1. Role of gluconeogenesis
    • synthesis of glucose from non-carbohydrate sources (requires E from metabolism of fats and source of carbons)
    • essential for maintaining blood glucose concentrations
    • meets the body's demands for glucose when carbohydrate stores are limited (fasting and starvation)
    • occurs primarily in the liver (90%) and kidney (10%)
    • liver and kidney have G-6-phosphatase activity: allows release of glucose into blood stream
  2. Substrates (or precursors) for GNG
    • Lactate produced by anaerobic glycolysis (in RBC's and exercising skeletal muscle)
    • Glucogenic aminoacids (from muscle)
    • glycerol
    • TCA intermediates
  3. Lactate as a substrate for GNG
    • Lactate from exercising muscle diffuses into the blood stream
    • in the liver lactate is converted to pyruvate by lactate dehydrogenase
    • produces NADH in the cytoplasm
  4. Glycerol as a substrate for GNG
    Glycerol from breakdown of Triglycerides
  5. Amino acids as precursors for GNG
    • amino acids can undergo transamination rxns
    • amino group transferred to a-ketoglutarate
    • end product is pyruvate or TCA intermediates
    • (pyruvate, oxaloacetate, fumarate, succinyl CoA, a-ketoglutarate, acetoacetate and acetyl CoA)
  6. can GNG from fats?
  7. GNG vs Glycolysis
    • Gluconeogenesis is NOT the reversal of glycolysis
    • GNG:
    • 2 pyruvate + 4 ATP + 2 GTP + 2 NADG + 2 H+ + 4 H2O --> Glucose + 4 ADP + 2 GDP + 2 NAD+ + 6 Pi

    • Glycolysis:
    • Glucose + 2 ADP + 2 Pi + 2 NAD+ --> 2 pyruvate + 2 ATP + 2 H+ + 2 NADH + 2 H2O
  8. 3 irreversible steps of glycolysis
    • Glucose + ATP ----hexokinase---> glucose 6-phosphate + ADP
    • Fructose 6-phosphate + ATP ---PFK---> fructose 1,6-bisphosphate + ADP
    • Phosphoenolpyruvate + ADP --Pyruvate kinase--> pyruvate + ATP

    **these 3 reactions have a large -delta G (free energy change) and must be bypassed**
  9. Bypass reactions (compare to glycolysis)
    • 1) pyruvate carboxylase & PEP carboxykinase bypass pyruvate kinase step
    • 2) Fructose 1,6 biphosphatase bypasses phosphofructokinase step
    • 3) glucose 6 phosphatase bypasses hexokinase step

    • these provide for a spontaneous pathway in the direction of glucose synthesis
    • -delta G is in the direction of sugar synthesis
  10. 1st bypass (GNG)
    • pyruvate is first converted to oxaloacetate
    • carboxylation rxn requiring energy
    • CO2 is added by pyruvate carboxylase (a mitochondrial enzyme)
    • *most enzymes for GNG are cytoplasmic* this is an exception
    • pyruvate + ATP + CO2 + H2O --> oxaloacetate + ADP + Pi + 2H+
    • Oxaloacetate is shuttled into the cytosol and converted to phosphoenol pyruvate (PEP)
  11. Oxaloacetate converted to PEP
    • OAA is synthesized in the mitochondria
    • OAA cannot diffue out of the mitochondria
    • converted to malate and shuttled into the cytoplasm then converted back to OAA
    • uses a specific malate transport system
  12. 2nd bypass (GNG)
    Fructose 1,6 biphosphatase bypasses phosphofructokinase step
  13. 3rd bypass (GNG)
    • Glucose 6 phosphatase bypasses hexokinase stem
    • G-6-Pase is pimarily an enzyme of liver (and kidneys)
    • in hepatocytes the glucose-6-phosphatase reactions allows the liver to supply the blood with free glucose
    • muscle cells lack G-6-Pase and direct G-6-P to glycogen sunthesis
    • G-6-Pase is located on the membrane of the ER
    • hydrolysis of G-6-P released glucose into the lumen of the ER
    • Glucose is packaged into vesicles for transport
  14. Reciprocal regulation of GNG and glycolysis
    • GNG expends 6 P bonds of ATP and GTP
    • Glycolysis yields 2 P bonds of ATP
    • if 2 pathways runs concurrently becomes a futile cycle: must be regulated!!
    • when GNG is on, glycolysis should be off
    • when energy stores are high, glycolysis should be off
    • when energy stores are low, glucose should be rapidly degraded to provide energy
    • Regulation occurs at the sites of the irreversible reactions!!!
  15. Irreversible reactions provide regulation
    • GNG (Fructose 1,6 bisphosphatase) is inhibited by AMP
    • stimulated by citrate (signaling the cell to store energy as glucose)
    • Glycolysis (phosphofructose kinase) is inhbited by ATP, citrate and stimulated by AMP (indicates low energy)
    • Fructose 2,6-bisphosphate reciprocally controls these 2 enzymes
    • levels are controlled by glucagon and insulin
    • levels are low during startvation-stimulates GNA
    • levels are high during the fed state accelerates glyc.
  16. Regulation of GNG
    Glucose enters liver after meal--> insulin mediates dephosphorylation of PFK-2--> GNG is inhibited
  17. Irreversible reactions provide regulation (PEP to Pyruvate)
    • PEP to pyruvate: pyruvate kinase (glycolysis)
    • enzyme inhibited by acetyl-CoA, ATP and alanine
    • signals that energy levels are high
    • also controlled by phosphorylation by glucagon and insulin (pyruvate kinase is inhibited during starvation)
    • Pyruvate to oxaloacetate: pyruvate carboxylase (GNG)
    • stimulated by acetyl CoA and inhibited by ADP (more GNG when energy levels are high)
    • PEP carboxykinase is similarly inhibited by ADP levels
Card Set
Metabolism- Gluconeogenesis