Transducer Mechanism Flashcards
Most transmembrane signaling is accomplished by a small number of different molecular mechanisms (transducer mechanisms).
Large number of receptors share these handful of transducer mechanisms to generate an integrated response.
Mainly 4 (four) major categories:
GPCR
Receptors with intrinsic ion channel
Enzyme linked receptors
Transcription factors (receptors for gene expression)
GPCR is a large family of cell membrane receptors linked to the effector enzyme/channel/carrier proteins through?
one or more GTP activated proteins (G-proteins i.e. guanine nucleotide binding protein)
GPCR Are cell surface receptors called?
seven-transmembrane domain receptors
And they Mediate the majority of cellular responses to external stimuli
All receptors have common pattern of structural organization
The molecule has _____ membrane spanning hydrophobic amino acid segments ____
7 α-helical
3 extra and 3 intracellular loops
The ligands that bind and activate these GPCR receptors include 4 things? And they vary in size from small molecules to peptides to large proteins.
light-sensitive compounds
odours
hormones
neurotransmitters,
The body consists of certain molecules (essentially proteins) that specialize in recognizing ___ ___and thereafter, translate them into ___ ___.
Chemical signals
Cellular responses
Ion-Channels: Are modulated through 2 different means
- ligand-gated (ionotropic receptors)
Ion-channels open/close when ligands bind to the receptor - voltage-gated: the rate and direction of ion movement through the pore is governed by the electrochemical gradient (as measured by its concentration on either side of the membrane as well as the membrane potential) for each ion.
Drug action is more complex than it is perceived t/f
True
Drug action is more complex than it is perceived? T/F
True
Transmembrane Ligand-gated ion channels (Ionotropic):
Similar in structure to other ion-channels but incorporates a_____ _____ ______
Responsible for the regulation of the flow of ions across cell membranes
ligand-binding site, which is found extracellularly.
Ionotropic Ligand-binding and channel opening occur on a millisecond timescale, so they are confined to _______
Eg?
excitable tissues-
CNS, NMJ, autonomic ganglia.
Receptors mediate diverse functions, including neurotransmission, cardiac conduction and muscle contraction
For example, stimulation of nicotinic receptor by ACh results in sodium influx,then?
generation of an action potential, and activation of contraction in skeletal muscle.
Also, Benzodiazepines, enhance stimulation of gamma-aminobutyric acid (GABA), resulting in increased chloride influx and.
hyperpolarization of the respective cell
G-protein coupled receptors Are membrane bound receptors which bound to effector system through _____ (______)
They do not have _______ but activate a _______ that in turn activates a _______that in turn will activate something else.
G-proteins (Metabotropic)
Channels
G-protein
secondary messenger,
G-proteins are ______ _____ molecules having 3 subunits ___ ___ & ___
Based on α-subunit, they are further classified into 3 main varieties ____ ____ _____
hetero trimeric
α, β and ϒ. (known as Gα, Gβ, and Gϒ, respectively).
Gs, Gi and Gq.
Gi
Gs
Gq
Inhibits adenylyl cyclase
Stimulates Adenylyl cyclase
Stimulates phospholipase C
Note: G12/13 regulate actin cytoskeletal remodeling in cells during during movement and migration, including cancer cell metastasis.
The consequence of GPCR depends on ______
Ligands include:
The type of G protein
adenosine, bradykinin, endothelin, opioid peptides, dopamine, serotonin, acetylcholine (muscarinic effects), histamine, chemokines, eicosanoids, adrenaline, noradrenaline etc.
The G proteins are rendered inactive when **reversibly bound to_________, but active when bound to _____________
Guanosine diphosphate (GDP)
Guanosine triphosphate (GTP).
The inactive G protein is bound to the receptor in its ______ state.
Once the receptor is recognized, the receptor changes in its _______ and therefore, mechanically activates the G protein, which does what?
The receptor can now _________ or _______
Inactive
Conformation
detaches from the receptor.
activate another G protein or switch back to its inactive state.
What happens upon activation of GPCR
Upon activation, the subunits of the G protein dissociate from the receptor, as well as from each other to yield Gα-GTP monomer and a tightly interacting Gβϒ dimer, which are now free to modulate the activity of other intracellular proteins.
List the gprotein, effector and substrate of GPCR
Gs: Adenylyl cyclase increase - Beta-receptors,
H2, D1 through cAMP
Gi: Adenylyl cyclase decrease - Muscarinic M2
D2, alpha-2 through cAMP
Gq: Phospholipase C increase - Alph-1, H1, M1,
M3 through IP3& DAG
Go: Ca++ channel decrease - K+ channel in heart, sm
A number of G proteins have been identified; among the important ones are: and what they do
Gs: Adenylyl cyclase activation – Ca++ channel opening
Gi: Adenylyl cyclase inhibition – K+ channel opening
Go: Ca++ channel inhibition
Gq: Phospholipase C activation
Alpha 2-adrenergic Gi, Go
GABA B Gi, Go
5-HT1 Gi, Go
5-HT2 Gq
H2 Gs
Targets for G-proteins and their next task
Adenylate cyclase: the enzyme responsible for cAMP formation.
Phospholipase C: the enzyme responsible for inositol phosphate and diacyglycerol formation
Ion channels: particularly calcium and potassium channels.
How does G alpha work
Gα binds to and stimulate enzymes such as adenylate cyclase, which produces the cyclic nucleotide cAMP from ATP, which diffuses easily and serves as a “second messenger.”
Other proteins may interact with Gβϒ to further modulate the signal.
cAMP regulates many aspects of cellular functions such as:3
These different activities are via a common pathway i.e.?
cell division and cell differentiation
ion channels
ion transport
activation of protein kinases by cAMP
Adenylyl Cyclase-cAMP Pathway:
Activation of adenylyl cyclase results in intracellular accumulation of second messenger cAMP, which functions through protein kinase, which phosphorylates and alters the function of many enzymes, ion channels, transporters, transcription factors and structural proteins
This altering of functions in adenylyl cyclase - cAMP Pathway results in
increased contractility/impulse generation (heart)
relaxation (smooth muscle)
glycogenolysis
Lipolysis
inhibition of secretion/mediator release
hormone secretion, among others.
Responses opposite to the above-listed are produced when adenylyl cyclase is inhibited through inhibitory Gi-protein.
cAMP activity is terminated by_______ , which hydrolyse it to _____
Phosphodiesterases
5-AMP
Activation of phospholipase C catalyzes the cleavage of membrane-bound _______ into the second messenger ______ & _______
Phophatidylinositol 4,5 biphosphate (PIP2)
inositol (1, 4, 5) triphosphate (IP3)
and diacylglycerol (DAG).
IP3 acts on IP3 receptors found in the membrane of the____ ______ to elicit _____ release from the ER.
DAG diffuses along the plasma membrane to activate______ _______ __
The resultant effect of both messengers is.
endoplasmic reticulum (ER)
Ca++
protein kinase C
increased intracellular calcium ion
The activated GPCR conformation lasts long enough to allow one bound agonist molecule to _________, which does what ?
activate several G-proteins
amplifies the signal.
The GTP is hydrolysed to GDP by _______ activity of the______, therefore, there is reassociation with Gβϒ dimer to form the “resting” G-protein.
GTPase
α-subunit
What are the 3 important biological functions that are controlled by enzyme linked receptors.
metabolism,
growth and
differentiation
In Intracellullar receptors: primary targets are
________ which cause ______ & _________
transcription factors, which cause the transcription of DNA into RNA and translation of RNA into an array of proteins.
GPCR - 3 Major Pathways
Adenylyl cyclase: cAMP pathway
Phospholipase C: IP3-DAG pathway
Channel regulation
cAMP is a second messenger, used for intracellular signal transduction, such as transferring into cells, the effects of hormones like glucagon and adrenaline, which cannot pass through the plasma membrane.
Which protein kinase does it activate and which one doesn’t require cAMP
It is also involved in the activation of protein kinases. PKA (protein kinase A) is known as cAMP-dependent protein kinase. PKC does not require cAMP.
Activation of phospholipase C by the activated GTPα subunit of Gq hydrolysis the membrane phospholipid phosphatidyl inositol 4,5-bisphosphate (PIP2) to generate the second messenger, inositol 1, 4,5,- trisphosphate (IP3) and diacylglycerol (DAG).
The water soluble IP3 diffuses to the cytosol and does what?
mobilizes Ca++ from ER depots.
The lipophilic DAG remains within the membrane, but recruits protein kinase C (PKc) and activates it with the help of Ca++
Activated PKc phosphorylates many intracellular proteins and mediates various physiological responses.
Triggered by IP3, the release Ca++ (3rd messenger) acts through calmodulin, PKc, and other effectors to mediate/modulate contraction, secretion, transmitter release, neuronal excitability among other effects.
Main results of IP3- DAG pathway
Main Results:
Mediates /modulates contraction
Secretion/transmitter release
Neuronal excitability
Intracellular movements
Eicosanoid synthesis
Cell Proliferation
Responses are opposite in case of PLc inhibition
Main results of adenylyl cyclase: cAMP pathway
Increased contracblity or heart impulse a
Relaxation of smooth muscles
lipolysis
Glycogenolysis
Modulation or junctional transmission
Opens specific type of Cat+ channel = cyclic nucleotide gated channel (CNG) :heart brain and kidney
Responses are opposite in case of AC inhibition
In channel regulation
Activated G-proteins can open or close ion channels – Ca++, Na+ or K+ etc.
These effects may be without intervention of any of above mentioned 2nd messengers – cAMP or IP/DAG
Bring about depolarization, hyperpolarization or Ca ++ changes etc.
In channel regulation
Gs:?
Go&Gi: ?
Gs – Ca++ channels in myocardium and skeletal muscles
Go and Gi – open K+ channel in heart and muscle and close Ca+ in neurones
Most useful drugs in clinical medicine act by?
The natural ligands include acetylcholine, serotonin, aminobutyric acid (GABA), and the excitatory amino acids (eg, glycine, aspartate, and glutamate)
mimicking or blocking the actions of endogenous ligands that regulate the flow of ions through plasma membrane channels
Drug receptor binding forces
Covalent
Ionic
Hydrogen
Can see wahala
Covalent bond: Strong and in many cases not reversible under biological conditions.
Ionic bond: Weaker than covalent, but stronger than the others
Hydrogen bond: Stronger than Van-der-Waals
Van-der-Waals
5 theories have been put out to explain the relationship between drug binding and response
1.
Occupancy theory
response is proportional to the fraction of occupied receptors; maximal response occurs when all the receptors are occupied.
2
Rate theory
response is proportional to the rate of Drug-Receptor complex dissociation.
Here, duration of receptor dissociation determines whether a molecule is agonist, partial agonist or antagonist.
3
Stephenson’s theory:
response is proportional to the fraction of occupied receptor and the intrinsic activity.
4
Ariens’ theory:
response is a function of affinity; maximum response can be produced without 100 % receptor occupation.
5
The induced-fit theory:
binding site is not necessarily complementary with the ligand conformation. Rather binding produces a plastic molding of both the ligand and the receptor as a dynamic process (thus nullifying the obsolete “key and lock” concept)
Therefore, agonist induces a conformational change; antagonist, no conformational change and partial agonist induces partial conformational change.
6
Macromolecular Perturbation theory: when a drug-receptor interaction occurs, one of two general types of macromolecular pertubation is possible. Either a specific conformational pertubation (by agonist), which leads to a biological response.
Or a non-specific conformational pertubation (antagonist), leads to no biologic response.
Drugs that use enzyme linked receptor is
Insulin
Cytokines
Drugs that work through intracellular receptors
Steroids
Oestrogen
