FOB 5 signal transduction

• what is signal transduction?

  the process of receiving and acting on extracellular signalling to coordinate tasks such as growth metabolism and cell-specific functional outputs. property of all living cells
• how are signals detected

  signals are detected by receptors found outside the cell (hydrophilic, cannot cross membrane) or within the plasma membrane, cytoplasm, nuclear envelope or within the nucleus (hydrophobic, can cross membrane)
• different signals and responses

  many different signals and responses but there are few ways to transduce these signals
• 4 properties of signal transduction systems

  specificity 1- signal molecules fit into binding site specifically, other molecules do not fit. amplification 2- when enzymes activate enzymes the number of affected molecules increases geometrically in an enzyme cascade. desensitization/adaptation 3- receptor activation triggers a feedback circuit that shuts off the receptor or removes it from the cell surface. integration 4- when two signals have opposite effects on a metabolic characteristic the regulatory outcome results from the integrated input of both receptors
• g-protein-coupled receptor


  will bind to ligand and then activate and enzyme which intiates transduction. the receptor can then activate different enzymes in the cell. adenylyl cyclase and phospholipase c.
• receptor enzyme


  tyrosine kinase (an enzyme that adds a phosphate), when ligand binds it activates receptor which induces a signal cascade (acts as an enzyme). it adds a phosphate group to tyrosine amino acids, phosphorylates specific proteins which activates the protein. phosphate changes charge which changes the structure of protein.
• gated ion channel

  channel opnes or closes in response to ligand allowing ions to enter or exit cell
• nuclear receptors


  ligand enter the cell and binds to nuclear envelope or receptor, or enter nucleus a bind to receptor, causing activation or inactivation of gene expression
• Phosphatases

  • Dephosphorylate
  • Remove a phosphate group from specific residues in a protein
• Addition of phosphate group

  1. Conformational change in structure

  2. Activation/inactivation of protein
• Protein phosphorylation

  Reversible phosphorylation of proteins is an important regulatory mechanism
• ADP

  Adenosine diphosphate
• ATP

  Nitrogenous base with 3 phosphates connected by a ribose sugar
• GDP

  Guanosine diphosphate
• Kinase

  Takes a phosphate from ATP or GTP forming ADP or GDP and changes the hydroxyl group of the amino acid for a phosphate group
• GTP

  Guanosine with 3 phosphates connected by a ribose sugar
• Amino acids commonly phosphorylated

  • Serine
  • Threonine
  • Tyrosine
  • Histidine
• Kinases

  • Phosphorylating enzymes
  • Add a phosphate group from specific residues in a protein
• cAMP


  cyclic adenosine monophosphate, the phosphate fuses with the ribose ring backbone to form a cyclic structure
• phosphorylation cascade


  allows for signal amplification. ligand binds to receptor activating the first kinase enzyme which activates another kinase through the process of phosphorylation. amplifies because each phosphorylated kinase can begin another cascade and branch off into another pathway causing different outcomes
• Conformational change - 2 g protein moa

  Causes conformational change in the g-protein (which has lipid tails anchoring it in the cell membrane) associated with the receptor and the alpha subunits swaps GDP for GTP which causes a further conformational change causing the a subunit to dissociate from the beta and gamma subunits; this is the activate form
• 1. protein coupled receptor MOA

  Ligand binds to external surface of 7 transmembrane receptor causing conformational change on external and internal (c terminus) surface of the receptor
• Activation of g-protein coupled receptor- MOA 3

  Activates a specific enzyme (adenylyl cyclase) which induces a signalling cascade, and converts ATP to cAMP this is the second messenger which does a variety of things to lead to phosphorylation of proteins and induce a cellular response (activates PKA protein kinase A which phosphorylates proteins)
• different types of G-protein receptors

  greatest variation in ligands and the enzyme that g protein activates
• how does cAMP activate protein kinase a

  contains 2 catalytic subunits held together by regulatory subunits attached to AKAP protein. regulatory subunits sit in the substrate binding cleft of the catalytic subunit. cAMP binds to regulatory subunits allowing catalytic subunits to dissociate and the cleft is available.
• inactivation of signal transduction (ephinephrine) 4

  concentration of epinephrine in external environment falls below Ka of that receptor , hormone no longer binds, inactivating it, signal transduction ceases. self-inactivation of a-subunit. it intrinsically converts GTP to GDP, changes the subunit back to inactive form. convert cAMP through hydrolysis to 5'-AMP which reduces cAMP levels causing kinases to revert to inactive form. phosphatases reverse the phosphorylation of signalling proteins inactivating them.
• desensitisation of signal transduction (epinephrine)

  beta and gamma subunit from g protein left behind attract BARK b-adrenergic receptor kinase, which phosphorylates the c terminus of receptor which attract beta-arrestin (Barr) to bind which causes endocytosis of the receptor (removed from the cell membrane and moves into the cytoplasm) this reduces the number of receptors and therefore strength of signal that can be produced. the receptor is eventually returned to the membrane usually when ligand concentration in extracellular space decreases
• ligand binding differing affects on cells (adrenaline example)


  is the receptor present? the subtype of the receptor present (a or b adrenergic receptor)? the type of g protein present at the receptor (excitatory or inhibitory)? the set of target proteins present (specific to each cell)? compartmentalization of the receptor and its target ligands (effectors, are they in the compartment of cytoplasm with pka in order to be phosphorlyated)?
• Glycerophospholipid signalling PIP2


  second class of G-protein coupled receptors: activation of phospholipase C (PLC) by G protein cleaves specific phospholipids. commonly: phosphatidylinositol 4,5 - diphosphate (PIP2). PIp2 is broken into inositol triphosphate (IP3) and diacylglycerol (DAG). IP3 (water soluble) diffuses and binds to the endoplasmic reticulum causing the opening of Ca2+ channels (increasing cytosolic Ca 2+ levels) inducing numerous outputs including activation of protein kinase C (PKC). DAG also activates PKC which phosphorylates proteins
• receptor tyrosine kinase - general summary

  integral membrane proteins with three domains. extracellular ligand-binding, trans-membrane and intracellular domain (contains enzymatic activity - a tyrosine-specific protein kinase). when ligand binds the kinase becomes active leading to the phosphorylation of tyrosine residues within target proteins, changing their activity state. for example an insulin receptor
• insulin


  hormone synthesised/released from the beta cells of the pancreas in response to high glucose levels. insulin causes: modulation of genes involved in cell growth and division, stimulation of glycogen synthesis, uptake of glucose by liver, muscle, and adipose cells. in a cell, glucose if first metabolised for energy then further glucose is stored as glycogen (storage is limited as glycogen is water soluble) then energy is stored as fat
• insulin receptor


  tyrosine chianse receptor composed of 2 alpha subunits that protude outside the membrane and bind to ligand. 2 beta subunits that span the plasma membrane and protrude into the cytoplasm and contain tyrosine kinase activity
• insulin signal transduction 


  ligand (insulin) binding. conformational change in a then b subunits. b then autophosphorylates causing a final conformational change so tyrosine kinase can now bind target proteins and phosphorylate amino acids. IRS1 (insulin receptor substrate 1) is phosphorylated and activates. pathways differ at this point depending on which response is needed
• insulin signal transduction growth

  IRS 1 then initiates a protein complex in MAPK process. This complex induces a phosphorylation cascade which ends with a signalling kinase entering the nucleus and phosphorylates specific transcription factors and activates them which activates specific genes for cell growth
• insulin signalling process - glycogen synthesis

  IRS 1 activates PI3K whcih converts PIP2 to PIP3 which binds to protein kinase B PKB. PKB is then phosphorylated by PDK1 which then phosphorylates glycogen synthase kinase 3 GSK3. GSK3 normally inhibits Glycogen synthase, so glycogen can now be synthesised
• insulin signalling process - glucose uptake 


  IRS 1 activates PI3K which converts PIP2 to PIP3 which binds to protein kinase B PKB. PDK1 then phosphorylates PKB and stimulates the movement of GLUT4 transporter (glucose transporter) from internal membrane vesicles to insert in the plasma membrane allowing for the uptake of glucose in the environment.
• G-protein coupled receptor pathway vs a receptor tyrosine kinase pathway similar

  both receptors bind a ligand on the external surface of the cell membrane. both undergo conformational change when binded. both receptors phosphorylated and gain at least one phosphate group. both receptors have internal and external components. induce signal cascade
• G-protein coupled receptor pathway vs a receptor tyrosine kinase pathway different

  G receptor crosses in and out the cell 7 times, where K has an external and internal subunit. G protein does not itself have enzymatic properties while K does. part of the alpha subunit of G protein splits off once a ligand has bonded to activate and enzyme, K does not.
• gated ion channels


  open and close in response to voltage or the binding of a ligand. e.g: voltage-gated channels: Na+, K+, Cl- and Ca2+ (respond to changes in electrochemical gradient) . ligand gated channels: acetylcholine receptor AChR. the channel opens when ACh (neurotransmitter released by nuerons)
• voltage-gated ion channels

  conventional model: voltage sensors move in and out membrane like a peg in the hole. paddle model: paddles composed of a-helical hairpin, pivot against the membrane like levers. transporter-like model: voltage sensors do not translocate across the membrane but simply pivot along their longitudinal axis. voltage sensors are positively charged and move into the negative inside of the cell when closed, but when outside of the cell is more negative than inside the gates open. open positive out. closed positive in.