CELL SIGNALLING

Question 1

Function of cell signalling

Answer 1

Ensures that cellular activites occur in the right cells, at the right time and in proper coordination with other cells

Question 2

How is singalling cells able to coordinate over short and long ranges, various taregt cells’ diverse physiological functions

Answer 2

• Short range: by cell junction or cell to cell recognition
• Long range: endocrine signalling, paracrine signalling, autocrine signalling, synaptic signalling, neutroendocrine signalling

Question 3

Define signal reception

Answer 3

detection of an extracellular signal molecule

Question 4

Describe signal reception(ligand-receptor interaction)

Answer 4

• signal binds to specific complementary site on receptor protein located at cell’s surface/insdie target cell
• forms a ligand-receptor complex
• receptor protein undergoes a conformational change
• activates receptor

Question 5

What is a ligand

Answer 5

any molecule that binds to a specific site on another molecule, often a larger one

Question 6

Types of signal receptor proteins and how they function

Answer 6

• receptor channel: ligand binding opens or closes the channel
• Receptor-enzyme: activates an intracellular enzyme
• G protein-coupled receptor:
  1. opens ion channel
  2. alters enzyme activity
• Integrin receptor: alters cytoskeleton

Question 7

Define signal transduction

Answer 7

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

Question 8

What are second messengers and their functions

Answer 8

non-protein molecules that through diffusion rapidly relay signal from cell surface into cell interior

Question 9

Describe signal transduction

Answer 9

• activation of receptor initiates transduction
• which is a multistep signal transduction pathway consisting of a series of relay molecules
• each relay molecule in pathway acts by altering conformation and hence activating or inhibiting protein immediately downstream
• conformational changes usually brought about by phosphorylation, relay proteins in signal transduction pathway are sequentially phosphorylated
• forms a phosphorylation cascade that transmits signal received at cell surface into the cell

Question 10

Function of cell surface receptors

Answer 10

to bind to signal molecules to initiate signal transduction within cells

Question 11

Describe action of protein kinase

Answer 11

enzyme that catalyses the transfer of phosphate group from ATP to a protein

Question 12

Describe action of protein phosphatase

Answer 12

removes phosphate group from protein

Question 13

Describe phosphorylation cascade

Answer 13

• Protein kinase phosphorylates, activating protein kinases, switching on signal transduction pathway
• Protein phosphatase dephosphorylates, deactivating protein kinases, turning off signal transduction pathway
• protein kinases available for reuse
• activity of protein regulated by phosphorylation depends on balance between active protein kinase and active protein phosphatase
• sequential protein phosphorylation, each bringing a conformational change
• changes protein from active/inactive form to inactive/active form

Question 14

Describe the generic cell signalling process

Answer 14

• specific ligand recognises and binds to complementary signal-binding site of receptor protein
• forms ligand-receptor complex
• changes 3D conformation of receptor protein
• activates receptor protein
• initiates signal transduction pathway - multistep pathway
  1. relay molecules operate sequentially
  2. act by altering 3D conformation of protein immediately downstream, activating or deactivating it
  3. alters 3D conformation usually by phosphorylation
  4. relay molecules phosphorylated sequentially
  5. forms phosphorylation cascade

OR
1. stimulates increase in cytosolic concentration of second messengers
2. spread throughout cytosol by diffusion
3. stimulate a variety of cellular activities, enables cells to mount large-scale, coordinates cellular response following stimulation by single extracellular signal molecule

• results in regulation of cellular activity, either in the cytoplasm or the nucleus
• in the cytoplasm:
  1. alteration of enzyme activitiy
  2. rearrangement of cytoskeleton
• in the nucleus: altering gene expression

Question 15

what does G protein stand for

Answer 15

Guasonine triphosphate protein

Question 16

Describe the primary, secondary and tetiary structures of G protein

Answer 16

primary: single polypeptide chain
secondary: fold into 7 α-helices held together by inter-helical segments
tetiary:
* hydrophobic interaction between seven transmembrane α-helics -> barrel shape of GPLR
* hydrogen bonds and highly conserved disulfide bond between non-helical segments stabilise GPLR
* seven α-helices form membrane-embedded domain
* N-terminus + 3 inter-helical segments = extracellular domain, contains the signal-binding site
* C-terminus + 3 inter-helical segements = intracellular domain, contains the G-protein interaction site

Question 17

Link structures of GPLR to its function

Answer 17

Structure: interhelical loops and N and C termini mainly consist of hydrophilic amino acid residues
Function: intracelluar and extracellular domain soluble in aqueous medium, able to interact with water-soluble ligand and G-protein

Structure: seven intermembrane α-helices are mainly formed by hydrophobic amino acids
Function: hydrophobic interactions able to exist between α-helices and with fatty acid tail of phospholipids of membrane bilayer, allowing transmembrane domain of GPLR to be stabilised and embedded within the membrane bilayer

Structure: extracellular domain contains specific amino acids at ligand binding site
Function: allows ligand-binding site to have specific 3D conformation to bind to and interact with specific ligands, resulting in a huge diversity of ligands that GPLR can bind to

Structure: intracellular domain contains specific amino acids at G-protein binding site
Function: allows G-protein binding site to have specific 3D conformation to bind to and activate G-protein

Structure: binding of ligand to ligand-binding site of GPLR leads to conformational change of GPLR
Function: allows GPLR to initiate signal transduction pathway by activating G-protein

Question 18

Process of GPLR signalling

Answer 18

1. signal molecule binds to extracellular signal-binding site of GPLR, causing a change in its 3D conformation, activating it
2. Increases affinity of GPLR for G protein, cytoplasmic side of GPLR binds to an inactive G protein, causing a GTP molecule to displace the GDP molecule bound to the G protein
3. G protein is activated
4. Dissociates from the GPLR and diffuses along the membrane
5. Binds to a target potein, usually an enzyme, and alters its activity
6. The altering of its activity initiates a cascade of signal transduction pathway by triggering the next step in the signal transduction pathway:
   * cytosolic production of cAMP
   * cytosolic production of inositol triphosphate (IP3)and calcium ions, serves as second messengers
7. Last activated molecule in signal transduction pathway triggers to cellular response
8. Intrinsic GTPase activtiy of G protein hydrolyses its bound GTP to GDP, inactivating the G protein. Signal molecule also dissociates from the GPLR
9. G protein dissociates from the target protein, returning target protein to its original activity. G protein is available for reuse

Question 19

Role of cyclic adenosine Monophosphate (cAMP) in signal transduction pathway

Answer 19

activation of serine/threonine kinase called protein kinase A that phosphorylates various other proteins depending on the cell type, allowing moany different cellular activities to be triggered, hence allowing cell to mount a large-scaled, coordinated celluar response to a single, extracellular signal molecule

Question 20

Role of inotisol triphosphate (IP3) and calcium ions as second messengers in GPLR signalling

Answer 20

1. inotisol trisphosphate (IP3) binds to an IP3 gated calcium channel on ER membrane, causing it to open and calcium ions to diffuse out into cytosol, increasing cytoplasmic concentration of calcium ions
2. triggers many cellular responses via activation of calcium sensitve proteins

Question 21

What are G-Protein Linked Receptors(GPLR)

Answer 21

extracellular receptor proteins that work with G proteins

Question 22

How is Ca2+ ions released in cells

Answer 22

• binding of signal molecule to GPLR leads to activation of G protein and consquently activation of phospholipase C
• cleaves part of plasma membrane known as PIP2 to form diacylglycerol(DAG) and inositol triphosphate(IP3)
• DAG functions in second messenger in other pathways
• IP3 quickly diffuses in cytosol and binds to IP3 gated calcium channel, causing it to open
• Ca2+ ions diffuse into out of ER lumen down conc gradient into cytosol
• increase in cytosolic concentration of Ca2+ cause activation of proteins such as calmodulin in one or more signalling pathways, triggering cellular repsonse

Question 23

Structure of receptor tyrosine kinase

Answer 23

1. Extracellular signal binding site
2. α-helix spanning membrane
3. intracellular tail containing tyrosine and tyrosine-kinase domain

Question 24

Process of receptor tyrosine kinase cell signallling

Answer 24

1. signal molecule binds to one subunit of RTK
2. results in subunit aggregation and dimerisation
3. dimerisation leads to activation of tyrosine kinase activity of receptor, resulting in authophosphorylation or cross-phosphorylation
4. each tyrosine kinase domain transfers a phosphate group from ATP to tyorosine on its own tail or on other polypeptide subunit
5. RTK is fully activated
6. RTK is able to bind cytoplasmic relay proteins, altering their activity, localising it or altering its ability to interact with other signalling molecules
7. Each relay molecule recognises and binds to specific phosphorylated tyrosine, becoming activated due to conformational change
8. Relay molecule triggers a transduction pathway, trigerring cascade of signal transduction events
9. when final protein in signal transduction pathway is activated, cellular response is activated
10. multiple transduction pathways can be trigerred by activation of a single RTK as each tyrosine is recognised by a different relay protein, triggering multiple different signal transduction pathways

Question 25

Advantages and significance of cell signalling

Answer 25

1. Signal amplication 2. Regulation of cell signalling 3. Specificity of cell signalling

Question 26

What is signal amplification

Answer 26

enhancing of signal strength as signal is relayed through a transduction pathway

Question 27

What does signal amplification mean for the cell

Answer 27

* at each catalytic step in the transduction pathway, there is a greater amount of activated product than in the preceding step * a small number of signal molecules is needed to ellicit a cellular response * cellular response is large as there is a large amount of activated molecules produced at the end of the signal transduction cascade

Question 28

Why is signal amplification able to occur

Answer 28

1. transduction pathway consists of many steps between signal reception and cellular response 2. persistence of proteins in pathway in active form long enough to process numerous molecules of substrate before they become inactive again

Question 29

Why and how does regulation of cell signalling occur

Answer 29

Purpose: for cell to continually respond to incoming signals, molecular changes must last only a short time -> there must be reversibility in the changes signal produces, leading to signal termination How: - protein phosphatase: desphosphorylation of active protein kinases, inactivating it, impedes transduction pathway downstream of affected proteins - intrinsic GTPase activity: rapid hydrolysis of GTP into GDP, inactivating G protein - phosphodiesterase activity: catalyses conversion of cAMP to AMP, decreases concentration of cAMP in cell

Question 30

Specificity

Answer 30

* involves specific combination of signalling proteins, receptor proteins, relay proteins involved in transduction protein, proteins involved in cellular reponse * two cells that respond differently to same signal differ in one or more proteins that receives, transduces or respond to the signal

Question 31

How is blood glucose concentration regulated

Answer 31

* negative feedback system, regulated by antagonistic hormomes * insulin and glucagon oppose each others actions * net effect (lowering or increasing blood glucose concentration) determined by ratio between insulin and glucagon

Question 32

Signal molecules involved in regulation of blood glucose concentration and their functions

Answer 32

Insulin: * promotes cellular uptake of blood glucose into liver, skeletal muscles * stimulates mechanism to lower high blood glucose concentration Glucagon: * promotes secretion of glucose through hydrolysis of liver glycogen * stimulates mechanism to increase low blood glucose concentration

Question 33

Mechanism in fed state

Answer 33

* blood glucose concentration increases above **set poin**t of 90mg/100ml * detected by islet of Langerhans in pancreas * β-cells of islet of Langerhans stimulated and increase secretion of insluin * Glucagon secretion by α-cells of islet of Langerhans inhibited * adipose, skeletal muscle, liver cells main effector cells * insulin secreted into and travels into blood stream * binds to receptor tyorsine kinase insulin receptor, activating it, causing it to phosphorylate intracellular enzymes *

Question 34

Cellular responses due to insulin in fed state

Answer 34

* upon activation of insulin receptor, activation of signal transduction pathway which triggers migration and fusion of these vesicles containing extra glucose transporters with cell surface membrane in liver and adipose cells * increase number of glucose transporters on cell surface membrane * increase uptake of glucose into cell by facilitated diffusion * increased rate of glycolysis, for production of ATP in insulin-dependent effector cells, utilising more glucose * stimulate glycogenesis in liver and skeletal muscle cells to increase where glucose is used in the synthesis of glycogen, activated insulin protein activates glucokinases that phosphorylates glucose to form glucose-6-phosphate which is used in the synthesis of glycogen * inhibits glycogenolysis which is the breaking down of glycogen into glucose in liver and skeletal muscle cells * increase absorption of amino acids and protein synthesis in liver and skeletal muscle cells, inhibits gluconeogenesis which is the conversion of amino acids into glucose * stimulates lipogenesis in adipose and liver cells which is the formation of triglycerides from excess glucose by increasing adsorption of glucose into adipocytes

Question 35

Outcome of insulin

Answer 35

* decrease in blood glucose concentration back to set point of 90mg/100ml acts as a negative feedback signal to decrease stimulation of β-cells of islet of Langerhans * negative feedback mechanism prevents further release of insulin, no further decrease in blood glucose concentration

Question 36

Mechanism of insulin

Answer 36

1. insulin binds to insulin receptor, activating tyrosine kinase activity of insulin receptor 2. triggering autophosphorylation, activating insulin receptor 3. specific relay proteins bind to specific phosphorylated tyrosine on receptor, becoming activated 4. activated relay proteins activate specific signal transduction pathway 5. downstream activated relay proteins stimulate the migration of cytoplasmic vesicles with glucose transporters(GLUT 4) on its membrane to CSM 6. fuses with CSM, increases number of glucose transporters on CSM, increase uptake of glucose into cell by facilitated diffusion 7. downstream activated relay proteins activate glycogen synthase 8. catalyses conversion of glucose into glycogen, increase rate of glycogenesis

Question 37

Mechanism of glucagon

Answer 37

1. when blood glucose concentration decreases below set point, α-cells of islet of Langerhans stimulated to increase secretion of glucagon 2. glucagon secretion by β-cells of islet of Langerhans is inhibited 3. glucagon is screted into and travels in the bloodstream to target cells, liver cells, adipose cells and skeletal muscle cells 4. Glucagon binds to signal-binding site of specific GPLR, causing it to change conformation and activtiating it 5. Actviated GPLR binds to specific G protein, activating it 6. G protein dissociates from GPLR and binds to adenylyl cyclase, activating it 7. Active adenylyl cyclase catalyses the synthesis of a large amount of intracelluar cAMP 8. cAMP binds to and activates Protein Kinase A 9. Protein kinase A phosphorylates glycogen synthase, inhibiting it, reducing glycogenesis where glucose is converted to glycogen 10. Protein kinase A phophorylates glycorgen phosphorylase kinase, activating it 11. glycogen phorphorylase kinase then phosphorylates glycogen phosphorylase, activating it 12. glycogen phosphorylase stimulates the hydrolysis of glycogen into glucose, increasing rate of glycogenolysis, allowing increases diffusion of glucose into bloodstream, increasing blood glucose concentration back to set point