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H2 Biology > Cell Signalling > Flashcards

Cell Signalling Flashcards

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1
Q

state the three stages of cell signalling

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

define signal reception in cell signalling

A

the target cell’s detection of an extracellular signal molecule

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3
Q

what occurs during signal transduction in cell signalling?

A

when a signal molecule binds to a specific receptor protein located at the cell’s surface or inside the target cell (eg. hormones, neurotransmitters)
ligand-receptor interaction is highly specific (ligand being the signal molecule)

ligand binds to specific complementary site on receptor to form ligand-receptor complex, receptor protein undergoes conformational change (for most receptors, change in conformation directly activates the receptor)

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4
Q

define signal transduction in cell signalling

A

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

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5
Q

what occurs during signal transduction in cell signalling?

A

formation of activated ligand-receptor complex changes the conformation of receptor protein, initiating the process of transduction
single step for signaling mediated by intracellular receptors
multistep signal transduction pathway for cell surface membrane receptors, with a series of relay molecules (usually enzymes in specific sequence)
- each protein alters conformation of protein immediately downstream, activating or inhibiting it
- usually by phosphorylation, forming a phosphorylation cascade
- may also involve non-protein molecules (second messengers, through diffusion)

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6
Q

define cellular response in cell signalling

A

signal transduction pathway eventually leads to the regulation of one or more cellular activities (cytoplasmic or nuclear response)

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7
Q

what is the difference between effects of cytoplasmic and nuclear responses in cell signalling?

A

cytoplasmic response causes changes in:
- regulation of enzyme activity
- cytoskeletal arrangement

nuclear response causes changes in gene expression, like:
- turning specific genes in the nucleus on or off (control of Eukaryotic Gene Expression)

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8
Q

what types of molecules bind to cell surface / membrane receptors?

A

hydrophilic / water-soluble / polar molecules cannot diffuse across hydrophobic core of cell membrane, bind to specific complementary sites on cell surface receptor proteins (eg. peptide hormones and neurotransmitters)

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9
Q

what are the 2 main types of cell surface receptors, and how do they transmit extracellular signal information?

A
  1. G-protein linked receptors
  2. Receptor tyrosine kinases

transmit extracellular signal information into the cell via conformational changes or subunit aggregation, activating the receptors that initiate one or more intracellular signal transduction pathways

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10
Q

what enzymes catalyse phosphorylation and dephosphorylation?

A

phosphorylation: protein kinase (PK)
dephosphorylation: protein phosphates (PP)

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11
Q

what is phosphorylation and what happens in the process?

A

phosphorylation transfer of phosphate groups from ATP to a protein
- phosphorylates and activates protein kinases, turning on signal transduction pathway

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12
Q

what is dephosphorylation and what happens in the process?

A

dephosphorylation: removal of phosphate groups from proteins
- dephosphorylates and inactivates protein kinases, turning off signal transduction pathway when initial signal is no longer present, allows protein kinases to be available for reuse

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13
Q

what does the activity of a protein regulated by phosphorylation depend on at any given moment?

A

balance in the cell between active kinase molecules and active phosphatase molecules
phosphorylation / dephosphorylation system acts as molecular switch

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14
Q

what is the purpose of second messengers in cell signalling?

A
  • transmit the message carried by extracellular signal molecule (the first messenger) into target cell’s interior
  • binding of first messenger onto receptors stimulates an increase in the concentration of second messengers
  • small, water-soluble second messengers can readily spread throughout cytosol by diffusion
    enable cells to mount large-scale, coordinated response
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15
Q

what are second messengers (type of molecule and common examples)

A

non-protein signal molecules
- small, non-protein, water-soluble molecules or ions
- cyclic AMP (cAMP) and Ca2+ (calcium ions) + inositol triphosphate (IP3) and diacylglycerol (DAG)

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16
Q

what is the G-protein linked receptor, and its function?

A

most common type of cell surface receptor (transmembrane)
linked to G proteins (Guanosine triphosphate GTP binding protein), on cytoplasmic side of cell membrane
mediates the passge

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17
Q

describe the tertiary structure of G-protein linked receptors

A
  • hydrophobic interactions between seven transmembrane alpha-helices result in barrel-shape conformation
  • hydrogen bonds and disulfide linkage between inter-helical loops stabilise protein
  • N-terminus and 3 non-helical segments form extracellular domain (binding site for signal molecule)
  • 7 alpha-helices form membrane-embedded domain
  • C-terminus and 3 non-helical segments form intracellular domain (binding site for G protein)
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18
Q

describe the primary and secondary structure of G-protein linked receptors

A

primary: one polypeptide chain
secondary: seven alpha-helices span the cell membrane, connected by non-helical segments

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19
Q

how does the structure of the G-protein linked receptor relate to its function (5)?

A
  • hydrophilic aa residues from inter-helical loops and N and C termini: soluble in aqueous medium and can interact with water soluble ligands and G-protein
  • hydrophobic aa residues in seven transmembrane alpha-helices, hydrophobic interactions between alpha-helices and hydrophobic fatty acid talks of bilayer: stabilises and embeds protein
  • extracellular domain has specific aa at signal-binding site: specific 3D conformation, interaction with specific ligand
  • intracellular domain has specific aa at G-protein interaction site: specific 3D conformation, bind and activate G-protein
  • formation of ligand-GPLR complex causes conformational change that allows it to interact with G protein: GPLR can initiate signal transduction pathways via activation of G-protein
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20
Q

what determines if the G-protein (and thus the GPLR) is active or inactive?

A

if GDP is bound to the G-protein, it is inactive
if GTP is bound to the G-protein, it is active

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21
Q

how does cell signalling occur with a G-protein linked receptor?

A

signal reception: the signal molecule (ligand) binds to the extracellular side of the GPLR, causing change in receptor conformation, activating the GPLR

signal transduction:
- increased affinity for G-protein, cytoplasmic side of GPLR binds to inactive G protein
- binding of G protein to GPLR causes GTP to replace GDP bound to G protein, activating the protein
- activated protein dissociates from GPLR, diffuses along membrane
- activated G protein binds to target protein (usually an enzyme), altering protein activity
- initiates cascade of signal transduction events, including production of second messengers

cellular response: cytoplasmic or nuclear cellular response triggered

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22
Q

what occurs after cell signalling is completed by a GPLR (how does the cell signalling pathway deactivate)?

A

intrinsic GTPase activity of G protein hydrolyses its bound GTP (guanosine triphosphate) to GDP (guanosine diphosphate), inactivating G protein
signal molecule also dissociates from GPLR
inactive G protein leaves enzyme, returns to original inactive state, so G protein is available for reuse

23
Q

what is the process of the activation of cyclic adenosine monophosphate, and its effect?

A
  • adenylyl cyclase (enzyme embedded in plasma membrane) converts ATP to CAMP in response to extracellular signal molecule that binds to a GPLR (eg. epinephrine)
  • active G protein activates adenylyl cyclase, catalysing synthesis of many more cAMP molecules, concentration x20 (signal amplification)
  • immediate effect: activation of a serine / threonine kinase called protein kinase A, initiating a phosphorylation cascade
24
Q

why does the number of cAMP molecules not persist for long in the absence of the extracellular signal molecule?

A

phosphodiesterase (enzyme) converts cAMP to AMP

25
Q

what are the four ways low cytosolic Ca2+ is maintained?

A
  1. calcium ATPase in ER membrane r Ca2+ removes from cytosol into ER lumen
  2. calcium ATPase in plasma membrane actively pumps Ca2+ from cytosol into extracellular fluid
  3. sodium calcium exchangers (transmembrane pumps) couples export of Ca2+ with facilitated diffusion of Na+ into cytosol
  4. mitochondrial Ca2+ pumps moves Ca2+ into mitochondria
26
Q

why can Ca2+ function as a second messenger in cell signalling?

A

its concentration in the cytosol is normally much lower than the concentration outside the cell
brief increase to the cytosolic concentration of Ca2+ causes many cellular responses

27
Q

what mechanism causes cytosolic Ca2+ level to rise, and what other second messengers does the same pathway lead to?

A

mechanism that releases Ca2+ from cell’s endoplasmic reticulum
other second messengers: inositol triphosphate (IP3) and diacylglycerol (DAG), produced by cleavage of phospholipid known as PIP2

28
Q

how does the activation of a G protein produce IP3 and DAG?

A

signal molecule binds to GPLR, activating G protein and phospholipase C
phospholipase C (PLC) cleaves PIP2 (plasma membrane phospholipid) into DAG (second messenger in other pathways) and IP3 (diffuses through cytosol)

29
Q

what are receptor tyrosine kinases (RTKs), and its key features?

A
  • major class of cell surface / membrane receptors that possess enzymatic activity
  • tyrosine kinase: part of protein extended into cytoplasm, catalyses transfer of phosphate group from ATP to aa tyrosine on a substrate protein
  • can trigger more than one (ten or more) different signal transduction pathways from a single ligand-binding event, activate several different cellular responses, significant functional diversity
30
Q

before the signal molecule binds, what do receptor tyrosine kinases exist as?

A

two individual polypeptide units, each with:
- extracellular signal-binding site
- alpha-helix spanning membrane
- intracellular tail with multiple tyrosines and a tyrosine kinase domain

31
Q

what is the flow of cell signalling, with signal reception involving a receptor tyrosine kinase?

A
  • signal molecule (ligand) binds to one of the two RTK subunits, causing receptor aggregation (dimersation of both subunits to form one RTK)
  • dimerisation leads to activation of tyrosine kinase activity of receptor
  • autophosphorylation / cross-phosphorylation: where each tyrosine kinase domain adds a phosphate from an ATP to a tyrosine on the tail of its own or the other subunit
  • RTK is fully activated after phosphorylation, binds cytoplasmic relay proteins to each specific phosphorylated tyrosine and alters their activity by causing conformational change, triggering transduction pathway and initiating cascade of signal transduction events
  • triggers cytoplasmic or nuclear response
32
Q

what are the three advantages of cell signalling’s multistep pathways?

A
  1. signal amplification: signal amplified, small number of extracellular signal molecules produce large cellular response
  2. coordination and regulation
  3. specificity of response: same signal molecule causes different responses via specific combination of signalling / relay proteins
33
Q

what is signal amplification in cell signalling?

A

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

34
Q

what are the three key features of signal amplification?

A
  1. number of activated products is much greater than in the preceding step
  2. small number of extracellular signal molecules can elicit cellular response
  3. response of target cell is large
35
Q

what are the two reasons the amplification effect is possible in cell signalling?

A
  1. presence of multiple steps in signal transuduction pathway (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
36
Q

what is the importance of signal termination in cell signalling?

A

for continuous response to incoming signals, each molecular change must last only a short time, so the changes produced by signals must be reversible
signal termination: receptor and each component of signal transduction returned to inactive states

37
Q

what mechanisms result in signal termination for cell signalling?

A
  • protein phosphatase activity: protein phosphates catalyse dephosphorylation and inactivation of protein kinases / other relay proteins, impeding transduction pathway downstream
  • intrinsic GTPase activity of G protein: GTPase rapidly catalyses hydrolysis of bound GTP to GDP, inactivating G protein
  • phosphodiesterase activity: enzymes catalyse conversion of cAMP to AMP, decreasing the concentration of active cAMP
38
Q

what are the properties of endocrine glands in the context of cell signalling?

A

secretes hormones directly into bloodstream (ductless glands)

39
Q

what are the properties of hormones in cell signaling?

A

effective in low concentration
slow effect on target cells
sustained, prolonged and long-lasting effect
binds to receptors on target cells

40
Q

functions of hormones in cell signalling

A

alter cellular operations by changing types, activities, and/or quantities of important enzymes and structural proteins
- stimulates synthesis of enzyme or protein
- increasing or decreasing rate of synthesis of protein
- activating or inactivating existing enzyme by altering specific 3D conformation

41
Q

what different types of hormones make use of what types of receptors?

A

water-soluble = cell surface
lipid-soluble: intracellular

42
Q

what is the set point of blood glucose, and by what type of system is it regulated?

A

90mg / 100ml of blood
negative feedback system
(when set point restored, corrective mechanisms switched off)

43
Q

what two antagonistic hormones are involved in the regulation of blood glucose concentration?

A

insulin and glucagon: oppose each other’s action
* ratio of the amounts of both hormones determines their net effects, not their absolute amounts

44
Q

what are the target cells of glucagon and insulin

A

liver, muscle, adipose

45
Q

how is blood glucose concentration regulated when it is higher than the set point?

A
  • change is detected by the islets of Langerhans of the pancreas
  • beta cells secrete more insulin, alpha cells secrete less glucagon
  • insulin secreted is transported in the bloodstream to the target cells, binds to cell-surface insulin receptors (RTK) on plasma membrane, initiates phosphorylation cascade to cause various effects that decrease blood glucose concentration back to set point of 90mg / 100ml of blood
  • negative feedback mechanisms prevent further insulin release, by decreasing stimulation of beta cells
  • less insulin released, no further decrease in blood glucose concentration
46
Q

what are the effects of insulin on its target cells?

A

accelerate rate of glucose uptake into cells
increase number of glucose transporters in cell membrane

extra glucose transporters normally sequestered in vesicle membranes migrate to fuse with plasma membrane

more glucose utilisation by:
- increasing rate of GLYCOLYSIS (glucose breakdown)
- increasing rate of GLYCOGENESIS (by activating GLUCOKINASE)
- inhibiting GLYCOGENOLYSIS (glycogen breakdown)
stimulates aa absorption and protein synthesis in muscle
inhibits GLYCONEOGENESIS (aa to glucose)
stimulates LIPOGENESIS in adipose (triglyceride formation)

47
Q

how is blood glucose concentration regulated when it is lower than the set point?

A
  • change detected by islets of Langerhans of pancreas
  • alpha cells secrete more glucagon, beta cells secrete less insulin
  • glucagon secreted travels in bloodstream to cell-surface glucagon receptors (GPLR) on target cells
  • adenylate cyclase activated to produce cAMP from ATP
  • cAMP acts as second messenger, initiating various cellular responses
  • these responses increase blood glucose concentration back up to the set point of 90mg / 100ml of blood
  • negative feedback mechanisms prevent further release of glucagon, increase in blood glucose - reduces stimulation of alpha cells
  • less glucagon released, no further increase in blood glucose concentration
48
Q

what are the effects of glucagon on its target cells?

A

stimulates GLYCOGENOLYSIS (breakdown of glycogen to glucose)
inhibits GLYCOGENESIS (synthesis of glycogen)
stimulates GLUCONEOGENESIS (aa from bloodstream synthesised into glucose, released back)
stimulates LIPOLYSIS (triglyceride breakdown, more fatty acids for use instead of glucose as energy source)

49
Q

how does insulin lead to cytoplasmic responses of glucose uptake and storage in cell signalling?

A

insulin binds to receptor tyrosine kinase (insulin receptor), activating receptor’s tyrosine kinases’ activity, resulting in auto and cross phosphorylation (two subunits aggregate and dimerase, becoming activated)
relay protein specific to insulin receptor binds to specific phosphorylated tyrosine on RTK tail, activing relay protein and triggering signal transduction pathway
activated down-stream relay protein stimulates:
- migration of cytoplasmic vesicles with GLUT-4 glucose transporters to the cell membrane
- activation of glycogen synthase (glycogenesis, glucose to glycogen)

50
Q

how does glucose lead to cytoplasmic responses of glucose uptake and storage in cell signalling?

A

binding of glucose to glucose-specific receptor proteins (GPLR) causes GTP to replace GDP, activating the G protein
activated G protein activates enzyme adenylyl cyclase, which catalyses synthesis of large amounts of intracellular cAMP
cAMP binds to and activates protein kinase A
active protein kinase A (OR Ca2+ from Ca2+ / IP3 pathway) phosphorylates glycogen phosphorylase kinase, activating it
active protein kinase A phosphorylates glycogen synthase, INHIBITING catalytic activity, preventing conversion of glucose to glycogen (glycogenesis)
active glycogen phosphorylase kinase phosphorylates glycogen phosphorylase, activating it (more glycogenolysis, glycogen into glucose)
glucose diffuses into bloodstream

51
Q

what enzyme synthesises cyclic AMP?

A

adenylyl cyclase

52
Q

what is typically the immediate effect of cAMP?

A

activation of serine / threonine kinase called protein kinase A

53
Q

How does IP3 increase Ca2+ concentration in the cell?

A

IP3 binds to IP3 gated calcium channel in ER membrane, opening it
Ca2+ diffuse out of ER (down concentration gradient), raising cytosolic Ca2+ level, activating next protein like calmodulin

H2 Biology (24 decks)

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