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GLUCONEOGENESIS

Summary of handout:

Comparison with glycolysis, unique and shared enzymes

Role of biotin in gluconeogenesis (and comparison with vitamin K which is not involved in gluconeogenesis)

"Reversal" of pyruvate kinase. Participation of the mitochondria

"Reversal" of Phosphofructokinase

"Reversal" of hexokinase

The Cori and alanine cycles

Regulation. Role of insulin and glucagon in glycolysis and gluconeogenesis.Glycogenic and ketogenic compounds

Metabolic role of gluconeogenesis

Ferchmin 2015

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The overall reaction of glycolysis is:

COOH | Glucose + 2 ADP + 2 Pi + 2 NAD+ 2 CO +➔ 2 ATP + 2 NADH + 2H+ + 2 H2O | CH3

ΔG°'= -20 Kcal/mole

COOH |2 CO + 4 ATP + 2 GTP + 2 NADH + 2H+ + 2 H2O glucose + 4 ADP + 2 GDP + 6 P➔ i + 2 NAD+ | CH3

ΔG°'= -9 Kcal/mole

COMPARISON BETWEEN GLYCOLYSIS AND GLUCONEOGENESIS

The overall reaction of gluconeogenesis is:

Glycolysis yield 2 ATP/glucose plus a net dissipated -20 Kcal/mole. Gluconeogenesis is really bad news, it consumes the equivalent of 6 ATP/glucosesynthesized. Why would be a need for such a metabolic pathway?

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Gluconeogenesis is the synthesis of glucose from precursors that are not sugars, like lactate, pyruvate, glycerol or glycogenic amino acids. The synthesis of glucose from other sugars simply is not gluconeogenesis. The neo means de novo from non-carbohydrate molecules. There is no gluconeogenesis from fatty acids except the rare ones with odd number of carbons that have a minute contribution to the synthesis of glucose of rather academic value just to stimulate your intellect! Fatty acids contribute to the fasting organism with ATP through β-oxidation and oxidation of ketone bodies in the Krebs cycle. Ketone bodies only partially substitute for glucose and are synthesized by a pathway different from gluconeogenesis. Ketone bodies are potentially dangerous in the absence of glucose (you will study this later). In conclusion: lipids can spare glucose because they provide for ATP that otherwise would be synthesized in glycolysis. But lipids do not substitute glucose. We need about l60 grams of glucose per day, 120 grams are needed for the brain and 40 grams for muscle, erythrocytes, eye lens cells, kidneys medulla, etc. Approximately 200 grams are stored in hepatic glycogen. Gluconeogenesis provides the necessary glucose during fast. The complete gluconeogenesis occurs in liver and a small fraction in kidney. Since glycolysis is irreversible gluconeogenesis cannot be the reversal of glycolysis. The enzymes that catalyze the irreversible reactions in glycolysis are overridden in various ingenious ways in gluconeogenesis.

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We will study gluconeogenesis by comparing it with glycolysis

How do your reverse an irreversible metabolic step?

How do your reverse an irreversible metabolic step?

By using an enzyme that catalyzes the opposite also irreversible step!!!

By using an enzyme that catalyzes the opposite also irreversible step!!!

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The last glycolytic step catalyzed by pyruvate kinase is irreversible, the free energy change is high, -7.5 Kcal/mole. To reverse this step in gluconeogenesis two enzymes are used and the process takes place in two cellular compartments.

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Enzymatic differences between glycolysis and gluconeogenesisa) Regulatory enzymes__________________________________________________________________Glycolysis Gluconeogenesis__________________________________________________________________Hexokinase Glucose 6-phosphatase

Phosphofructokinase 1 Fructose 1,6-bisphosphatase

Pyruvate Pyruvate kinase carboxylase

Phosphoenolpyruvate carboxykinase

__________________________________________________________________b) The remaining enzymes are shared by both pathways__________________________________________________________________Essential concept: Pathways for breakdown and synthesis of a particular metabolite are always different, utilizing unique enzymes in one or more steps. The difference usually is in the regulatory enzymes.

Pyruvate carboxylase is located in liver mitochondria

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First we will consider the reversal in gluconeogenesis of the exergonic glycolyticreaction catalyzed by pyruvate kinase. The reaction is shown below:

The above exergonic reaction is overcome by an input of energy andof two complex reactions that regenerate phosphoenolpyruvate.The two enzymes involved are:

a) Pyruvate carboxylase

b) Phosphoenolpyruvate carboxykinase

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PYRUVATE CARBOXYLASE is exclusively hepatic.The reaction catalyzed by pyruvate carboxylase takes place in 2 steps:

STEP 1: Enz-Biotin + ATP + CO2 Enz-Carboxybiotin + ADP + Pi➔

This first step requires CH3-CO-CoA (acetyl~S-CoA)

STEP 2: Enz-Carboxybiotin + pyruvate Enz-Biotin + oxaloacetate➔

This is an anaplerotic reaction (re-supplying). It provides oxaloacetate for the Krebs cycle and for gluconeogenesis. The requirement for CH3-CO~S-CoA is a manifestation of the need of oxaloacetate for the TCA cycle or the abundance of CH3-CO-CoA produced by a lipid rich diet that calls for storage of glucogenic intermediaries.

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The next step is the synthesis of phosphoenolpyruvic acid from oxaloacetate

The synthesis of PEPA reverses the effect of pyruvate kinase

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Gluconeogenesis takes place in the cytosol and in the mitochondria. There are two pathways to generate PEPA (phosphoenolpyruvic acid). In both pathways NADH must be generated to allow the activity of glyceraldehyde-3-phosphate dehydrogenase in the reduction of 3-phosphoglyceric acid.

From PEPA to fructose-1,6-bisphosphate all the steps are shared by glycolysis and gluconeogenesis and are reversible.

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This graph represents the relationshipbetween the activity of both enzymes and the energy status of a muscle cell.

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From previous page, we can see the relationship between phosphofructokinase and fructose-1,6-phosphatase

In this point we have a metabolic cycle or futile cycle that “wastes” energy but provides more leverage for regulation

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Integration of gluconeogenesis and glycolysis

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There is a fundamental difference between the role of glycolysis in the “peripheral” organs and the liver.In liver the role of glycolysis is to make you FAT!!!! In muscle is to make you run!!!

Ethanol and fatty acids are not glucogenic (odd number fatty acids contribute insignificantly to gluconeogenesis).

Glycerol, the ketoacids of most amino acids, lactate and pyruvate ARE glucogenic.

Galactose, fructose, etc are not glucogenic. They are monosaccharides in equilibrium with glucose!