catalyze the exchange of GTP for GDP
provides amplification
7TM receptors are also known as GPCRS because they signal through G proteins
many therapeutics target GPCR signaling pathways
Step 1: Activated G Proteins Transmit Signals by Binding to Other Proteins
adenylate cyclase (AC): enzyme that converts ATP to cAMP
second messenger
activated Gα binds AC
Gα binding favors a catalytically active conformation of AC
provides another level of amplification
Step 2: cAMP activates protein Kinase A
protein kinase A (PKA); phosphorylate specific Ser and Thr residues in target proteins to alter their activity
consists of 2 regulatory (R) and 2 catalytic (C) chains
inactive in the absence of cAMP
binding of cAMP to R chains relieves the inhibition of C chains → become catalytically activated when free
provides a higher and widespread level of signal amplification to stimulate physiological response
Step 3: Epinephrine Signaling: Heterotrimeric G Proteins Transmit Signals and Reset Themselves
adrenergic receptors: cell surface proteins of 2 classes
α-adrenergic receptors
β-adrenergic receptors
epinephrine (adrenaline): hormone secreted by mammalian adrenal glands in response to internal and external stressors
binds to β-adrenergic receptors → exerts “fight-or-flight” response
on GTP binding → Gα dissociates and binds to adenylate cyclase → AC activation
AC activation (increase in cAMP conc.) = activated PKA
troponin complex: blocks the ability of myosin to bind to actin → preventing muscle contraction
in cardiac muscle cells: PKA phosphorylate troponin I → weakens troponin binding to actin → relieves inhibition of troponin = muscle contraction
Step 4: G Protein Spontaneously Reset Themselves Through GTP Hydrolysis
Gα subunits promote the hydrolysis of bound GTP to GDP + Pi
intrinsic GTPase activity
hydrolysis is slow → allows for the activation of downstream components for deactivation
after GTP hydrolysis and the release of Pi, GDP-bound form of the Gα reassociate with Gβγ to re-form the inactive heterotrimeric protein
Step 5: Termination of β-Adrenergic Receptor Signaling
resetting the hormone-bound activated receptor to prevent continuous activation of G proteins occurs via 2 processes:
hormone dissociation
G-protein receptor kinases phosphorylate Ser and Thr residues in receptors, then β-arrestins bind and block the receptors ability to activated G proteins
Some 7TM Receptors Activate the Phosphoinositide Cascade
α-adrenergic receptors = receptors that activate Gαq (G protein that binds to and activates the enzyme phospholipase C when in its GTP form)
expressed in muscle cells that control pupil dilation and line small blood vessels in skin
bind epinephrine
homologous to β-adrenergic receptors with very similar 3D structures
phospholipase C = catalyzes cleaves of PIP2 (cell membrane phospholipid) into IP3 and DAG
IP3: soluble second messenger that diffuses away from the membrane
causes rapid release of calcium from the ER through specific IP3-gate calcium channel proteins
calcium is also signaling molecule and second messenger
binds calmodulin (CaM) → induces substantial conformational changes on Ef hands = exposedshydrophobic surfaces that can bind proteins such as kinases
contains four EF-hand motifs that can each bind a single calcium ion
EF-hands: calcium bind motifs that consists of a helix, loop, and second helix
DAG: second messenger that remains in the plasma membrane
binds and activates protein kinase C
PKC requires calcium to bind DAG
Pathway
IP diffuses away from membrane toward receptors on ER
causes rapid release of calcium from ER
calcium binds PKC or calmodulin
DAG binds and activates protein kinase C
PKC requires calcium to bind DAG
Some Receptors Signal Through G Proteins that Inhibit rather than Stimulate Adenylate Cyclase
α2 - adrenergic receptors = class of 7TM receptor that signals through Gαi
Gαi = G protein that binds and inhibits adenylate cyclase
expressed by some blood vessel smooth muscle cells
activation of this pathway results in decreased cAMP
7TM Receptors Trigger Signaling Through G Proteins in Many Other Cell Types
ex: adenosine A1 receptor signals through Gαi
slows heartbeat due to decreased level of cAMP
decreases the release of certain neurotransmitters in the brain
caffeine is an adenosine receptor antagonist
Insulin Signaling: Phosphorylation Cascades are Central to Many Signal-Transduction Processes
insulin: hormone release in response to increased blood-glucose levels after a meal
leads to the movements of glucose transporters to the surfaces of fat and muscle cells
insulin receptor is a protein tyrosine kinase that is autoinhibited prior to insulin binding
homodimer of heterodimers
each α chain lies outside of the cell
each β lies inside of the cell
tyrosine kinase inside of the cell
insulin binding changes the conformation of the extracellular domains of the insulin receptor
conformational change brings the protein kinase domains together
cross-phorphylation of tyrosine residues activate insulin receptor kinase
flexible activation loop of one kinase subunit fits into the active site of the other kinase subunit
after phosphorylation, Tyr resides cannot fit into the active site
activation loops swing out, which opens the active site and activates the kinase
activated kinase phosphorylate insulin receptor site proteins, membrane-associated anchor proteins
phosphoinositide 3-kinase (PI3K): lipid kinase
add a phosphoryl group to the 3-position of inositol in PIP2 to form PIP3
bind to phosphorylated IRS proteins and are drawn to the membrane
insulin signaling pathway results in PDK1 protein kinase activation
PIP3 molecules serve as gathering points for other protein, continuing a kinase cascade
insulin signaling is terminated by the action of phosphatases
protein tyrosine phosphatases remove phosphate from Tyr residues on the insulin receptor and the IRS adaptor proteins
protein serine phosphatases remove phosphates from activated protein kinases such as PKB
lipid phosphatases remove phosphates from inositol lipids such as PIP3
Net Effects = Increased Glucose Uptake
insulin binding to the receptor induces cross-phosphorylation and activation of IR-Kinase
IR-kinase phosphorylation of IRS leads to recruitment of PI3 Kinases, which produces PIP3
PIP3 activates PDK1 kinase, which phosphorylates other kinases downstream
pathway leads to an increase in glucose transporters on cell surfaces, which promotes glucose uptake, utilization, and storage in the cells
Epidermal Growth Factor (EGF) and EGF Receptor Signaling
epidermal growth factor (EGF): signaling polypeptide that stimulates the growth of epidermal and epithelial cells
released into regions where wounding occurs
EGF receptors: monomeric receptor tyrosine kinases that bind a single molecule of EGF in its extracellular domain
EGF binding to the receptor induces receptor dimerizaiton
EGF binding to the receptor results in the activation of Ras
Ras: small G protein that exchanges GDP for GTP and undergoes a conformation change
Net Effects
EGF is a signaling molecule
EGF binding to EGFR activates Ras → activates Raf, activates MEKs (protein kinase)
ERKs phosphorylate transcription factors in the nucleus and other protein
net effect: gene expression changes promote growth/wound healing
Termination
protein phosphatases remove phosphoryl group form:
Tyr resides on the EGF receptor
Ser, Thr, and Tyr residues in the protein kinases that participate in the signaling cascade
Ras possesses intrinsic GTPase activity
GTPase-activating proteins (GAPs) = proteins that interact with small G proteins in the GTP form and facilitate GTP hydrolysis
Defects in Signal-Transduction Pathways = Cancer and Other Diseases
many cancers contain mutations in genes that encode signal-transduction proteins
ex: genes encoding Ras proteins are commonly mutated in human tumors
most common Ras mutations lead to a loss of the ability to hydrolyze GTP, trapping Ras in the “on” position
EGF(R) family members are over-expressed in many cancers
many therapeutics target proteins in the aberrant processes
Sensory Systems
based on specialized signal-transduction pathways
ex: most odorants are small organic compounds
~ 400 different odorant receptors in the human genome
individual receptors bind to specifically to certain odorant molecules
shape of molecule determines which receptors they bind to and hence perception of their odors
olfactory signal transduction cascade is similar to the β-adrenergic receptor cascade
odorant binding to an OR on the neuronal surface initiates a signal-transduction cascade that results in an action potential
rise in cAMP activates a nonspecific cell channel
calcium and other cations flow into the cell → depolarizing the cell and initiating an AP
vision relies on specialized 7TM receptor ot singal a response to absorbed like
based on the absorption of visible light by photoreceptor cells in the eye
rhodopsin: photoreceptor that absorbs light efficiently in the middle of the visible
part of 7TM-receptor family
consists of the protein opsin linked to 11-cis-retinal (prosthetic group) via lysine residue
light absorption induces isomerization of bound 11-cis-retinal of rhodopsin to its all-trans form
nitrogen atom from the linked Lys reside shifts, leading to the closing of ion channels and the generation of a nerve impulse
transducin: heterotrimeric G protein that activates a specific cGMP phosphodiesterase that hydrolyzes cGMP to GMP
causes cGMP-gated ion channels to close
activated by all-trans form of rhodposin