Cell signalling

• Stage 1: reception

  • specific ligand binds to extracellular binding site of a specific receptor
  • ligand complementary to shape and charge of ligand binding site
  • ligand binding causes a change in conformation in the intracellular domain
• stage 2: signal transduction

  • conformational change in intracellular domain activates receptor protein
  • phosphorylation cascade
  • signal amplification
• stage 3: cellular response

  • possible responses: regulation of gene expression, regulation of metabolic pathways, changes in cytoskeleton
  • diff cell types can have diff responses to the same signal
  • same cells can have the same response to different signals
• termination of cellular response:

  • termination at reception: ligand degraded by enzymes, removal of bound ligand after cellular response triggered, endocytosis of ligand-receptor complex to prevent signal transduction
  • termination during pathway: increase activity of phosphatases to dephosphorylate proteins, production of inhibitors
• advantages of pathway:

  • signal amplification, small number of signal molecules for large cellular response
  • multiple responses to 1 signal molecule
  • multiple checkpoints for regulation
  • ensures specificity
  • activate genes in nucleus without need for ligand to move into nucleus
• Second messengers: small, non-protein, water-soluble molecules or ions
• G-protein system:

  • g-protein coupled receptor
  • g-proteins
• g-protein coupled receptor (GPCR):

  • 7 alpha helices spanning the membrane
  • extracellular ligand binding site
  • intracellular site that associates with g-protein
• g-proteins:

  • cytoplasmic side
  • made of 3 subunits
  • intrinsic GTPase activity, hydrolyses GTP to GDP
• G-protein system pathway:

  1. Ligand binding, conformational change in intracellular domain
  2. inactive g-protein carries GDP, bound to receptor, GTP displaces GDP, activating the G-protein
  3. activated g-protein dissociates from receptor, moves along cell membrane
  4. g-protein binds to and activates adenyl cyclase
  5. adenyl cyclase catalyses formation of cAMP from ATP, cAMP amplifies signal
• g-protein signalling pathway for glucagon:

  1. cAMP binds to protein kinase A, phosphorylation cascade
  2. glycogen phosphorylase activated, causes breakdown of glycogen to glucose-1-phosphate
• G-protein return to inactive:

  • g-protein catalyses hydrolysis of bound GTP to GDP, inactivating it
  • G-protein dissociates from enzyme (adenyl cyclase), available for reuse
• Receptor tyrosine kinase:

  • single a-helix in transmembrane domain
  • intracellular domain with several tyrosine residues
  • exists as 2 separate subunits or linked dimer
• RTK reception and initiation:

  1. binding of signal molecule
  2. dimerisation
  3. activation of intrinsic tyrosine kinase activity of both subunits
  4. each tyrosine kinase catalyses attachment of phosphate to a specific tyrosine residue of the other subunit (cross phosphorylation), becomes linked dimer
• Cellular responses for glucagon:

  • glycogen phosphorylase activated, causes breakdown of glycogen to glucose-1-phosphate, gives glucose, increase rate of glycogenolysis
  • increase rate of gluconeogenesis (glucose synthesis) by breaking down fats and proteins in liver cells
  • increase rate of lipid breakdown into fatty acids
• Reception and initiation of insulin receptor:

  • exists as linked dimer
  • binding of insulin molecule, causes each tyrosine kinase in the 2 subunits to catalyse attachment of phosphase group to a specific tyrosine residue on the other subunit
  • dimer recognised by specific relay proteins in cell
  • each relay protein binds to specific phosphorylated tyrosine, conformational change, bound relay protein activated
  • each activated dimmer can activate 10 or more diff relay proteins simultaneously (can trigger more than 1 pathway at once)
• cellular response (insulin):

  • stimulation of glucose uptake in muscles and adipose tissues via translocation of vesicles containing GLUT4 transport proteins to the plasma membranes, glucose can pump/diffuse into cells
  • increase rate of glycogenesis (glucose to glycogen) in liver cells and muscle cells by activating glycogen synthase
  • increase use of excess glucose as substrate for cellular respiration, increase glucose oxidation
  • increase rate of conversion of excess glucose to fatty acids, fatty acids used to synthesise triglycerides that are transported to adipose cells for storage
• insulin is secreted from beta cells in the islets of Langerhans of the pancreas
• glucagon is secreted from alpha cells in islets of Langerhans of the pancreas
• increase in blood glucose concentration above 90mg/100ml, insulin secretion stimulated, glucagon secretion inhibited
• decrease in blood glucose concentration below set point of 90mg/100ml, glucagon secretion stimulated, insulin secretion inhibited
• Vesicles with glucose transporters (GLUT4) embedded in vesicle membrane move towards and fuse with cell surface membrane
• protein phosphorylation: addition of phosphate groups by kinase enzymes, activation
• protein dephosphorylation: removal of phosphate groups from proteins by hydrolysis, action by phosphatase enzymes, deactivation
• examples of signal amplification:

  • in G-protein pathway: adenyl cyclase convert ATP to cAMP, each adenyl cyclase produces many molecules of cAMP
  • in G-protein pathway: activation of protein kinase A that can phosphorylate and activate many other enzymes
  • in RTK: kinase enzymes phosphorylates and activates many downstream relay proteins