Intracellular Signalling Pathways Flashcards

Question

Are receptors in the membrane and G protein fixed?

Answer 1

Receptors in the memb and the G protein are fixed - lipid tails which insert the subunits into the membranes - can move along the inner surface of membrane

Answer 2

The α-GTP and/or βγ interaction with effectors lasts until the α subunit GTPase activity hydrolyses GTP back to GDP. α-GDP and βγ subunits then reform an inactive heterotrimeric complex.

Answer 3

1-10 seconds

Answer 4

The human genome encodes 20 Gα (alpha), 5 Gβ (beta) and 12+ Gγ (gamma) proteins Therefore, there are >1000 possible Gα-βγ protein combinations

Answer 5

Activated GPCRs preferentially interact with specific types of G protein. The Gα subunit is a primary determinant. In turn, Gα subunits and Gβγ subunits interact with specific effector proteins. In this way, an extracellular signal, working via a specific GPCR, will activate a single, or small sub-population of G proteins and effectors in the cell to bring about a specific cellular response.

Answer 6

Adrenaline or noradrenaline

Answer 7

Acetylcholine

Answer 8

Phospholipase c (increase activity)

Answer 9

Adenylyl cyclase (INHIBITORY)

Answer 10

Adenylyl cyclase (increase activity)

Answer 11

M2/M4 muscarinic

Answer 12

M1/M3 muscarinic - Airway smooth muscle | Parasympathetic - release Ach in airways - pathway - causes bronchoconstriction

Answer 13

Cholera toxin - CTx | Pertussis toxin - PTx

Answer 14

Ptxi modifies alpha i subunit which corrupts its Normal function Lock GDP, cannot exchange for GTP Lock in off state Cant get protein to release GDP, so GTP cant bind so never get consequence of this pathway Pertussis toxin (PTx) ‘uncouples’ G GPCRs from mediating signal transduction events

Answer 15

Covalently modifies alpha s Affects function CTx prevents alpha s from possessing GTPase activity Alpha can be switched on, it can bind GTP and signal But it cannot switch off Keeps signalling CAMP levels rise Causes water secretion Gut produces lots of water that it doesn't normally Watery diarrhoea Dehydration in person suffering

Answer 16

Enzymes or ion channels

Answer 17

Converts cellular ATP to cAMP the prototypic second messenger molecule

Answer 18

Takes a minor membrane phospholipid, PIP2 and converts it to 2 second messenger molecules - IP3 and DAG

Answer 19

Phosphorylates PIP2 to PIP3

Answer 20

Convert cyclic GMP to 5'-GMP

Answer 21

Voltage operated Ca2+ channels - VOCCs | G Protein-Regulated Inwardly-Rectifying K+ Channels - GIRKs

Answer 22

beta adrenoceptors D1 dopamine receptors H2 histamine receptors

Answer 23

G protein coupled receptor in celll which preferentially couples to Gs protein A ligand binds to receptor, receptor interacts with G protein, prevents loss of GDP Alpha then introduced to adenylyl cyclase, activates it, causes it to use more ATP to make moree cAMP So camp levels go up Camp has a number of binding partners such as camp dependedentprotein kinase (PKA)

Answer 24

Alpha 2 adrenoceptors D2 dopamine receptors Mu opioid receptors

Answer 25

Agonist prefer alpha i GTP builds Then reduced Ac activity Inhibits action of athother Gs mechanism This pathway stops AC activity increasing Camp levels go down, PKA not activated

Answer 26

cAMP dependent protein kinase Epacs (guanine nucleotide exchange factors) Cyclic-nucleotide-gated ion channels (CNGs)

Answer 27

Phospholipase c activity GDP to GDP exchange on a Aq released Interacts with phospholipase c C then interacts with PIP2 Pip 2 hydrolysed by phospholipase c This forms head group (IP3) - lipid part stays in membrane Lipid part is one second messenger (DAG) Head group can go into cytoplasm and finds IP3 receptor - this then activates it - calcium then released from store to cytoplasm Dag activated by high ca and interacts with PKC

Answer 28

Alpha1 adrenoceptors M1 muscarinic receptors H1 histamine receptors

Answer 29

Signal amplification is a key feature of many cell signalling pathways. For example, a few molecules of adrenaline binding to cell surface β-adrenoceptors may cause a relatively massive cellular response The β-adrenoceptor -> Gs protein -> adenylyl cyclase part of the cascade causes relatively little amplification. Nevertheless, activation of adenylyl cyclase generates many molecules of cyclic AMP which then activate the enzyme PKA.

Answer 30

The force with which the heart contracts

Answer 31

The rate at which the heart contracts

Answer 32

Adrenaline and NA can interact with beta 1 adrenoceptors

Answer 33

Bind to receptor, gdp to Gtp A releases, AC makes camp Activates protein kinase 10 or 11 key substrates for this kinase One of the substrates is a VOCC in memb of ventricular cell Each heart beat cause voltage change - depolarises memb - allowing pulse of ca to enter cell - ca triggers contraction of ventricular cell - uses ca induced ca release When we phosphorylase voccs, each time memb depolarises, a bigger pulse of ca is allowed to enter cell Then there is a bigger rise in ca and a greater contraction By modding voccs covalently, can regulate ca entry

Answer 34

NA (and to some extent adrenaline) can interact with vascular smooth muscle alpha 1 adrenoceptors to cause vasoconstriction

Answer 35

ACh can interact with bronchiolar smooth muscle M3 muscarinic receptors to cause bronchoconstriction

Answer 36

Vasoconstriction Bronchoconstriction Contracting gastrointestinal and Genito-unirary smooth muscles Inotropy

Answer 37

G protein coupled receptors in the right place - presynaptically in this case - specialised region that releases neurotransmitter Synaptic terminal - juxtaposed with what it releases neutotransmitter onto Membrane contains presynaptic gpcrs Vesicles waiting to be released When nerve depolarised, depolarisation invades nerve terminal - opening of VOCCs High levels of ca locally Neurotransmitter vesicles fuse with memb to release neurotransmitter

Answer 38

If presynaptic receptors activated by morphine (e.g. opioids receptors in pain pathway) Binds to activate G proteins Gdp-> gtP then a and bg released Betagamma binds directly to voccs Next time depolarised - channel opens - much less ca influx Weaker influx, less vesicle fusion, inhibited ability of terminal to release neurotransmitter, this can be so powerful it can shut down neurotransmitter release

Answer 39

The as-GTP group called Gs, carries the signal from the receptor bound by the adrenaline ligand by a beta-adrenoceptor for the endogenous signalling molecule. The as-GTP group then activates the effector enzyme adenylyl cyclase. This in turn increases levels of cAMP which goes off to activate the enzyme Protein Kinase A or PKA. This goes on to phosphorylate a range of proteins acting as a switch. Depending on the protein target, this phosphorylation can either increase or decrease its activity. The as-GTP group carries the signal from the receptor part in this case a beta-adrenoceptor for the endogenous signalling molecule adrenaline and the effector enzyme adenylyl cyclase. Gs activation can also lead to other effects apart from Adenylyl Cyclase activation. These include effects on ion channels and other enzymes and signalling pathways.

Answer 40

In addition to the originally elucidated activation of adenylyl cyclase by Gs to generate cyclic AMP, its converse regulatory inhibitory version exists. This is the Gi form of ai–GTP. Like Gs, Gi proteins have additional effects independent of adenylyl cyclase inhibition, including effects on ion channels and signalling pathways involved in growth and differentiation.

Answer 41

Gq protein exert their actions on effectors other than adenylyl cyclase. The Gq protein family preferentially interact with the membrane bound enzyme phospholipase C. This causes hydrolysis of a minor plasma membrane phospholipid. This is phosphatidylinositol 4,5-bisphosphate or PIP2. This in turn generates two second messengers: - Inositol 1,4,5-trisphosphate (InsP3) - Diacylglycerol (DAG). The signal transduction pathways fed by Gs Gi and Gq all employ the general three component Receptor-G protein-Effector system. Some examples are given in Table 1 below:

Answer 42

Adenylyl Cyclase as described above, is an integral plasma membrane protein that can be either activated (via Gs) or inhibited (via Gi) by activation of different receptors. An example of this is adrenaline or noradrenaline. These ligands can act at b-adrenoceptors to activate adenylyl cyclase. Conversely, they can also act at a2-adrenoceptors to inhibit adenylyl cyclase. Therefore, the same ligand can have opposite effects depending on the receptor type. As described above, the adenylyl cyclase enzyme acts to hydrolyse cellular ATP to generate cyclic AMP. Cyclic AMP then interacts with a specific protein kinase, the cyclic AMP- dependent protein kinase A, or PKA. PKA then in turn phosphorylates a variety of other proteins within the cell to increase or decrease their levels of activity. Therefore, in this way receptors which activate adenylyl cyclase and increase cellular cyclic AMP levels can cause increased glycogenolysis and gluconeogenesis in the liver and increased lipolysis in adipose tissue. It can also cause relaxation in a variety of types of smooth muscle and positive inotropic and chronotropic effects in the heart.

Answer 43

D1 Gs stimulatory | D2 Gi inhibitory

Answer 44

Phospholipase C, or PLC, is the effector enzyme in another near-ubiquitous cell signalling pathway involving hydrolysis of a minor plasma membrane phospholipid, phosphatidylinositol 4,5-bisphosphate. This known as PIP2. The hydrolysis of PIP2 generates two second messengers; Inositol 1,4,5- trisphosphate (InsP3 or IP3) and diacylglycerol (DAG). IP3 exerts its effects by interacting with specific intracellular receptors on the endoplasmic reticulum (ER) to allow Ca2+ to leave the lumen of the ER and enter the cytoplasm. One consequence of this is the activation of Ca2+- sensitive protein kinases. DAG also interacts with a family of protein kinases (Protein Kinase C’s or PKC), which are activated by this second messenger.

Answer 45

A large number of G protein-coupled receptors can activate this pathway This includes Acetylcholine via M1 and M3 muscarinic cholinoceptors; Histamine via H1-receptors; Noradrenaline via 1-adrenoceptors; serotonin via 5-HT2- receptors). Phospholipase C activation is mediated by a distinct family of G protein collectively termed Gq. This signalling pathway is responsible for an array of important responses including vascular, GI tract and airways smooth muscle contraction, mast cell degranulation and platelet aggregation.

Answer 46

The intrinsic rate at which the sinoatrial (SA) node fires an action potential can be affected by acetylcholine released by the parasympathetic nerves. The predominant acetylcholine receptor population in the SA node is M2- muscarinic cholinoceptors; activation of these receptors increases the open probability of K+-channels which have been shown to be directly regulated by both Gai-GTP (and perhaps the γ-subunits simultaneously released). Although M2-muscarinic cholinoceptor activation will also inhibit adenylyl cyclase activity, it is not known whether this has any functional consequences. The increased plasma membrane K+-permeability causes a hyperpolarization which slows (and if strong enough prevents) the intrinsic firing rate resulting in a negative chronotropic effect.

Answer 47

Sympathetic innervation of the cardiac ventricles (and/or circulating adrenaline) can influence the force of contraction (inotropy). Activation of b-adrenoceptors (predominantly b1-adrenoceptors) increases both cyclic AMP formation and the open probability of voltage-operated Ca2+-channels (VOCCs). The increase in Ca2+ influx is brought about by two complementary mechanisms. B1-adrenoceptors activate adenylyl cyclase via Gas-GTP and the increase in cyclic AMP activates cyclic AMP-dependent protein kinase which can phosphorylate and activate the VOCC. In addition, Gas -GTP can interact directly with VOCCs - thus the direct and indirect actions at the level of VOCCs reinforce each other and cause an increase in the magnitude of Ca2+-entry resulting in a positive inotropic effect.

Answer 48

Sympathetic release of noradrenaline acts on b1-adrenoceptors to stimulate phospholipase C and phosphoinositide turnover via a Gq protein. The immediate effect is the generation of InsP3 which releases ER Ca2+ and initiates a contractile response. The role of DAG is less clear, but it is thought that activation of protein kinase C by DAG and the phosphorylation of key target proteins are important for sustaining the vasoconstrictor response to noradrenaline.

Answer 49

G protein-coupled receptors located pre-synaptically can influence the release of neurotransmitters at the synapse. For example, pre-synaptic μ- opioid receptors can be stimulated, either by endogenous opioids, or by analgesics such as morphine, to couple to Gαi proteins. The Gβγ subunits liberated from the Gαi-βγ heterotrimer interact with voltage-operated Ca2+ channels (VOCCs) to reduce the entry of Ca2+ through these channels. This decrease in Ca2+ influx inhibits the release of neurotransmitter from the pre-synaptic terminal, since neurotransmitter release is a Ca2+-dependent process.

Answer 50

cAMP dependent protein kinase

Answer 51

cGMP dependent protein kinase

Answer 52

Protein kinase C

Answer 53

Ca2+ or calmodulin dependent protein kinase

Answer 54

Each protein kinase causes phosphorylation of a distinct family of target proteins. These include enzymes, ion channels, transporters, structural proteins, whose activities are increased, decreased or unaltered by this covalent modification.