Metabolic Regulation

• Signal Distances

  • Contact depending
  • Paracrine
  • Synaptic
  • Endocrine
• Contact depending


  Membrane bound signal molecule binds to ligand on surface of signalling cell
• Paracrine


  Allows cells to communicate with each other by releasing signalling molecules that bind to and activate surrounding cells
• Synaptic


  Neuron has long extension of axon in very close contact with target cell (but not touching)
• Endocrine


  Signals (hormones) secreted and enters bloodstream to deliver to far away cells
• General Characteristics of Signalling Pathways

  • Specificity
  • Amplification
  • Modularity
  • Transient
• Specificity


  Signal molecule fits binding site on its complementary receptor
• Amplification


  When enzymes activate enzymes the number of affected molecules increases geometrically in an enzyme cascade
• Modularity


  Proteins with multivalent affinities form diverse signalling complexes from interchangeable parts
• Transient


  Signalling molecules expressed and degraded very quickly
• G Protein-coupled Receptors

  • Closely associated with a guanine nucleotide-binding protein (G protein)
  • When it binds GTP it is active, when it hydrolyses GTP it is inactive
• Adrenergic receptor

  1. Epinephrine binds on extracellular portion of protein

  2. Hormone-receptor complex causes the GDP bound to Gsa to be replaced by GTP

  3. Activated Gsa separates from GsBy, moves to adenylyl cyclase and activates it

  4. Adenylyl cyclase catalyses the formation of cAMP (from ATP)

  5. cAMP activates PKA

  6. Phosphorylation of cellular proteins by PKA causes the cellular response to epinephrine

  7. cAMP is degraded -> reversing the activation of PKA
• Normal RTK Activation

  Requires two receptors that both have a signal bound -> must be pulled together by signal molecule -> receptors able to react with each other
• Dominant-Negative inhibition by Mutant RTK


  One inherited gene for RTK is mutated -> cannot activate
• Insulin Receptor


  1. Upon binding two sides of bridge are pulled together -> causes conformational change to pull intracellular tyrosine-kinases together -> activation

  2. Inactive: activation loop blocks substrate binding site (unphosphorylated)

  3. Active: activation loop moves dramatically -> allowing room for target protein in substrate binding site (triply phosphorylated)
• Reoccurring Features in Regulation

  • Compartmentalisation
  • Allosteric Regulation
  • Specialisation of Organs
  • Covalent Regulation
  • Enzyme Levels
• Compartmentalisation


  Different competing reactions occurring in different compartments keeps them separate & facilitate their control
• Allosteric Regulation


  Enzymes catalysing committed & usually irreversible steps
• Specialisation of Organs


  Metabolism of brain, liver, muscle & adipose
• Allosteric Regulation in Glycolysis: PFK-1

  1. PFK-1 Structure: homo-tetramer, 4 identical subunits (each with active site & regulatory site)

  2. PFK-1 Inhibitors: ATP, citrate

  3. PFK-1 Activators: AMP, ADP, F-2,6-bisP
• Pyruvate Kinase Inhibitors

  Alanine, ATP
• Adipose

  • Fuel Source: Glucose
  • Energy Store: >80% of total available energy
  • Metabolism: active during starvation
• Brain

  • Fuel Source: Glucose, Ketone bodies
  • Energy Store: No stores
  • Metabolism: 60% of total GNG glucose daily
• Liver

  • Fuel Source: Glucose, Fatty Acids, Ketone Bodies, Amino Acids
  • Energy Store: Glycogen (25% of total)
  • Metabolism: active during starvation making glucose, oxidises FAs, synthesises ketone & others
• Skeletal Muscle

  • Fuel Source: Glucose, Fatty acids, Ketone bodies
  • Energy Store: Glycogen (75% of total)
  • Metabolism: Consumes fatty acids when resting (85% energy), heart prefer ketone
• Glucagon Signalling Steps

  1. Glucagon (1° messenger)

  2. Glucagon Receptor

  3. Guanyl Nucleotide (G) Protein

  4. cAMP (2° messenger)

  5. cAMP-dependent protein kinase / Protein Kinase A (PKA)
• Glucagon (1° messenger)


  Secreted into the blood when blood [glucose] drops <4.5 mM
• Glucagon Receptor

  7 transmembrane helices
• cAMP (2° messenger)


  G proteins activate adenylate cyclase -> produces cyclic AMP
• cAMP-dependent protein kinase / Protein Kinase A (PKA)


  cAMP activates PKA allosterically -> activated PKA phosphorylates several protein targets on Ser/Thr residues
• Glucagon Effect in the Liver: Protein Kinase A Activations

  1. Bifunctional PFK-2/FBPase-2 Enzyme -> kinase domain becomes phosphorylated by PKA -> stop producing F-2,6-bisP -> breakdown F-2,6-bisP

  2. Pyruvate kinase -> inhibiting PK activity shuts down conversion of PEP to pyruvate -> slowing feed into TCA cycle -> promoting gluconeogenesis

  3. Glycogen synthase -> inhibiting GS activity -> prevents adding UDP-glucose to glycogen granule

  4. Phosphorylase Kinase -> activates glycogen phosphorylase -> increases breakdown of glycogen -> glucose produced

  5. CREB Protein -> controls gene expression -> transcription of PEPCK gene -> product more PEPCK -> converts OAA to PEP -> increase gluconeogenesis
• Adrenaline Effect

  1. In liver: switches on gluconeogenesis (make glucose) & glycogenolysis (liberate glucose)

  2. In muscle: switches on glycolysis (use glucose) and glycogenolysis (liberate glucose)
• Adrenaline Effect in Liver

  • PFK-1 Inhibited via decrease [F-2,6-bisP]
  • PK: Ser residue phosphorylated
• Adrenaline Effect in Muscle

  • PFK-1 Activated via increase [F-2,6-bisP] (different isoform than liver)
  • PK: No covalent modification -> pyruvate feeds into TCA cycle & oxidative phosphorylation for prolonged muscle usage
  • Neuronal Signalling in Muscle via Ca2+ as a 2° messenger. Ach receptor -> stimulates Ca2+ release -> phosphorylase kinase activated by Ca2+ -> more glycogen phosphorylase -> more glucose from glycogen -> more ATP
• Insulin Synthesis

  preproinsulin -> single polypeptide -> signal sequence cleaved when entering ER -> folds in oxidising environment to form -S-S- bonds -> C peptide cleaved -> leaves A and B chains
• Signalling Process: insulin binds to receptor

  Conformational change -> transautophorsphorylation of two copies of receptor -> recruits IRS-1 which is phosphorylated -> becomes docking site for other signalling molecules -> 2 pathways signalised at once
• Insulin Signalling Effect

  1. Phosphorylation of Grb2 protein -> MAPK pathway -> cascade reaction -> impact gene expression to upregulate protein synthesis

  2. Phosphorylation of PI-3K -> cascading reaction -> phosphorylation of lipid messenger on cell membrane (PIP2) -> PIP3 concentration increase -> activation of glycogen synthase & increased GLUT4 to membrane (muscle & adipose)
• 3 Targets of Pathways Regulated by Insulin

  • GLUT4 to Membrane: muscle & adipose
  • Glycogen Synthesis: liver & muscle
  • Glycogen Breakdown: liver
• GLUT4 to Membrane: muscle & adipose

  PIP3 activates PKB (Protein Kinase B) -> phosphorylates many targets -> leads to movement of GLUT4 stores from internal vesicles -> add to cell membrane
• Glycogen Synthesis: liver & muscle

  Inactivated by phosphorylation -> cannot convert glycogen synthase to active form by phosphorylation so GS remains active