12 - Gluconeogenesis

• What is the activity of Pyruvate Kinase increased by?

  Allosteric binding of:

  - phosphoenolpyruvate (substrate)

  - Fructose-1,6-bisphosphate (earlier substrate)
• What is this type of control?

  Positive feed forward control
• What substances inhibit Pyruvate kinase?

  ATP and the alternate fuel sources citrate, alanine and long-chain fatty acids
• What are some of the isozymic forms of pyruvate kinase?

  - L type in the liver and M type in the muscle
• How does allosteric regulation effect the isoenzymes?

  It effects both of them the same
• What alternate form of regulation is the L-type subject to?

  Phosphorylation makes the enzyme less active > this is to reduce consumption of glucose by the liver if concentration is low
• Why is Phosphofructokinase the main control point in glycolysis?

  It is the first committed step (Step 1 produces Glucose-6-Phosphate, which can also be used in glycogenesis or the Penrose-phosphate pathway to produce NADH
• Why is glycolysis regulated at more then one point?

  Because glycolysis produces intermediates which are precursors to other compounds, such as lipids and amino acids. These can be syphoned into other synthetic pathways
• Where is the glucose store of the body?

  Liver glycogen, as glucose stored in muscles cannot be released as the muscle lacks glucose-6-phosphatase
• How much glycogen does the liver contain?

  50-120g
• How long is this store sufficient for without food?

  A day or so
• How do we generate glucose in survival periods of starvation?

  Gluconeogenesis
• What are the requirements for gluconeogenesis within cells?

  - The avalibility of precursors (pyruvate, lactate, amino acids and glycerol)

  - Spare energy (ATP and GTP)

  - Enzymes needed to catalyse the reactions in the liver and kidney
• How does gluconeogenesis compare to glycolysis?

  While not identical, it is analogous, as Pyruvate is converted to glucose, and the regulator steps are the same
• How does gluconeogenesis get around irreversible steps?

  By additional reactions
• What are the three steps in gluconeogenesis?

  1. Pyruvate > Phosphoenolpyruvate

  2. Fructose-1,6-bisphosphate > Fructose-6-phosphate

  3. Glucose-6-Phosphate> Glucose
• How is step 10 reversed in the mitochondria?

  1. Pyruvate > Oxaloacetate (pyruvate carboxylase)

  2. Oxaloacetate > maleate (mitochondrial malate dehydrogenase)

  Leaves mitochondria

  3. Malate > Oxaloacetate (cytoplasmic malate dehydrogenase)

  4. Oxaloacetate > Phosphoenolpyruvate (Phoshoenolpyruvate carboxykinase)
• What is consumed in this process?

  ATP, Acetyl CoA, NADH, NAD+ and GTP
• How is Phosphoenolpyruvate reversed to Fructose-1,6-bisphosphate?

  Straightforward reversal of steps 9 to 4, but requires energy input of ATP at step 7 and NADH at step 6
• How are steps 3 and 1 reversed to converted fructose-1,6-bisphosphate to glucose?

  - Step 3 is reversed by Fructose-1,6-bisphosphatase

  - Step 2 is reversed as normal by G6P isomerase

  - In the liver and kidney only, G6P is converted to glucose by Glucose-6-phosphatase
• What does gluconeogenesis require overall?

  6 High energy phosphate bonds to produce 1 glucose molecule, as it is an energetically unfavourable process
• How are Gluconeogenesis and Glycolysis regulated?

  Reciprocally, if one pathway is active, the other is relatively inactive
• What is the key point of control in gluconeogenesis?

  Fructose-1,6-bisphosphate > Fructose-6-phosphate by Fructose-1,6-biphosphatase
• What increases the activity of Fructose-1,6-bisphosphatase?

  Citrate
• What decreases the activty of Fructose-1,6-bisphosphate?

  AMP and F-2,6-BP
• What is another point of control in glucogenogenesis?

  Pyruvate to Phosphoenolpyruvate by Pyruvate Carboxylase and PEP Carboxykinase
• What activates the enzymes?

  Acetyl-CoA and ATP (high energy state)
• What deactivates the enzymes?

  AMP (Low energy state)
• What is the effect of Fructose-2,6-bisphosphate?

  Stimulates glycolysis and inhibits gluconeogenesis
• What is Fructose bisphosphatase 2?

  Same as PFK2 (bifunctional enzyme) but converts Fructose-2,6-bisphosphate to fructose-6-phosphate
• What is the function of PFK2/FBP2 controlled by?

  Phosphorylation
• What is the enzyme when hyperphosphorylated?

  Fructose Bisphosphatase 2 > promotes gluconeogenesis
• What is the enzyme when hypophosphorylated?

  Phosphofructokinase 2 > Promotes glycolysis
• What is the effect of Glucogon on the bifuctional enzyme?

  - Activates Cyclic AMP dependant protein kinase

  - Bifuctional enzyme phosphorylated > FBP-2 activity

  - Gluconeogenesis, blood glucose rises
• What is the effect of insulin on the bifuctional enzyme?

  - Protein phosphatase 2A activated

  - Bifunctional enzyme hypophosphorylated, PFK2 activity

  - Glycolysis, blood glucose decreases
• Describe the cori cycle

  The liver and muscle cooperate metabolically, e.g, while the liver undergoes gluconeogenesis, the muslce undergoes glycolysis, and they supply each other with the molecules needed to synthesis this