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• Gluconeogenesis


  The sequence of reactions that converts pyruvate to glucose
• Gluconeogenesis


  • Takes place mostly in liver and kidneys
  • To maintain adequate glucose levels in the blood
  • Uses ATP and GTP
  • Shares many steps with glycolysis but crucially not all
• Glycolysis has three irreversible steps
• Reversible steps


  ΔG close to 0 so changes in q controls direction of reaction
• Irreversible steps


  ΔG << 0 so changes in q not big enough to reverse reaction
• Steps with large negative ΔG are irreversible under physiological conditions and hence often used as regulatory steps
• Reversible steps shared in two opposing pathways e.g. glycolysis and gluconeogenesis are not good candidates for regulation
• Reversing the irreversible


  1. Fructose-1,6-bisP and glucose-6-P are hydrolysed

  2. Glucose + ATP → glucose-6-phosphate and ADP

  3. Glucose-6-P + H2O → Glucose + Pi
• Futile cycles


  1. Fructose-6-P + ATP → Fructose-1,6-bisP + ADP

  2. Fructose-1,6-bisP + H2O → Fructose-6-P + Pi

  3. Net: ATP + H2O → ADP + Pi
• Metabolic pathways tightly regulated to avoid futile cycles
• PFK and Fructose-1,6-bisphosphatase both tightly and oppositely regulated
• Pyruvate to PEP


  1. Pyruvate → Oxaloacetate

  2. Oxaloacetate + ATP → PEP + ADP + Pi

  3. Pyruvate carboxylase

  4. PEP carboxykinase
• Both pyruvate carboxylase and PEP carboxykinase are regulated, e.g. inactivated by ADP
• Starting materials for gluconeogenesis


  • Lactate
  • Some amino acids
  • Pyruvate
  • Some amino acids
  • Oxaloacetate
  • Glycerol
  • Dihydroxyacetone
  • Triacylglycerides (fat release one molecule of glycerol)
• Gluconeogenesis


  • Occurs in the cytosol
  • Linear pathway
  • Entry points: Pyruvate, oxaloacetate, glyceraldehyde-3P
  • End products: Glucose
  • Linked to pentose phosphate pathway and others
  • 11 steps, most shared with glycolysis
  • Oppositely regulated to glycolysis
• Pentose phosphate pathway


  The sequence of reactions that converts glucose to pentoses with the production of NADPH
• Pentose phosphate pathway


  • Pentoses: Ribose-5-P needed for DNA and RNA (and ATP, NAD+, FAD, CoA) synthesis
  • NADPH: Reductive biosynthesis e.g. fatty acids (membranes, adipose tissue), reduces oxidative stress
  • Dual purpose: NADPH and ribose-5-P production
• PPP: Key Steps


  1. Oxidative stage: G-6-P to ribulose-5-P giving 2 NADPH

  2. Non-oxidative stage: Ribulose-5-P to ribose-5-P and glycolytic intermediates
• PPP: oxidative steps


  1. Glucose 6-P → Ribulose 5-P + CO2

  2. Step 1: Glucose 6-P Dehydrogenase oxidises to an intramolecular ester, forms NADPH

  3. Step 2: Gluconolactonase hydrolyses intramolecular ester

  4. Step 3: 6-P gluconate DH oxidatively decarboxylates, forms NADPH, releases CO2
• Control of the PPP - availability of substrate, Glucose 6-P DH controls glucose-6-P entry into the PPP, The level of NADP+ controls glucose-6-P-DH activity
• PPP: Non-oxidative stage


  1. Ribulose 5-P → Ribose 5-P

  2. Strategy: ketose - aldose isomerisation as for phosphoglucose isomerase
• Non-dividing cells may have high NADPH but low ribose requirement, e.g. adipose tissue, red blood cells
• Carbohydrate interconversions


  1. 3 Ribulose-5-P → 2 F-6-P + 1 G-3-P → (2.5) G-6-P

  2. Transketolase (C2 transfer)

  3. Transaldolase (C3 transfer)

  4. Transketolase (C2 transfer)
• The PPP as a cyclic pathway


  1. Fructose 6-P + glyceraldehyde 3-P used to make glucose 6-P

  2. Recycle to glucose by reversing some reactions of glycolysis

  3. Glucose 6-P + 12 NADP+ + 7H2O → 6 CO2 + 12 NADPH + 12 H+ + Pi
• The PPP has several modes


  • Mode 1: G6P is completely converted to ribose 5P, no NADPH production
  • Mode 2: G6P is completely oxidised to CO2, 12 NADPH per glucose
  • Mode 3: NADPH is needed but not ribose-5-P
  • Mode 4: Cells need NADPH and ATP
• Favism


  Recessive X-linked disorder, hemolytic anemia when eating fava beans and sometimes other legumes, same response also seen to certain drugs, Glucose-6-P dehydrogenase deficiency, i.e. the PPP pathway is compromised
• Favism compromises NADPH but not Ribose-5-P production via mode 1
• NADPH is needed to help with oxidative stress as well as for reductive biosynthesis (e.g. fatty acids), Red blood cells exposed to oxidative stress are more prone to hemolysis
• Favism deficiency is benign when no oxidative stress
• PPP - overview


  • Oxidises glucose to ribulose + CO2
  • Oxidation allows reduction of 2 NADP+ → 2 NADPH
  • Produces ribose for nucleic acids
  • Branched pathway: forms a cycle with some steps of the glycolytic pathway
  • Located in cytosol
  • Intermediates are phosphorylated
  • 3 steps in oxidative stage, each catalyzed by a different enzyme
  • 5 steps in the link to glycolysis, 2 key enzymes
• Overview


  • Dietary fructose and galactose enter glycolysis
  • Gluconeogenesis share most steps with glycolysis
  • Three key steps have different pathway
  • Glycolysis and gluconeogenesis are oppositely regulated
  • The PPP pathway generates NADPH and/or ribose-5-P in response to cellular needs
  • Glucose-6-P dehydrogenase is key regulatory enzyme
  • PPP pathway regulated through availability of substrates