Glycolysis and GluconeogenesisDr M. D. Lloyd5W 2.13; M.D.Lloyd@bath.ac.uk
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);
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.