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23.4 Digestion of Carbohydrates23.5 Glycolysis: Oxidation of Glucose23.6 Pathways for Pyruvate

Chapter 23 Metabolic Pathways for Carbohydrates

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In the mouth, salivary amylase hydrolyzes α-glycosidic bonds in polysaccharides to give smaller polysaccharides (dextrins), maltose, and some glucose.In the small intestine, pancreatic amylasehydrolyzes dextrins to maltose and glucose.The disaccharides maltose, lactose, and sucrose are hydrolyzed to monosaccharides.The monosaccharides enter the bloodstream for transport to the cells.

Stage 1: Digestion of Carbohydrates

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Digestion of Carbohydrates

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GlycolysisIn Stage 2, the metabolic pathway called glycolysisdegrades glucose (6C) to pyruvate (3C).

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Glycolysis: Energy-InvestmentIn reactions 1-5 of glycolysis:

Energy is used to add phosphate groups to glucose and fructose. Glucose is converted to two three-carbon molecules.

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Glycolysis: Energy Investment

1

2

3

4

5 5

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Glycolysis: Energy-ProductionIn reactions 7 and 10, the hydrolysis of phosphates in the triosephosphates generates four ATP molecules.

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Glycolysis: Reactions 6-10

6

7

8

9

10

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Glycolysis generates 2 ATP and 2 NADH.Two ATP are used in energy-investment to add phosphate groups to glucose and fructose-6-phosphate.Four ATP are formed in energy-generation by direct transfers of phosphate groups to four ADP.

Glucose + 2ADP + 2Pi + 2NAD+

2Pyruvate + 2ATP + 2NADH + 4H+

Glycolysis: Overall Reaction

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Regulation of GlycolysisReaction 1 Hexokinase is inhibited by high levels of glucose-6-phosphate, which prevents the phosphorylation of glucose.Reaction 3 Phosphofructokinase, an allostericenzyme, is inhibited by high levels of ATP and activated by high levels of ADP and AMP.Reaction 10 Pyruvate kinase, another allostericenzyme is inhibited by high levels of ATP or acetyl CoA.

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Learning CheckIn glycolysis, what compounds provide phosphate groups for the production of ATP?

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SolutionIn glycolysis, what compounds provide phosphate groups for the production of ATP?In reaction 7, phosphate groups from two 1,3-bisphosphoglycerate molecules are used to form two ATP.In reaction 10, phosphate groups from two phosphoenolpyruvate molecules are used to form two more ATP.

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When oxygen is present in the cell, (aerobic conditions), pyruvate from glycolysis is decarboxylated to produce acetyl CoA and CO2.

O||

CH3—C—COO- + HS—CoA + NAD+

pyruvateO||

CH3—C—S—CoA + CO2 + NADH + H+

acetyl CoA

23.5 The Fate of Pyruvate

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When oxygen is not available (anaerobic conditions), pyruvate is reduced to lactate, which replenishes NAD+ to continue glycolysis.

O lactate|| dehydrogenase

CH3—C—COO- + NADH + H+

pyruvateOH|

CH3—CH—COO- + NAD+

lactate

Lactate Formation

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Under anaerobic conditions (strenuous exercise):Oxygen in the muscles is depleted.Lactate accumulates in the muscles.Muscles tire and become painful. Rest is needed to repay the oxygen debt and to reform pyruvate in the liver.

Lactate in Muscles

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Pathways for Pyruvate

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Match the following terms with the descriptions:1) Catabolic reactions 2) Coenzymes3) Glycolysis 4) LactateA. Produced during anaerobic conditions.B. Reaction series that converts glucose to pyruvate.C. Metabolic reactions that break down large

molecules to smaller molecules + energy.D. Substances that remove or add H atoms in

oxidation and reduction reactions.

Learning Check

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Match the following terms with the descriptions:1) Catabolic reactions 2) Coenzymes3) Glycolysis 4) Lactate

A. 4 Produced during anaerobic conditions.B. 3 Reaction series that converts glucose to pyruvate.C. 1 Metabolic reactions that break down large

molecules to smaller molecules + energy.D. 2 Substances that remove or add H atoms in

oxidation and reduction reactions.

Solution

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23.7 Regulation of Glucose Metabolism and energy Production

High glucose levels and insulin promote glycolysis. Low glucose levels and glucagonpromote gluconeogenesis.

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23.10 Glycogen Metabolism: Glycogenesis and Glycogenolysis

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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-PhosphateGlucose is converted to glucose-6-phosphate using ATP.

Glucose-6-phosphate

O

OHOH

OH

OH

CH2OP

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Formation of Glucose-1-PhosphateGlucose-6-phosphate is convertedto glucose-1-phosphate.

Glucose-6-phosphate Glucose-1-phosphate

O

OOH

OH

OH

CH2OH

P

O

OHOH

OH

OH

CH2OP

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UDP-GlucoseUTP converts glucose-1-phosphate to UDP-glucose and pyrophosphate (PPi).

UDP-glucose

O

OOH

OH

OH

CH2OH

P

O

O-O P

O

O-O CH2

O

OHOH

N

N

O

H

O

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Glycogenesis: GlycogenThe 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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GlycogenolysisGlycogenolysis:

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-phosphateGlucose-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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Learning CheckMatch each description with:1) Glycogenesis 2) GlycogenolysisA. Activated by low levels of blood glucose.B. Converts glucose-1-phosphate to glucose-6-

phosphate.C. Activated by high levels of glucose-6-phosphate.D. Glucose + UTP UDP-glucose + PPi

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SolutionMatch each description with:1) Glycogenesis 2) GlycogenolysisA. 2 Activated by low levels of blood glucose.B. 2 Converts glucose-1-phosphate to glucose-6-

phosphate.C. 1 Activated by high levels of glucose-6-phosphate.D. 1 Glucose + UTP UDP-glucose + PPi

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Utilization of GlucoseGlucose:

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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23.11 Gluconeogenesis:Glucose Synthesis

Gluconeogenesis is:The synthesis of glucose from carbon atoms of noncarbohydratecompounds.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 HexokinaseReaction 3 PhosphofructokinaseReaction 10 Pyruvate 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 FormationA 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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Learning CheckIdentify each process as:1) glycolysis 2) glycogenesis3) glycogenolysis 4) gluconeogenesis

A. The synthesis of glucose from noncarbohydrates.

B. The breakdown of glycogen into glucose.C. The oxidation of glucose to two pyruvate.D. The synthesis of glycogen from glucose.

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SolutionIdentify each process as:1) glycolysis 2) glycogenesis3) glycogenolysis 4) gluconeogenesis

A. 4 The synthesis of glucose from noncarbohydrates.

B. 3 The breakdown of glycogen into glucose.C. 1 The oxidation of glucose to two pyruvate.D. 2 The synthesis of glycogen from glucose.

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Tutorials:Glycolysis URLshttp://instruct1.cit.cornell.edu/courses/biomi290/ASM/glycolysis.dcrhttp://programs.northlandcollege.edu/biology/Biology1111/animations/glycolysis.htmlhttp://www.tcd.ie/Biochemistry/IUBMB-Nicholson/swf/glycolysis.swf