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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

• irreversible

• regulation

essential for

the subsequent

cleavage

Substrate-level

phophorylation

Substrate-level

phophorylation

2. Oxidation and substrate-level phosphorylation

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:

Energy yield 2 mol of ATP

dissociation and

formation of H+

Daily lactate production 115 (g/d)

Erythrocytes 29

Skin 20

Brain 17

Sceletal muscle 16

Renal medulla 15

Intestinal mucosa 8

Other tissues 10

Major tissues of lactate production:

(in a resting state)

Cori cycle:

Lactate can be further metabolized by:

heart, sceletal muscle

Lactate dehydrogenase: a tetramer (subunits M and H)

Lactate dehydrogenase

Pyruvate + NADH + H+ lactate + NAD+ LD

5 isoenzymes:

Heart (lactate)

Muscle (pyruvate)

Biosynthetic functions of glycolysis:

Regulation

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)

• tissue-specific isoenzymes

(low Km, a high afinity)

• glucokinase (high Km)

• the rate-limiting, allosteric enzyme

• tissue-specific isoenzymes

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 high-

protein 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

Phosphoenolpyruvate-

carboxykinase

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 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

Pictures used in the presentation:

Marks´ Basic Medical Biochemistry, A Clinical Approach, third edition, 2009 (M.

Lieberman, A.D. Marks)