cell signalling

Question 1

what is cell signalling?

Answer 1

cell signalling is the process by which cells communicate with one another.

Question 2

what are the 3 stages of cell signalling?

Answer 2

1. signal reception
2. signal transduction
3. cellular response

Question 3

define and describe signal reception.

Answer 3

signal reception refers to the target cell’s detection of an extracellular signal molecule.

• a signal is detected when a signal molecule (ligand) binds to a specific receptor protein located at the cell’s surface or inside the target cell.
• ligand-receptor interaction is highly specific -> a ligand binds to a specific complementary site on the target cell’s receptor to form a ligand-receptor complex. this causes the receptor protein to undergo a conformation change (which for usually activates the receptor)

there are 2 types of signal receptor proteins - intracellular (not in syllabus) and extracellular/cell surface/membrane receptors

Question 4

define and describe signal transduction.

Answer 4

signal transduction is the process by which a target cell converts an extracellular signal into an intracellular signal that results in a specific cellular response

• the formation of the activated ligand-receptor complex changes the conformation of the receptor protein, initiating transduction
• transduction sometimes occurs in a single step (for signalling mediated by intracellular receptors). but usually, for cell surface membrane receptors, transduction occurs in a multistep signal transduction pathway consisting of a series of relay molecules
• the relay molecules are usually enzymes that operate in a specific sequence; each protein in the pathway typically acts by altering the conformation of and activating/inhibiting the protein immediately downstream. conformational changes are usually brought about by phosphorylation, creating a phosphorylation cascade
• transduction may also involves non-protein second messengers that relay the signal from the cell surface into the cell interior through diffusion

Question 5

describe cellular response.

Answer 5

• a signal transduction pathway eventually leads to the regulation of one or more cellular activities
• a response may occur in the cytoplasm (cytoplasmic response) or may involve action in the nucleus (nuclear response)
• cytoplasmic response mainly changes cell metabolism
• nuclear response involves changes in gene expression, such as turning specific genes on or off in the nucleus

Question 6

why are membrane receptors hydrophilic?

Answer 6

the receptors are hydrophilic and are unable to diffuse across the hydrophobic core of the cell membrane,
allowing ligand molecules to bind to specific sites on cell surface receptor proteins.

Question 7

what are the 4 main types of cell surface receptors?

Answer 7

• G-protein linked receptors (GPLR)
• receptor tyrosine kinases (RTK)
• ion channel receptors (FYI)
• integrin receptors (FYI)

Question 8

what is phosphorylation?

Answer 8

phosphorylation is the process by which a protein kinase (PK) transfers phosphate groups from ATP to a protein.
PK phosphorylates and activates (downstream) protein kinases, turning on the signal transduction pathway

phosphorylation does not always lead to activation of a protein

the signal is transmitted by a cascade of sequential protein phosphorylation, each bringing with it a conformational change. this changes a protein from an inactive form to an active form

Question 9

what is dephosphorylation?

Answer 9

dephosphorylation is the process by which a protein phosphatase (PP) removes phosphate groups from proteins
PP dephosphorylates and thus inactivates protein kinases, turning off the signal transduction pathway when the initial signal is no longer present -> protein kinases are available for reuse

Question 10

what are second messengers and what do they do?

Answer 10

second messengers are non-protein signal molecules (include small, non-protein, water soluble molecules/ions)

second messengers transmit the message carried by the first messenger into the target cell’s interior.

• binding of first messenger onto receptors stimulates an increase in conc of second messengers
• small & water soluble second messengers can readily spread throughout the cytosol by diffusion
• binding of second messengers to relay proteins alter the latter’s behaviour
• second messengers enable cells to mount a large-scale, coordinated response following stimulation by a single extracellular signal molecule

Question 11

describe the structure of g-protein linked receptors (GPLR)

Answer 11

primary structure - each GPLR protein is made up of 1 polypeptide chain

secondary structure - the single polypeptide chain comprises of 7 α-helices spanning the cell membrane, connected by non-helical segments

tertiary structure - hydrophobic interactions between the 7 transmembrane α-helices result in a barrel shape conformation. hydrogen bonds and a highly conserved disulfide linkage between the non-helical segments also stabilise the protein.

the N terminus and 3 non-helical segments form the extracellular domain of GPLR. the seven α-helices form the membrane-embedded domain. the C-terminus and 3 other non-helical segments form the intracellular domain. GPLR has different binding sites for the specific signal molecule and G protein

Question 12

how do the properties of GPLR amino acid residues aid their function?

Answer 12

GPLR a.a. residues that form the inter-helical loops and N&C termini are hydrophilic - enables extracellular & intracellular domains to be soluble in aqueous mediums and also interact with water-soluble ligands (outside) and G-protein (inside)
hydrophobic GPLR a.a. residues are primarily found in the 7 transmembrane α-helices - enables the membrane-embedded domain to be stabilised and embedded within the membrane bilayer.

Question 13

how does the structure of binding/interaction site of GPLR relate to its function?

Answer 13

extracellular domain contains specific a.a. at signal-binding site - enables signal-binding site to have specific 3D conformation that allows for interaction with specific ligand, resulting in a huge diversity of GPLRs that different ligands can bind to.
intracellular domain contains specific a.a. at G-protein interaction site - enables G-protein interaction site to have specific 3D conformation to bind and activate G-protein

binding of ligand to GPLR causes a conformational change in protein, allowing it to interact with G-protein -> enables GPLR to initiate signal transduction pathways via activation of G-protein

Question 14

describe how signalling is mediated by a GPLR in response to signal molecule binding.

Answer 14

1. the signal molecule binds to the extracellular side of the GPLR and causes a change in receptor conformation, activating the GPLR
2. with an increased affinity for the G protein, the cytoplasmic side of the GPLR binds an inactive G protein, causing a GTP to displace the GDP bound to the G protein. this activates the G protein
3. the activated G protein dissociates from the GPLR and diffuses along the membrane,
4. the activated G protein binds to a target protein, usually an enzyme, altering target protein activity
5. a change in target protein (enzyme) activity initiates a cascade of signal transduction events by triggering the next step in the transduction pathway inside the cell, including the production of cyclic AMP OR IP3 and release of Ca2+ (these 3 serve as 2nd messengers)
6. the last activated molecule in the transduction pathway triggers a cellular response.
7. the intrinsic GTPase activity of the G protein soon hydrolyses its bound GTP to GDP, so that the G protein is inactive again, the signal molecule has also dissociated from the GPLR
8. the inactive G protein leaves the enzyme, which returns to its original inactive state. the G protein is now available for reuse

Question 15

what does adenylyl cyclase do?

Answer 15

adenylyl cyclase converts ATP to cAMP in response to an extracellular signal molecule that binds to GPLR

Question 16

how is adenylyl cyclase activated?

Answer 16

1. an extracellular signal molecule (eg epinephrine) binds to and activates a GPLR, which activates a specific G protein.
2. the active G protein activates adenylyl cyclase, which catalyses the synthesis of many molecules of cAMP. this boosts the normal cellular concentration of cAMP by 20-fold in a matter of seconds
3. the immediate effect of cAMP is usually the activation of protein kinase A, which phosphorylates various other proteins, depending on the cell type.

the number of cAMP molecules does not persist for long in the absence of the hormone as phosphodiesterase (another enzyme) converts cAMP to AMP

Question 17

how is low cytosolic Ca2+ maintained?

not as imp js read through

Answer 17

1. calcium ATPase in the ER membrane sequester Ca2+ from cytosol into the ER lumen
2. calcium ATPase in the plasma membrane actively pump Ca2+ from the cytosol into the extracellular fluid
3. sodium calcium exchangers in the plasms membrane couples export of Ca2+ with the facilitated diffution of Na+ into the cytosol
4. mitochondrial Ca2+ pumps move Ca2+ into mitochondria

Question 18

how does IP3 (inositol trisphosphate) stimulate the release of calcium from the ER?

according to bio lecturer, ‘2nd priority’

Answer 18

1. a signal molecule binds to the receptor, leading to the activation of G protein and consequently the enzyme phospholipase C (PLC)
2. PLC cleaves a plasma membrane phospholipid called PIP2 into DAG and IP3.
3. DAG functions as a second messenger in other pathways
4. IP3 quickly diffuses through the cytosol and binds to an IP3 gated calcium channel in the ER membarne, causing it to open.
5. calcium ions diffuse out of the ER down a conc gradient, raising the cytosolic Ca2+ level.
6. Ca2+ activate the next protein such as calmodulin in one or more signalling pathways, producing a cellular response

Question 19

what are structural features of a receptor tyrosine kinase protein?

note: RTK exists as 2 individual polypeptide subunits

Answer 19

each RTK subunit comprises of…
1. an extracellular signal-binding site
2. an α-helix spanning the membrane
3. an intracellular tail containing multiple tyrosines and a tyrosine kinase domain

Question 20

how does RTK work in response to a signal molecule binding?

Answer 20

1. when the signal molecule/ligand binds to a subunit of the RTK, aggregation and dimerisation occurs
2. dimerisation leads to the activation of tyrosine kinase activity of the receptor, resulting in autophosphorylation
3. each tyrosine kinase domain adds a phosphate from an ATP molecule to a tyrosine on the tail of its own or the other polypeptide subunit
4. RTK is fully activated
5. the activated RTK binds cytoplasmic relay proteins. this alters their activity, localisation or ability to interact with other intracellular signalling proteins. each relay protein recognises and binds to a specific phosphorylated tyrosine. the bound relay protein becomes activated, in many cases undergoing a conformational change
6. each activated relay protein triggers a transduction pathway, initiating a cascade of signal transduction events inside the cell
7. the last activated molecule in each transduction pathway triggers a cellular response

Question 21

what are the advantages of cell signalling?

Answer 21

1. signal amplification - a small number of extracellular signal molecules can produce a large cellular response
2. more opportunities for coordination and regulation
3. the response is more specific as the same signalling molecule can lead to a variety of cellular responses via specific combination of signalling/relay proteins in each cell

Question 22

what is signal amplification?

Answer 22

signal amplification is the process of enhancing signal strength as the signal is relayed through a transduction pathway. as a consequence, the response is amplified.

Question 23

how does the signalling cascade amplify a signal during transduction?

Answer 23

• there are multiple steps in the transduction pathway. at each catalytic step in the cascade, the number of activated products is much greater than in the preceding step
• one ligand/signal molecule activates a large number of protein kinases to elicit a large cellular response
• active form of protein kinases persist in the pathway long enough to process numerous substrate molecule before they become inactive again

Question 24

what is signal termination and why must it occur?

Answer 24

signal termination is where the signal response is terminated by processes which return the receptor and each of the components of the signal transduction pathway to their inactive states.
signal termination must occur as a cell must be able to continually respond to incoming signals, so each molecular change in its signalling pathways must last only a short time.

Question 25

what are mechanisms resulting in signal termination?

Answer 25

1. **protein phosphatase** - catalyses dephosphorylation & inactivation of protein kinases, impeding the transduction pathway downstream of the affected proteins. 2. **intrinsic GTPase** - rapidly catalyses hydrolysis of bound GTP to GDP, inactivating G protein 3. **phosphodiesterase** - catalyses conversion of cAMP to AMP, decreasing concentration of cAMP

Question 26

why do 2 cells that receive the same signal have different signal transduction pathways?

Answer 26

- the **specific combination** of signalling proteins and proteins needed to carry out the response gives the cell great specificity in both the signals it detects the the responses it produces - different cells have **different types of receptors** that will bind to particular ligands - different cells have **different relay molecules** that will activate **different downstream molecules**, leading to different responses in cells with the same signal molecule.

Question 27

what are the 3 primary components of a response loop?

Answer 27

1. input signal - stimulus + receptor 2. integration of signal - involves integrating signal (usually nerve/endocrine cell) 3. output signal - effector + response | vocab! ## Footnote set point - the steady state range of values at which the system operates stimulus - fluctuations in normal condition above/below set point receptor - signals the extent of deviation from the set point integrating center - determines set point, evaluates & coordinates incoming signals and compares them with set point, decides on appropriate responses to restore set pooint effector - responds to commands by eleciting appropriate response(s) feedback mechanism - informs integrating centre of any change as a result of the response elicited by effector

Question 28

what is the difference between negative and positive feedback?

Answer 28

negative feedback - counteracts stimulus so the entire control mechanism is shut down (usually involved in homeostatic control) positive feedback - reinforces stimulus so it proceeds at faster rate

Question 29

what are endocrine glands?

Answer 29

endocrine glands secrete hormones directly into the bloodstream. the hormone producing cells of the glands are thus embedded within a network of blood capillaries. endocrine glands are also known as ductless glands as they release hormones directly into the surrounding extracellular fluid, which will transport hormones to target cells. (compared to exocrine glands which store hormones in ducts before releasing them)

Question 30

what are hormones?

Answer 30

hormones are chemical messengers secreted by endocrine glands and cells that are transported by blood to act on specific target cells away from where they are synthesised. | there are 2 types of mechanisms of action for hormones: ## Footnote water-soluble hormones: make use of cell surface/membrane receptor signalling lipid-soluble hormones: make use of intracellular receptor signalling

Question 31

what are properties of hormones?

Answer 31

1. effective in **low conc** 2. effect on target cells may be **slow** to appear due to time taken to travel through the bloodstream to target cells 3. effect on target cells is **sustained, prolonged & long lasting**. removal requires inactivation by the liver & excretion by kidneys 4. binds to **receptors** on target cell, and only target cells for a given hormone have the specific receptor that recognises and binds to it

Question 32

what are functions of hormones?

Answer 32

- hormones alter cellular operations by changing the types, activites and/or quantities of important enzymes and structural proteins. - this is done by stimulating the synthesis of an enzyme/structural protein not already present in the cell OR increasing/decreasing rate of synthesis of a particular enzyme/protein by changing its rate of gene expression OR activating/inactivating an existing enzyme by altering its specific 3D dimensional conformation

Question 33

# insulin -> β cells -> RTK describe how a response is elicited when blood glucose concentration is higher than set point.

Answer 33

**stimulus** - blood glu conc is higher than set point of 90mg/100ml **receptor** - change is detected by islets of langerhans in pancreas **integrating center** - β cells of islets of langerhans are stimulated to secrete **more insulin.** secretion of glucagon by α cells is inhibited **effectors** - liver, skeletal muscle and adipose cells **response** - insulin is secreted into & travels in the bloodstream. insulin binds to **cell-surface insulin receptors** on the plasma membrane of insulin-depedent effector cells. upon binding of insulin, the insulin receptor, which is a **RTK**, phosphorylates intracellular enzymes to bring about the following effects. (another flashcard) **outcome** - cellular mechanisms decrease blood glu conc to set point. once set point is reached, **negative feedback** mechanisms prevent further release of insulin. decrease in blood glu conc acts as negative feedback signal to result in **decreased stimulation of β cells**, so less insulin is released and there is no further decrease in blood sugar conc

Question 35

describe the effects of insulin when blood glucose concentration is above set point.

Answer 35

**effects of insulin** 1. *ACCELERATED RATE OF GLUCOSE UPTAKE* extra glucose transporters (GLUT4) in muscle & adipose cells are **normally sequestered** in membranes of vesicles within the cytoplasm. upon activation of insulin receptors, a signal transduction pathway is activated to stimulate **migration & fusion of these vesicles** with the plasma membrane to **increase rate of glucose uptake** via facilitated diffusion. this increases the no. of glucose transporters in the plasma membrane. 2. *ACCELERATED RATE OF GLUCOSE UTILIZATION AND STORAGE* rate of **glycolysis** (breakdown of glucose to produce ATP) is **increased**. **stimulates glycogenesis** (formation of glycogen from glucose) as activated insulin receptors activates **glucokinase** (phosphorylates glucose into a sugar that is used to synthesised glycogen) in liver & skeletal muscle cells. **inhibits glycogenolysis** (breakdown of glycogen to glucose) 3. *STIMULATES A.A. ABSORPTION AND PROTEIN SYNTHESIS* **inhibits gluconeogenesis** (conversion of amino acid into glucose) in skeletal muscle cells 4. *STIMULATES LIPOGENESIS* in adipose tissues, **lipogenesis** (triglyceride formation) **is stimulated** by increasing absorption of glucose into adipocytes where excess glucose is stored as triglycerides. **lipolysis is inhibited**

Question 36

# glucagon -> α cells -> GPLR describe how a response is elicited when blood glucose concentration is lower than set point.

Answer 36

**stimulus** - blood glu conc is lower than set point of 90mg/100ml **receptor** - change is detected by the islets of langerhans of the pancreas **integrating center** - α cells of the islets of langerhans are stimulated to secrete **more glucagon**. secretion of insulin by β cells is inhibited **effectors** - liver, skeletal muscle and adipose cells are main target cells **response** - glucagon is secreted and travels into the bloodstream, and binds to **cell-surface glucagon receptors** on the plasma membrane of target cells. upon binding of glucagon to its receptor, a GPLR, adenylate cyclase is activated to **produce cAMP from ATP.** cAMP acts as a second messenger to activate cytoplasmic enzymes to mobilize energy in the following ways (effects are in another flashcard) **outcome** - the cellular response mechanisms increase blood glu conc back up to set point. once set point is achieved, **negative feedback mechanisms** prevent further release of glucose. increase in blood glucose results in **decreased stimulation of α cells**. less glucagon is released and there is no further increase in blood glucose concentration

Question 37

describe the effects of glucagon when blood glucose concentration is below set point.

Answer 37

**effects of glucagon** 1. **stimulates glycogenolysis**, in liver & skeletal muscle cells, which is the breakdown of glycogen to glucose, increasing blood glucose conc. (protein kinase A, activated by cAMP, phosphorylates **glycogen phosphorylase kinase** which stimulates glycogenolysis) 2. **inhibits glycogenesis**, in liver & skeletal muscle cells, which is the synthesis of glycogen from glucose. (protein kinase A also phosphorylates **glycogen synthase**, inhibiting its catalytic activity and preventing glycogenesis) 3. **stimulates gluconeogenesis**, in liver cells, which is the process where glucose is synthesised from amino acids. amino acids absorbed from the bloodstream are converted into glucose in liver cells and released into the bloodstream. 4. **stimulates lipolysis**, in adipocytes, which is triglyceride breakdown. lipolysiswill take place under prolonged hypoglycemia (low blood glu conc in blood) and fatty acids are released into bloodstream for use as energy source by other cells. **inhibit lipogenesis**, which is triglyceride formation from glucose.

Question 38

describe the mechanism of insulin - RTK signalling

Answer 38

1. binding of insulin to insulin receptor activates the receptor's tyrosine kinase activity, leading to **autophosphorylation & receptor activation** 2. relay protein specific to insulin receptor binds to a **specific phosphorylated tyrosine** on the receptor and becomes activated 3. activated relay protein triggers a signal transduction pathway 4. activated downstream relay protein stimulates the movement of cytoplasmic vesicles carrying **GLUT4 glucose transporters**, fusion of these vesicles with the plasma membrane and **more glucose transporters** to mediate glucose uptake 5. downstream activated protein also leads to the activation of glycogen synthase, which catalyses **glycogenesis** (formation of glycogen from glucose 6. active glycogen synthase converts glucose to glycogen, decreasing blood glucose concentration

Question 39

describe the mechanism of glucagon - GPLR signalling

Answer 39

1. binding of hormone glucagon to specific GPLR activates a specific G protein 2. activated G protein activates **adenylyl cyclase**. active form of adenylyl cyclase catalyses the synthesis of large amounts of intracellular **cAMP** (second messenger) from ATP 3. cAMP binds and activates **protein kinase A** 4. active protein kinase A phosphorylates **glycogen phosphorylase kinase**, activating it. (can also be activated by Ca2+, released from Ca2+/IP3 pathway) 5. active protein kinase A also phosphorylates **glycogen synthase**, inhibiting its catalytic activity and preventing **glycogenesis** (conversion of glucose to glycogen) 6. activated glycogen phosphorylase kinase phosphorylates **glycogen phosphorylase**, the enzyme for breakdown of glycogen into glucose, converting it to its active form such that it **stimulates glycogenolysis**, the breakdown of glycogen to release glucose-1-phosphate