Biological signalling Flashcards
what are signals
- represent information that is detected by specific receotors
- Signals are converted to a cellular response which always involves a chemical process
-
what is signal transduction
Conversion of information into chemical change
what is specificity
- singal molecule fits binding site on its complementary receptor; other signlas do not fit
* Multicellular organisms have additional specificity: Some receptors are present in only certain cell types
what are the 3 factors accounting for sensitivity of signal transducers
- High affinity of receptors for signal molecules (Kd = 10-10 M)
**small [L] → large activation response (only need tiny amount bc hugre affinity)
- Cooperativity in the ligand-receptor interaction
- Once activated, signal amplification occurs through by enzyme cascades
explain amplification
Amplifications of several orders of magnitude within milliseconds = FAST RESPONSE
*one signalling molecule bidns to one receptor, receptor then activate then can activate other things)
- amplification= when enzymes activate enzymes, the number of affected molecules increases geometricaly in an enzyme cascade
what is modularity
- proteins with multivalent affinities from diverse signalling compleses from interchangable aprts
- phosphorylation provides reversible points of interaction
explain desensitization/adaptation
- receptor activation triggers a feedback circuit that shuts off the receptor or removes it from the cell surface
- When stimulus falls below a certain threshold the system again becomes sensitive
explain integration of signas
- when two signals have opposite efects on a metabolic characteristic such as the concentration of a second messengerX or the mebrane potential Vm the regualtory outcome results from the integrated input from both receptors
- Integration produces a unified response appropriate to the needs of the cell or organism
what is a G protein coupled recpetor
- external ligand [L] binding to receptor [r] activates an intracellular GTP bidning protein (G)
- this regualtes an enzyme that generates an intracellular second messanger
- ligand bidning usually causes confomrational change
-
what are receptor tyrosine kinases
0 integral mem rptoin on plasma memrbane
- ligand binds and activates tyrsoine kinase activity by autophosphorylation
*RTK become an active kinase itself
what are receptor guanylyl cyclase
- ligand binding to extracellular domain stimulates formation of second messenger cyclic GMP
what are gated ion channels
- open or close in resonde to concentration of signal ligand or memrbane protential
what is an adhesion receptor
- integrin
- binds molecules in extracellular matrixm changes conformation altering interaction with cytoskeleton
*if cell wants to move thru environment must removed the cytoskeleton, so you have integrin recpetors that can bind and interact with components of ECM leading to cahnges in shape of the cell
what are nuclear recetors
- hormone bidning allows the receptor to regulate the expression of specific genes
ion gradients in neurons and transmission of a nerve impulse
- Neuron cytosol has high [K+] and low [Na+]
- At rest, ΔΨ = -60 mV
Transmission of a Nerve Impulse
(i) action potential carries electrical signal down axon
(ii) neurotransmitter carries signal to next cell
explain Nicotinic acetylcholine receptor (AchR)
- Passage of electrical signal from motor neuron to muscle fiber at neuromuscular junction
- Acetylcholine released by motor neuron diffuses to plasma membrane of a myocyte → binds AchR
• Conformational change in AchR → opens
→ inward movement of cations (Na+, Ca2+)
→ triggers muscle contraction
explain ion gradeints in neural transmission
*ensures that even if second signal comes wont be effected unit mem prot back to normal
Acetylcholine (Ach) opens Ach receptors (ligand-gated Na+/Ca2+ channel)
- Na+ flows in (down gradient): depolarization
Adjacent voltage-gated Na+ channels open: Na+ rushesin→ ΔΨ=+30mV (<1msec)
Na+ channels inactivated; voltage-gated K+ channels open
K+ flows out (down gradient): ΔΨ = -75 mV (2 msec)
K+ channels inactivated, ΔΨ = -60 mV (3 msec)
Wave of de-/re-polarization travels along axon
explain action potential
- originally -60 (polarized)
- Na comes in goes to +30 (depolarized)
- K channels open and K goes out, -75 (hyperpolarized)
back to polarized
explain neural transmission
SNARE-regulated exocytosis releases neurotransmitter into synaptic cleft
1) wave of depolarization
- tons of voltage geted calcium channels at synpatic cleft, once wave of depolarz=iation comes the channels open and Ca comes into presynaptic terminal
- vesicles fuse and neurotransmitter is relseased into the synpatic cleft
- neurotransmitter goes to receptor on post synpatic neuron
structure of nicotinic acetylcholine receptor
- alpha helices called M2 amphipathic helices: surround channel
- AchR has 5 subunits with 4 helices in each
- 2 acetylcholine bidning sites on outside
how is the acetyl choline receptor activated
- activated by rotating its subunits
- takes 2 ach to bind causing conformational change and subunit rotates
- bulky hydrophobic leu side chains of M2 helices close the channel
- bindign fo two acetylchlone molecules causes twisting of the M2 helices
- M2 helices now have smaller polar residues lining the channel
what signals are transduced by Heterotrimeric G protein-coupled receptors (GPCRs)
- glucagon, histamines, melatonin, light, cannabinoids, epinephrine, opioids, oxytocin, serotonin and odorants
what are the 3 components of Heterotrimeric GPCR signaling, what seconadary messangers does it include
consists of 3 compoennts
- Plasma membrane receptor with 7 transmembrane helices
e. g. epinephrine receptor - Heterotrimericguanosinenucleotide-binding protein (G protein)
- Intracellular enzyme that generates a 2nd messenger
Second messagers include:
- cAMP, cGMP, inositol 1,4,5 trisphosphate (IP3)
how to turn off/on g proteins (basic)
- g protin are inactive when bound to GDP
- use GTP-GDP exchange factors which pop out GDP and put in GTP
- this activates the protein
*not a phosphorylation event to activate, but the deactivation is a dephosphorylation event
- g proteins have intrinisc ability to dephosphorylate
- to turn it off, dephosphorylate the GTP to GDP (intrinsic GTPase activity), GAP and RGS faciliate the dephosphorylation
exaplin the g protein activation cycle
- Gs is stimulatory G protein
- There are also Gi – inhibitory G proteins
- Note Gα and Gγ are anchored in the membrane by lipid tails
*beta subunit not anchored but assocaites the alpha and gamma subunit
- Gs (stimulatory) with GDP bound is turned off, it cannot activate adenylyl cyclase
- Contact of Gs iwth hormone receptor complex causes displacemnt of bound GDP by GTP
- Gs with GTP bound dissociates into α and bγ subunits, Gsα-GTP is turned on; it can activate adenylyl cyclase
- GTP bound to Gsα is hydrolyzed by the proteins intrinsic GTPase; Gsα and turns itself off. the inactive α subunit reassocaites with thebγ subunit
explain epinehrine (β-Adrenergic) signal transduction pathway (activation that catalyses formation of cAMP)
*transduced via g protein
Gα binds GDP at rest
Epinephrine binding receptor promotes GTP-binding by Gα
Gα dissociates from the receptor
Diffuses away, and binds and activates adenylate cyclase
- epinephrine bind to specific receptor
- hormone receptor or complex causes the GDP bound Gsα to be replaced by GTP, activating Gsα
- activated Gsα separates from Gβy moved to adenylyl cyclase and activates it. many Gsα subunits may be activated by one occupried receptor
- adenylyl cyclase catalyzes the formation of cAMP
Epinephrine (β-Adrenergic) Signal Transduction Pathway (steps 4-7)
- adenylly cyalse catalyzes formation of cAMP
- cAMP activates PKA
- phosphorylation of cellular proteins by PKA causes the cellular response to epinephrine
- cAMP is degraded, reversing the activation of PKA
how is the epinephrine receptor internalized
Internalization of the epinephrine receptor is induced by phosphorylation of the receptor by β-adrenergic receptor kinase (βARK) and subsequent binding of β-arrestin
- binding of epinephrine (E) to β-adrenergic receptor triggers dissociation of Gsβy from Gsα (not shown)
- Gsβy recruits βARK to the memrbane where it phosphorylates Ser residues at the carboxyl terminus of the receptor
- β- arrestin (βarr) binds to the phosphorylated carboxyl-terminal domain of the receptor
- Receptor-arrestin complex enters the cell yb endocytosis
- in endocytic vesible arrestin dissociates; receptor is dephosphorylated and returned to cell surface
explain signalling in receptor enzymes
Receptor enzymes are plasma membrane receptors
The ligand-binding domain is located outside the cell
Inside the cell is a catalytic domain
This catalytic domain can have: tyrosine kinase activity or guanylyl cyclase activity
Binding of the cognate ligand outside the cell activates this catalytic activity
explain the insulin receptor
Tetramer (α2β2)
- Insulin binds externally
- activates tyrosine kinase activity in intracellular domain
- β-chains are autophosphorylated (they autophosphorylate eachother)
- Opens up active site
explain the Inactive Insulin receptor tyrosine kinase
- When the insulin receptor is inactive, the tyrosine kinase catalytic site is blocked by its activation loop
- This loop has three tyrosines, including one which makes a key hydrogen bond with an aspartate
explain activation of Insulin receptor tyrosine kinase
When the insulin binds, the tyrosine kinase phosphorylates all three tyrosines
This stabilizes the loop in a conformation that no longer blocks the catalytic site
Substrate proteins can now bind and be phosphorylated
explain how the nsulin receptor regulates gene expression
- Phosphorylated IRS-1 is bound by SH2 domain of Grb-2 -Grb-2 binds Sos via SH3 domain (binds Pro on Sos) -Grb-2-bound Sos activates Ras (G-protein)
- GTP-bound (active) Ras activates a protein kinase cascade
- Raf-1 → MEK → ERK (MAPK family)
- ERK enters nucleus and phosphorylates transcription factors
how does kinase cascade mediates gene response to insulin
Kinases phosphorylate other kinases
This propagates and amplifies the signal
Also gives opportunities for signal integration
These kinase cascades are common in signaling
- insulin recepotr bind insulin and undergoes autophorphorylation on carboxyl terminal tyr residues
- insulin receptor phosphorylates IRS-1 on its tyr residues
- SH2 domain of Grb2 binds to phos-tyr of IRS-1. Sos binds to Grb2 then to Ras causing GDP release and GTP binding to Ras
- activated Ras bidns and activates Raf-1
- Raf-1 in a kinase that phosphorylates MEK on two ser residues activating it. MEK phosphorylates ERK on a Thr and a Tyr residue activating it
- ERK moves into the nuceleus and phosphorylates nuclear transcription factors such as Elk1 activating them
- phosphorylated Elk1 joins SRF to stimulate the transcription and translation of a set of genes needed for cell division
***HE IS MORE INTERESTED IN THE ABOVE 2 CARDS DONT REALLY NEED TO KNOW ALL THIS KNOW THERE IS A COMPLEX CASCADE OF SIGNALLING EVENTS
explain activation of glycogen synthase and glucose transporters by insulin
- IRS-1, phosphorylated by the insulin receptor activates P13K by binding to its SH2 domain. Pi3K converts PIP2 to PIP3.
- PKB bound to PIP3 is phosphorylated by PDK1 (not shown) this activated, PKB phosphorylates GSK3 on a ser residue inactivating it
- GSK3, inactivated by phosphorylation cannot convert glycogen synthase (GS) to its inactive form by phosphorylation so GS remains active
- synthesis of glycogen from glucose is accelerated
- PKB sitmulates movement of glucose transporter GLUT4 from internal membrane vesicles to the plasma membrane increasing the uptake of glucose
what does insulin fo in muscle cells
insulin acts in muscle cells to :
1) increase glucose transport by recruiting GLUT4 to membrane
2) induce synthesis of hexokinase
3) activate glycogen synthase by phosphorylation of GSK3
gove an exmaple of Protein-protein interactions and phosphorylation culminate in activation or inactivation of downstream enzymes
insulin signaling activates:
phosphoinositide 3-kinase (PI-3K)
protein kinase B (PKB)
Inactivates glycogen synthase kinase 3 (GSK3)
what are some effects of insulin:
i) Reduced phosphorylation of glycogen synthase, increased activity and glycogen synthesis by glycogen synthase
(ii) Movement of glucose transporter to plasma membrane
(iii) Modulation of insulin-responsive transcription factors
explain Nuclear hormone receptors
what is the structure of their ligands
Nuclear hormone receptors are transcription factors that are directly activated by hormone binding
Steroid hormones are based on a four-linked-ring structure (like cholesterol)
– e.g. estrogen, progesterone, cortisol
Thyroid hormones are built from iodinated tyrosine residues
– e.g. thyroxine
explain process of nuecler hormone receptors work
- hormone, carried to the targer tissue on serum binding proteins diffuses across the plasma membrane and binds to its specific receptor protein (Rec) in the nucleus
- hormone binding changes the conformation of Rec; it forms homo or heterodimers with other hormone receptor complexes and binds to specific regulatory regions called hormone response elements (HREs) in the DNA adjacent to specific genes
- Binding regulates transcription of the adjacent gene(s), increasing or decreasing the rate of mRNA formation
- Altered levels of the hormone regulated gene product produce the cellular response to the hormone
what do hormone receptors look like/how do tehy wrok
Conformational change is induced in nuclear hormone receptor
Receptor has zinc finger structures
Allows it to bind specific DNA
DNA sequences called hormone response element (HRE)
- contains a hormone binding region (varible in sequence and length), DNA binding residues (highly conserved), and transcription activation (varibale)
know that they ahve 2 zinc ginfers that protrude out the protein, loops have 4 cystine residues which cooridnate the binding of the zinc ion (this is what allows it to bind and interact with DNA)
**main thing to know
**66-68 residues
what are the mechanisms to detects and tranduce signlas
specificaity, amplification, modularity, desensitization/adaptation, integration of singlas
