Download Glycolysis and Gluconeogenesis

Glycolysis and
Gluconeogenesis
Alice Skoumalová
1. Glycolysis
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
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
Anaerobic glycolysis
a limited supply of O2
no mitochondria
increased demands for ATP
Lactic acidemia
in hypoxia
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 Km
Michaelis-Menten kinetics
1. Conversion of glucose 6-P to the triose
phosphates
2. Oxidation and substrate-level
phosphorylation
1. Conversion of glucose 6-P to the triose phosphates
essential for
the subsequent
cleavage
• irreversible
• regulation
2. Oxidation and substrate-level phosphorylation
Substrate-level
phophorylation
Substrate-level
phophorylation
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!)
Aerobic glycolysis:
 involving shuttles that transfer reducing equivalents across the mitochondrial
membrane
Glycerol 3-phosphate shuttle:
Malate-aspartate shuttle:
Anaerobic glycolysis:
dissociation and
formation of H+
Energy yield 2 mol of ATP
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
Cori cycle:
Lactate can be further metabolized by:
 heart, sceletal muscle
Lactate dehydrogenase: a tetramer (subunits M and H)
Lactate dehydrogenase
Pyruvate + NADH +
H+
LD
lactate + NAD+
5 isoenzymes:
Heart (lactate)
Muscle (pyruvate)
Biosynthetic functions of glycolysis:
Regulation
• 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)
the liver isoenzyme - inhibition by
cAMP-dependent protein kinase
(inhibition of glycolysis during
fasting)
Lactic acidemia:
increased NADH/NAD+ ratio
inhibition of pyruvate dehydrogenase
2. Gluconeogenesis
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
Precursors for gluconeogenesis
1. lactate (anaerobic glycolysis)
2. amino acids (muscle proteins)
3. glycerol (adipose tissue)
Conversion of pyruvate to phosphoenolpyruvate
1. Pyruvate → Oxaloacetate
 Pyruvate carboxylase
2. Oxaloacetate → PEP
 Phosphoenolpyruvatecarboxykinase
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)
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
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
Pictures used in the presentation:
Marks´ Basic Medical Biochemistry, A Clinical Approach, third edition, 2009 (M.
Lieberman, A.D. Marks)