Glycolysis and Gluconeogenesis. Alice Skoumalová

Transcription

1 Glycolysis and Gluconeogenesis Alice Skoumalová

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3 1. Glycolysis

4 Glucose: the universal fuel for human cells Sources: diet (the major sugar in our diet) internal glycogen stores blood (glucose homeostasis) Glucose oxidation: after a meal: almost all tissues during fasting: brain, erythrocytes

5 Glycolysis: oxidation and cleavage of glucose ATP generation (with and without oxygen) all cells in the cytosol (the reducing equivalents are transferred to the electron-transport chain by the shuttle) ATP is generated: 1. via substrate-level phosphorylation 2. from NADH 3. from oxidation of pyruvate Regulation of glycolysis: 1. Hexokinase 2. Phosphofructokinase 3. Pyruvate Kinase Generation of precursors for biosynthesis: fatty acids amino acids ribosis-5-p

6 Anaerobic glycolysis a limited supply of O 2 no mitochondria increased demands for ATP Lactic acidemia in hypoxia

7 Phosphorylation of glucose: irreversible Glucose 6-P: cannot be transported back across the plasma membrane a precursor for many pathways that uses glucose Hexokinases Glucokinase (liver, β-cell of the pancreas) high K m

8 Michaelis-Menten kinetics

9 1. Conversion of glucose 6-P to the triose phosphates 2. Oxidation and substrate-level phosphorylation

10 1. Conversion of glucose 6-P to the triose phosphates essential for the subsequent cleavage irreversible regulation

11 2. Oxidation and substrate-level phosphorylation Substrate-level phophorylation Substrate-level phophorylation

12 Summary of the glycolytic pathway: Glucosis + 2 NAD P i + 2 ADP 2 pyruvate + 2 NADH + 4 H ATP + 2 H 2 O G 0 = - 22 kcal (it cannot be reversed without the expenditure of energy!)

13 Aerobic glycolysis: involving shuttles that transfer reducing equivalents across the mitochondrial membrane

14 Glycerol 3-phosphate shuttle:

15 Malate-aspartate shuttle:

16 Anaerobic glycolysis: Energy yield 2 mol of ATP dissociation and formation of H +

17 Major tissues of lactate production: (in a resting state) Daily lactate production 115 (g/d) Erythrocytes 29 Skin 20 Brain 17 Sceletal muscle 16 Renal medulla 15 Intestinal mucosa 8 Other tissues 10

18 Cori cycle: Lactate can be further metabolized by: heart, sceletal muscle Lactate dehydrogenase: a tetramer (subunits M and H)

19 Lactate dehydrogenase LD Pyruvate + NADH + H + lactate + NAD + 5 isoenzymes: Heart (lactate) Muscle (pyruvate)

20 Biosynthetic functions of glycolysis:

21 Regulation

22 tissue-specific isoenzymes (low K m, a high afinity) glucokinase (high K m ) the rate-limiting, allosteric enzyme tissue-specific isoenzymes Fructose 2,6-bis-phosphate: is not an intermediate of glycolysis! Phosphofructokinase-2: inhibited through phosphorylation - camp-dependent protein kinase (inhibition of glycolysis during fasting-glucagon)

23 the liver isoenzyme - inhibition by camp-dependent protein kinase (inhibition of glycolysis during fasting) Lactic acidemia: increased NADH/NAD + ratio inhibition of pyruvate dehydrogenase

24 2. Gluconeogenesis

25 Gluconeogenesis: synthesis of glucose from noncarbohydrate precursors to maintain blood glucose levels during fasting liver, kidney fasting, prolonged exercise, a highprotein diet, stress Specific pathways: 1. Pyruvate Phosphoenolpyruvate 2. Fructose-1,6-P Fructose-6-P 3. Glucose-6-P Glucose

26 Precursors for gluconeogenesis 1. lactate (anaerobic glycolysis) 2. amino acids (muscle proteins) 3. glycerol (adipose tissue)

27 Conversion of pyruvate to phosphoenolpyruvate 1. Pyruvate Oxaloacetate Pyruvate carboxylase 2. Oxaloacetate PEP Phosphoenolpyruvatecarboxykinase

28 Conversion of phosphoenolpyruvate to glucose 3. Fructose-1,6-P Fructose-6-P Fructose 1,6-bisphosphatase (cytosol) 4. Glucose-6-P Glucose Glucose 6-phosphatase (ER)

29 Regulation of gluconeogenesis: concomitant inactivation of the glycolytic enzymes and activation of the enzymes of gluconeogenesis 1. Pyruvate PEP Phosphoenolpyruvate carboxykinase - induced by glucagon, epinephrine, and cortisol 2. Fructose 1,6-P Fructose 6-P Fructose 1,6-bisphosphatase - inhibited by fructose 2,6-P 3. Glucose 6-P Glucose Glucose 6-phosphatase - induced during fasting

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31 Summary Glycolysis Generation of ATP (with or without oxygen) The role of glycolysis in different tissues Lactate production Regulation Gluconeogenesis Activation during fasting, prolonged exercise, after a highprotein diet Precursors: lactate, glycerol, amino acids 3 key reactions: Pyruvate PEP Fructose-1,6-P Fructose-6-P Glucose-6-P Glucose Regulation

32 Pictures used in the presentation: Marks Basic Medical Biochemistry, A Clinical Approach, third edition, 2009 (M. Lieberman, A.D. Marks)