Glycolysis and Gluconeogenesis Dr M. D. Lloyd 5W 2.13;

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  • Glycolysis and GluconeogenesisDr M. D. Lloyd5W 2.13;

  • Steps in Glycolysis

  • Glycolysis of glucose is a central metabolic pathway and takes place in the cytosol;

    Energy (as 2 x ATP) has to be put in at the beginning;

    Most intermediates are phosphorylated (helps compartmentalisation)

    The products are 2 x pyruvate, 2 x NADH and 4 x ATP (energy);

    Net energy gain is 2 x NADH and 2 x ATP;

    Pyruvate is converted to lactate (anaerobic respiration) or completely oxidised to CO2 and H2O (Lectures 28 & 29) (aerobic respiration);

  • Aldolase splits a C6 phosphorylated sugar into two C3 phosphorylated sugars;

    DHAP and G-3-P can be intercoverted by Triose Phosphate Isomerase;

    Glyceraldehyde-3-phosphate (G-3-P) is further processed by glycolysisStructure of Triose Phosphate Isomerase

  • A Summary of Glycolysis

  • Anaerobic Metabolism of PyruvateIn the absence of O2, pyruvate is converted to lactate in humans. e.g. in muscle tissue;

    In other organisms (e.g. yeast) pyruvate is converted into ethanol or other products.

  • Reactions and Enzymes involved in glycolysis

  • Hexakinase is an induced fit enzyme.

    Binding of the substrate brings about a gross conformational change in the protein;

    Hexakinase traps glucose in the cell as glucose-6-phosphate (G-6-P);

    G-6-P is a key starting material for several pathways (therefore hexakinase is a secondary control point in glycolysis);

  • Most enzymes perform normal chemical reactions;

    Example from glycolysis: hexakinase. This reaction involves transfer of a phosphate group from a donor (ATP) to an acceptor (Glucose);

    Energy is put into the system (activation);

    Charged intermediates are produced (allows compartmentalisation);

  • Gluconeogenesis

  • Glucose is required for brain tissue and erythrocytes. Most humans require around 160 g of glucose per day;

    Can be synthesised from pyruvate, oxaloacetate or glycerol in the liver (also kidneys). These are derived from amino acids and fats;

    Gluconeogenesis is not a direct reversal of glycolysis. This is because the hexakinase, phosphofructokinase and pyruvate kinase reactions are effectively irreversible;

    Energy & reducing power needs to be put into the system.

  • Pyruvate carboxylase reactionMetabolic blocks are overcome by carboxylation of pyruvate followed by decarboxylation to phosphoenol-pyruvate;

    Carboxylation requires ATP (to synthesise carbamoyl phosphate) and thiamine (to capture CO2);

    Reaction is mitochondrial oxaloacetate exported to cytosol as malate (Lecture 31);

  • Phosphoenolpyruvate carboxykinaseReaction is cytosolic (as for rest of gluconeogenesis);

    Loss of CO2 from oxaloacetate drives formation of high-energy mixed anhydride bond (phosphoenol pyruvate formation);

    Overall DG = +0.83 kJ/mol for two steps compared to +31 kJ/mol for direct conversion.

  • Control of Glycolysis and Gluconeogenesis

  • The key regulatory site in glycolysis is phosphofructokinase (concommittent step);

    High levels of ATP (high energy) inhibit activity by decreasing the affinity for substrate;

    Citrate (intermediate in the TCA cycle, Lecture 28) signals high energy and increases effect of ATP (decreases activity);

    High levels of AMP (low energy) reduces the effect of ATP;

    Low pH inhibits activity (prevents lactic acidosis);

    Secondary control enzymes are hexakinase and pyruvate kinase.

  • Phosphofructokinase is allosterically regulated by Fructose-2,6-bisphosphate.

    F-2,6-BP is synthesised and degraded by phosphofructokinase 2 (PFK2);

    F-2,6-BP changes behaviour of phosphofructokinase from Sigmoidal to hyperbolic;

    The enzyme is bifunctional and has kinase (adds PO43-) and phosphatase (removes PO43-) activity;

    Activity of PF2 is hormonally regulated

  • Hormonal Control of Glycolysis

  • Control of gluconeogenesisGluconeogenesis produces glucose-6-phosphate prevents diffusion from cell and allows other metabolic uses;

    Hydrolysis of Glucose-6-phosphate by a phosphatase occurs in liver (and kidneys). Location is lumen of endoplasmic reticulum (ER);

    Glucose is produced from glucose-6-phosphate in response to low blood glucose levels (glucose homeostasis).

    Phosphatase requires calcium-binding stabilising protein and transporters for glucose and phosphate.

  • Control of glycolysis and gluconeogenesis

  • SummaryGlycolysis is a central pathway in the metabolism of sugars. Reactions take place in the cytosol;

    Intermediates are phosphorylated (prevents leakage of compounds into other compartments);

    Energy (2 x ATP) is put into the system. Energy (as pyruvate, 4 x ATP and 2 x NADH) come out of the pathway;

    Key control points are phosphofructokinase (allosteric and hormonal control), hexakinase and pyruvate kinase. Low energy in the cell increases flux and vice versa.

    Gluconeogenesis is not a complete reversal of the glycolytic pathway;

    Glycolysis and gluconeogenesis are reciprocally controlled.