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Adenylyl cyclase
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Cell surface receptors
G-protein Linked Receptor
Receptor Protein Tyrosin Kinase
Cytokine receptors and Nonreceptor Protein- Tyrosin
Kinase
Receptors linked to other Enzymatic activities
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G-protein Linked Receptor
They consist of single polypeptide
chain which have seven membrane
spanning alpha helices
These make Several sensory
receptor including Olfactoryreceptors, rhodopsin etc
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The switch is turned off when G protein hydrolyses its own
bound GTP converting it back to GDP.
But during this period the active protein has opportunity to
diffuse away from the receptor and deliver message to its
own downstream target.
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Most G protein linked receptors activate a chain of events
that alter the concentration of one or more small
intracellular mediators.
Cyclic AMP and Ca2+ ions are most widely used
mediators.
Cyclic AMP pathwayCyclic AMP is synthesiszed from ATP by membrane bound
enzyme adenylyl cyclase.
It is rapidly destroyed by one or more cyclic AMP
phospodiesterases which hydrolyze cyclic AMP to
adenosine 5` AMP.
Cyclic AMP dependant protein kinases mediates the effect
of cyclic AMP
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Protein kinases catalyzes the transfer
of the terminal phosphate group from
ATP to specific serine or threonine of
selected proteins (enzymes)
Protein Kinase A consists of a
complex of two catalytic and two
regulator subunits.
The binding of cyclic AMP alters the
confirmation of of regulatory subunits,
causing them to dissociate fromcomplex.
The realease catalytic subunit are
thereby activated to phosphorylate
specific proteins.
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The activity of an protein regulated by phosphorylation
depends on the balance at any instant between the
activities of kinases that phosphorylates it andphosphatases that are constantly dephosphorylating it
In some animal cells cyclic AMP activates the transcription
of specific genes. The genes contains a regulatory subunitcalled cyclic AMP response elements or CRP.
It is phosphorylated by protein kinases leading to activation
of genes.
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Receptor protein tyrosine Kinase
Unliked G linked receptors either surface receptors aredirectly linked to intracellular enzyme.
Largest family of such receptor is the receptor protein tyrosin
Kinase which phosphorylates their substrate proteins on
tyrosine residues.
More than 50 receptors protein Tyrosine Kinases have been
identified including EGF, NGF, PDGF, insulin and many
others.
N terminal extracellular ligand binding domain , a single
transmembrane alpha helix and a cytosolic C terminal domain
with protein tyrosine Kinase activity
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Most of them are single polypeptide chain although insulin
receptor and some related receptor are dimers
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Ligand binding induces the dimerization of receptor
Ligand inducced dimerization then lead to
autophosphorylation of the receptors as dimerised
polypeptide chain cross phosphorylate one another.
This auto-phosphorylation plays two key roles first
phosphorylation of Tyrosine residue within the catalytic
domain may play a regulatory role by increasing receptorprotein kinase activity.
Binding of ligand the extracellular domain of these receptors
activates ther cytosolic kinase domains resulting in
phosphorylation of both the receptors themselves and
intracellular target proteins that propagate the signal initiated
by growth factor binding.
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Phosphorylation of Tyrosine residue of catalytic domain
creates specific binding sites for additional proteins that
permits intracellular signals downstream of the activatedreceptor.
The association of these downstream signaling molecules
with receptor protein tyrosin kinases is mediated by Protein
domain that bind to specific phosphotyrosine kinase
SH2 domain consists of approximately a hundred amino
acids and bind to specific short peptide sequences
containing phospho tyrosine residues.
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Cytokine receptors function in association with nonreceptor protein Tyrosine Kinase
Cytokine receptors function in association with non receptorprotein tyrosine kinase, which are activated as a result of
ligand binding.
Ligand binding induces receptor diamerization and crossphosphorylation of associated nonreceptor protein tyrosine
kinase .
These activated Kinases then phosphorylates the receptors,
providing phosphotyrosine binding sites for the recruitment ofdownstream signaling molecules that contain SH2 domain.
Non receptor protein Tyrosine Kinase associated with
cytokine receptors fall into two major catagories. Many of
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Other family is Janus Kinase or Jak Family. They are
universally required for signaling from cytokine receptors.
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Receptor linked to other enzymatic activities.
These receptors include protein tyrosine phosphatase,protein serine Thrionine Kinase and Guanyl cyclase.
Protein Tyrosine phospahatases removes phosphate group
from phosphotyrosine residues and counter balance the
effect of protein Tyrosine Kinase.
Receptors for transforming growth factors and related
polypeptides are protein Kinases that phosphorylates Serine
and Therionine on substrate protein.
TGF is a family of polypeptide growth factor that controls
proliferation and differentiation of various growth cells.
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Binding of ligand to these receptors result in the assocaition of
two distinct polypeptide chans which are encoded by the
family of TGF rceptor gene family to form hetrodimers in
which receptor Kinases cross phosphorylates one another.
Some polypeptide ligands bind to receptor whose cytosolic
domain are guanylyl cycalse which catalyze formation of GMP.
The receptor has extracellular ligand domain, a single
transmembrane alpha helix and cytosolic domain with
catalytic activity.
Ligand binding stimulates cyclase activity leading to formation
of GMP a second messenger.
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Calcium ion Signaling
Resting concentration of Ca2+ is kept low in cytosol and this is
achieved n several ways.
Two pathways of ca2+ signaling have been well defined . First
one is studied in nerve cells in which depolarization of Plasma
membrane causes an influx of ca2+ into the nerve terminal
initiating the secretion of neurotransmitters.
Calcium ions enter through voltage gated ion channels that
open when PM of nerve terminals is depolarized by an invading
action potential.
In second Ubiquitous pathway the binding of ligand to receptor
on surface causes release of ca2+ from Endoplasmic reticulum.
This is done through another intracellular messenger molecule
inositol.
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Phospholipids and Ca2+
Phospthatidylinositol 4-5 bisphosphate (PIP2). PIP2 is a minor
component of inner leaflet of Plasma membrane. Severalgrowth hormones and growth factors stimulate hydrolysis of
PIP2 by phospholipase C which produces two distinct second
messenger diacylglycerol and inositol1,4,5triphosphate (IP3)
Diacylglycerol and IP3 stimulates distinct downstream signaling
pathways (Protein KinaseC and Ca2+ ion mobilization
respectively)
PIP2 hydrolysis is activated by both G protein coupled receptors
and protein Tyrosine Kinase.
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It happens because one form of Phospholipas C is stimulated by G
protein coupled receptors while a second enzyme (PLC-) contains
SH2 domain that mediates its association with activated receptorprotein Tyrosine Kinase.
Diacylglycerol activates Protein Kinase C family protein many of
which play role in cell growth and differentiation.
A good example is phorol esters action . Tumor promoting activity
of it is base on its ability to stimulate Protein Kinase C by acting as
anologue of diacylgylcerol. Protein Kinase C then activates other
intracellular targets, including MAP kinase pathaway, leading totranscription factor phosphorylation, change in gene expression
and stimulation of cell proliferation.
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Another target for protein kinases is the transcription factor NF-
kB, which is involved in a variety of response of various aspects of
immune response .
The activity of NF-kB is regulated by translocation from cytoplasm
to nucleus. In inactive sate NF-kB is retained in cytoplasm as a
complex with an inhibitory subunit called IkB.
Activation of proetin Kinase leads indirectly to Phosphorylation of
Ikb which targets Ikb for proteolytic degradation.
Distruction of IKB allows NF-kB to translocate to nucleus where itregulates transcription of its target gene.
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Whereas diacaylglycerol remains attached to Plasma membrane, IP3
is a small polar molecule that is released into cytosol where it acts to
signal the release of Ca2+ from intracellular stores.
In cells ca2+ concentrator is kept low by Ca2+ pumps that export Ca2+
from cells and into ER ER serves as intracellular Ca+ store. IP3 binds
to receptor that are ligand gated ion channels. As a a result cytosolicCa2+ level increase to about 1 micro meter which affects the activity
of several target proteins.
Many effects of Ca2+ are mediated by Ca2+ binding Camodulin which
is activated at Ca2+ concentration upto 0.5 M. Ca2+/ Calmodulin
then binds to a variety of target proteins including Protein Kinases.
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One example of such Ca/Calmodulin dependant protein kinase is
myosin light chain kinase, which signals actin myosin contraction
by phosphorylation of Myosin light chain.
Other such Protein Kinase include member of CAM kinase IInd
Family, which phosphorylate a numberof protein including
metabolizing enzyme, ion channels and Transcription factors
One form of CAM kinase Iind is particularly abundant in nervous
system. It regulates synthesis and release of neurotransmitters.
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CAM Kinase II can regulate phosphorylating transcription factor.
One of transcription factor is CREB which is phosphorylated at
the same site by protein kinase A
In addition to cleavage by phospholipase C, PIP2 can be
phosphorylated on 3 position of Inositol by enzyme phosphatidyl
inositide (PI) 3 kinase. Like Phospholipase C one PI3 Kinase is
activated by G proteins, while second has SH2 domain and is
activated by receptor protein tyrosin kinase..
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Ras, Raf and MAP Kinase pathways
MAP Kinases are family of protein serine/ Threonine Kinase
called so far Mitogen activated protein kinases
In yeast, MAP Kinase pathways control cativity of cellular
responses including mating , cell shape and sporulation.
In higher eukaryotes, MAP Kinases are ubiqutous regulators of
cell growth and differentiation
Best characterize form of MAP Kinase in mammalian cells belongto ERK that act through either G protein Kinases family that act
through either g Protein receptor or protein tyrosin kinase.
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