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How drugs works: Molecular
Objective/Learning outcome:
G-protein and role.
 Targets
for G-proteins.
Signal transduction via second-messengers.
Adenylate cyclase/cAMP system.
 Phospholipase C/insitol phosphate system
G-proteins coupled receptors:
They are called G-proteins because
of their interaction with the guanine
nucleotides GTP and GDP.
G-proteins consists of three
Guanine nucleotides bind to α-subunit,
which has enzymic activity, catalysing
the conversion of GTP to GDP. The βand -Y subunit remain together as a
All three subunits are anchored to the
membrane through a fatty acid chain,
coupled to the G-protein through a
reaction known as Prenylation.
G-protein is freely diffusible in the plain
of membrane, and they interact with
several different receptors and effectors
in an essentially promiscuous fashion.
In a resting state, the G-protein
exists as unattached αβY- trimer, with
GDP occupying the site on the αsubunit.
When a GPCR is occupied by an
agonist molecule, a conformational
change occurs, causing the bound
GDP to dissociate and to be replaced
with GTP ( GDP/GTP exchange),
Which in turn causes the dissociation
of the G-protein trimer, releasing αGTP and βY-subunits; these are the ‘
active’ forms of the G-protein,
These active forms diffuse into the
membrane and can associate with
various enzymes and ion channels,
causing activation or inactivation as
the case may be.
The process is terminated when the
hydrolysis of GTP to GDP occurs
through the GTPase activity of the αsubunit.
The resulting α-GDP then dissociates
from the effector and reunites with the
βY-subunits complex completing the
The mechanism results in,amplification
because a single agonist-receptor
complex can activate several G-protein
molecules in turn, and each of these
can remain associated with the effector
enzyme for long enough to produce
many molecules of product.
The product is often a second
messenger, and further amplification
occurs before the final cellular
response is produce. The main class
of G-protein are Gs, Gi, and Gp.
Targets for G-proteins:
Adenylate cyclase: the enzyme responsible
for cAMP formation
Phospholipase C: the enzyme responsible
for inostiol phosphate and diacylglycerol
Ion channels: particularly calcium and
potassium channels.
The adenylate cyclase:
cAMP is a nucleotide synthesized within
the cell from ATP by the action of a
membrane-bound enzyme, adenylate
cAMP is produced continuously and
inactivated by hydrolysis to 5’-AMP
through the action of a family of enzyme
known as phosphodiesterases.
Many different drugs hormones and
neurotransmitters act on GPCR and
produce their effects by increasing or
decreasing the catalytic activity of
adenylate cyclase, thus raising or lowering
the concentration of cAMP in within the
cAMP regulates many aspects of cellular
function including-: enzymes involved in
energy metabolism; cell division and cell
differentiation; ion channels; and
contractile proteins in smooth muscle.
The varied effects of cAMP are brought
about by a common mechanism namely
the activation of protein kinase by cAMP.
Protein kinase regulate the function of
many different cellular proteins by
catalyzing the phosphorylation of serine
and threonine residues using ATP as a
source of phosphate groups.
Phosphorylation can either activate or
inhibit target enzyme or ion channel.
Other examples of regulation by cAMPdependent protein kinase includes; the
increased activity of voltage-activated
calcium channels in heart muscle cells;
phosphorylation of these channels
increase the amount of Ca2+ entering the
cell during the action potential and, thus
increases the force of contraction of the
The phospholipase C/ inositol phosphate
The phosphoinositide system, an
important intracellular second messenger
system, was first discovered by Hokin and
Hokin in the 1950’s,recondite interests
centered on the mechanism of salt
secretion by nasal glands of seabirds.
They found that secretion was
accompanied by increased turnover of
minor class of membrane phosphoplipids
known as phosphoinositidies collectively
known as PI.
Michell Berridge found that many
hormones which produce an increase in
free intracellular Ca2+ concentration which
includes for example ; muscarinic
agonists and α- adrenoceptors agonists
acting on smooth muscle and salivary
glands and antidiuretic hormone
(vasopressin) acting on liver cells also
increase PI turnover.
Subsequently it was found that one
particular member of the PI family,
namely phosphatidylinositol 4,5bisphosphate (pip2), which has additional
phosphate group attached to the inositol
ring, plays a key role.
Pip2 is the substrate for a membranebound enzyme, PLCβ, which splits it into
diacylglycerol (DAG) and inostiol 1,4,5triphosphate (IP3) both of which functions
as second messengers.
Inositol phosphates and intracellular
IP3 water-soluble mediator
acts on specific IP3 receptors ligand gated calcium channel present on the
membrane of the endoplasmic reticulum.
IP3 is converted inside the cell to the
1,3,4,5-tetraphosphate,IP4,by a specific
IP3 role is to control the release of Ca2+
from the intracellular stores.
Diacylglycerol and protein kinase C (DAG):
DAG is highly lipophilic and remains in
the membrane.
The major effect of DAG is to activate a
membrane-bound protein kinase, protein
kinase C (PKC).
These PKC catalyses the phosphorylation
of a variety of intracellular proteins.
The various PKC isoforms,like tyrosine
kinase act on many different functional
proteins, such as ion channels, receptors,
enzymes (including other kinases) and
cytoskeletal proteins.
Kinases in general, play a central role in
signal transduction and control many
different aspects of cell function.
 The DAG-PKC link provides a
mechanism whereby GPCRs can moblise
this army of control freaks.
Ion channels as target for G-protein:
GPRCs can control ion channel
function directly by mechanism that do
not involve second messengers such as
cAMP or IP3 .e.g.
in cardiac muscle muscarinic
acetylcholine receptors are known to
enhance K+ permeability and similar
mechanism in neurons, where opiate
analgesics reduce excitability by opening
the potassium channels.
These actions are produced by direct
interaction between the G-protein subunit
and the channels without the involvement
of the second messengers.