Glycolysis and Gluconeogenesis

1
Glycolysis and Gluconeogenesis

• Alice Skoumalová

2
(No Transcript)
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 O2
• 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, ß-cell of the pancreas)
• high Km

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 2 Pi 2 ADP 2 pyruvate 2 NADH 4 H
2 ATP 2 H2O ?G0 - 22 kcal (it cannot
be reversed without the expenditure of energy!)
13

• Clinical correlations
• Hypoxemia (lack of oxygen in tissues)
• Acute hemorrhage (hypotension, lost of
  erythrocytes)
• - anaerobic glycolysis
• - lactate formation, metabolic acidosis
• Chronic obstructive pulmonary disease (an
  insuficient ventilation)
• - anaerobic glycolysis, lactate formation,
  metabolic acidosis
• - accumulation of CO2, respiratory acidosis
• Myocardial infarction (lack of oxygen in
  myocardium)
• - anaerobic glycolysis, lactate formation
• - lack of ATP

14

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

15
Glycerol 3-phosphate shuttle
16
Malate-aspartate shuttle
17
Anaerobic glycolysis
dissociation and formation of H
Energy yield 2 mol of ATP
18
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
19
Cori cycle

• Lactate can be further metabolized by
• heart, sceletal muscle
• Lactate dehydrogenase a tetramer (subunits M and
  H)

20
Lactate dehydrogenase
LD
Pyruvate NADH H lactate NAD
5 isoenzymes
Heart (lactate)
Muscle (pyruvate)
21
Biosynthetic functions of glycolysis
22
Clinical correlations Long-intensity exercise
(for example a sprint) - the need for ATP
exceeds the capacity of the mitochondria for
oxidative phosphorylation, anaerobic
glycolysis ? lactate formation, muscle
fatigue and pain - a training ? the amounts of
mitochondria and myoglobin increase
23
Regulation
24

• tissue-specific isoenzymes (low Km, a high
  afinity)
• glucokinase (high Km)

• 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)

25
the liver isoenzyme - inhibition by
cAMP-dependent protein kinase (inhibition of
glycolysis during fasting)
Lactic acidemia increased NADH/NAD
ratio inhibition of pyruvate dehydrogenase
26
2. Gluconeogenesis
27

• Gluconeogenesis
• synthesis of glucose from noncarbohydrate
  precursors ? to maintain blood glucose levels
  during fasting
• liver, kidney
• fasting, prolonged exercise, a high-protein
  diet, stress
• Specific pathways
• Pyruvate ? Phosphoenolpyruvate
• Fructose-1,6-P ? Fructose-6-P
• Glucose-6-P ? Glucose

28

• Precursors for gluconeogenesis
• lactate (anaerobic glycolysis)
• amino acids (muscle proteins)
• glycerol (adipose tissue)

29
Conversion of pyruvate to phosphoenolpyruvate

• 1. Pyruvate ? Oxaloacetate
• Pyruvate carboxylase
• 2. Oxaloacetate ? PEP
• Phosphoenolpyruvate-carboxykinase

30
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)

31
Clinical correlations Alcoholism - excessive
ethanol consumption ? increase NADH/NAD ratio
that drive the lactate dehydrogenase reaction
toward lactate - lack of precursors for
gluconeogenesis ? its inhibition - insuficient
diet - reduced glucose in the blood, consumption
of glycogen in the liver ? hypoglycemia
32

• 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

33
(No Transcript)
34

• 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 high-protein diet
• Precursors lactate, glycerol, amino acids
• 3 key reactions Pyruvate ? PEP Fructose-1,6
  -P? Fructose-6-P Glucose-6-P ? Glucose
• Regulation

35
Pictures used in the presentation Marks Basic
Medical Biochemistry, A Clinical Approach, third
edition, 2009 (M. Lieberman, A.D. Marks)