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23.7 Glycogen Metabolism
23.8 Gluconeogenesis: Glucose Synthesis
Chapter 23 Metabolic Pathways for Carbohydrates
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Glycogenesis
Glycogenesis: Stores glucose by converting glucose to
glycogen. Operates when high levels of glucose-6-
phosphate are formed in the first reaction of glycolysis.
Does not operate when energy stores (glycogen) are full, which means that additional glucose is converted to body fat.
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Diagram of Glycogenesis
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Formation of Glucose-6-Phosphate Glucose is converted to glucose-6-phosphate using ATP.
Glucose-6-phosphate
O
OH
OH
OH
OH
CH2OP
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Formation of Glucose-1-Phosphate Glucose-6-phosphate is converted to glucose-1-phosphate.
Glucose-6-phosphate Glucose-1-phosphate
O
O
OH
OH
OH
CH2OH
P
O
OH
OH
OH
OH
CH2OP
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UDP-Glucose UTP activates glucose-1-phosphate
to form UDP-glucose and pyrophosphate (PPi).
UDP-glucose
O
O
OH
OH
OH
CH2OH
P
O
O-
O P
O
O-
O CH2O
OHOH
N
N
O
H
O
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Glycogenesis: Glycogen The glucose in UDP-glucose adds to glycogen. UDP-Glucose + glycogen glycogen-glucose + UDP
The UDP reacts with ATP to regenerate UTP.UDP + ATP UTP + ADP
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Glycogenolysis
Glycogenolysisis the break down of glycogen to glucose.
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Glycogenolysis
Glycogenolysis: Is activated by glucagon (low blood glucose). Bonds glucose to phosphate to form glucose-1-
phosphate.Glycogen-glucose + Pi Glycogen + glucose-1-phosphate
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Isomerization of Glucose-1-phosphate
The glucose-1-phosphate isomerizes to glucose-6-phosphate, which enters glycolysis for energy production.
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Glucose-6-phosphate
Glucose-6-phosphate: Is not utilized by brain and skeletal muscle
because they lack glucose-6-phosphatase. Hydrolyzes to glucose in the liver and kidney,
where glucose-6-phosphatase is available providing free glucose for the brain and skeletal muscle.
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Utilization of Glucose
Glucose: Is the primary
energy source for the brain, skeletal muscle, and red blood cells.
Deficiency can impair the brain and nervous system.
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Gluconeogenesis: Glucose Synthesis
Gluconeogenesis is: The synthesis of
glucose from carbon atoms of noncarbohydrate compounds.
Required when glycogen stores are depleted.
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Gluconeogenesis: Glucose Synthesis
Carbon atoms for gluconeogenesis from lactate, some amino acids, and glycerol are converted to pyruvate or other intermediates.
Seven reactions are the reverse of glycolysis and use the same enzymes.
Three reactions are not reversible.Reaction 1 Hexokinase Reaction 3 PhosphofructokinaseReaction 10Pyruvate kinase
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Gluconeogenesis: Pyruvate to Phosphoenolpyruvate
Pyruvate adds a carbon to form oxaloacetate by two reactions that replace the reverse of reaction 10 of glycolysis.
Then a carbon is removed and a phosphate added to form phosphoenolpyruvate.
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Phosphoenolpyruvate to Fructose-1,6-bisphosphate
Phosphoenolpyruvate is converted to fructose-1,6-bisphosphate using the same enzymes in glycolysis.
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Glucose Formation A loss of a phosphate from fructose-1,6-
bisphosphate forms fructose-6-phosphate and Pi. A reversible reaction converts fructose-6-
phosphate to glucose-6-phosphate. The removal of phosphate from glucose-6-
phosphate forms glucose.
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Cori Cycle When anaerobic conditions occur in active
muscle, glycolysis produces lactate. The lactate moves through the blood stream to the
liver, where it is oxidized back to pyruvate. Gluconeogenesis converts pyruvate to glucose,
which is carried back to the muscles. The Cori cycle is the flow of lactate and glucose
between the muscles and the liver.
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Pathways for Glucose
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Regulation of Glycolysis and Gluconeogenesis
High glucose levels and insulin promote glycolysis. Low glucose levels and glucagon promote
gluconeogenesis.
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Ethanol Ethanol is not a carbohydrate, nor is it a precursor for the biosynthesis
of carbohydrates. However, ethanol can replace sizable amounts of carbohydrates as an
energy source when large amounts are ingested. It is present in the blood of most humans, being produced by intestinal
flora. People ingest ethanol in variable amounts in beverages and fermented
fruits. Ethanol is metabolized in the liver to acetate and adds to the caloric
content of the diet. Ethanol has an energy equivalent of 7 kcal/g. 100 mL of table wine has ethanol corresponding to about 72 kcal. A “jigger” of whiskey furnishes approximately 120 kcal.
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Ethanol continue: When ethanol is metabolized in the liver, alcohol dehydrogenase oxidizes it first to
acetaldehyde. CH3CH2OH + NAD+ → CH3CHO + NADH + H+
The acetaldehyde is oxidized further to acetate. CH3CHO + NAD+ + H2O → CH3COO- + NADH + H+
A small fraction of the alcohol may be oxidized by other systems: Cytochrome P450 oxidase (also involved in detoxification of many drugs); Catalase
The acetate produced from ethanol largely escapes from the liver and is converted to acetyl CoA and then to carbon dioxide by the way of the Krebs cycle.
The acetyl that stays in the liver may act as a precursor for lipid biosynthesis. A significant consequence of metabolism of ethanol in the liver is the twofold to
threefold increase in the NADH/NAD+ ratio. With higher concentrations of blood alcohol, the concentration of NADH remains high,
and the availability of NAD+ drops and limits both the further oxidation of ethanol and the normal functioning of other metabolic pathways, such as gluconeogenesis.
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“Fatty liver” Chronic consumption of significant amounts of
alcohol may lead to a “fatty liver”, in which the excess of triacylglyceride is deposited.
This is caused by several contributing factors: Reduced triacylglyceride secretion from the liver Reduced rates of fatty acid oxidation Increased rates of lipid biosynthesis
These processes are associated with the increased acetyl CoA and NADH/NAD+ ratio in the liver that results from ethanol oxidation.