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Home , Cori cycle

I Overview of , and Pentose Phosphate Pathway

Cori Cycle

Gluconeogenesis

Chapter 16 – part 3 Covered on Exam 3 (includes material from Chapter 20, p.589-590) Dr. Ray Control of • The flux through the glycolytic pathway must 1 be adjusted in response to conditions both inside and outside the cell. The rate of 2 conversion of into pyruvate is regulated to meet two major cellular needs: 3

1) the production of ATP, generated by the 4 degradation of glucose 2) the provision of building blocks for 5 synthetic reactions, such as the formation of fatty acids. 6 In metabolic pathways, catalyzing essentially irreversible 7 reactions are potential sites of control. 8 1. Which glycolytic enzymes are likely to be sites of control? 9 The ______reactions at steps ______, which is 10 near equilib in cellular concentrations) Energetics of Glycolysis • Most of the decrease in free energy in glycolysis takes place in the three essentially irreversible steps catalyzed by , phosphofructokinase, and . step ? 1

3

10

• The energy released in the anaerobic conversion of glucose into two molecules of pyruvate is only a portion of the total energy captured during complete aerobic glucose catabolism. 2. Where does most of the rest of the energy capture occur? 3. If glucose needed to be made, which steps would require the most amount of energy to overcome?

http://higheredbcs.wiley.com/legacy/college/boyer/0471661791/animations/animations.htm  Introduction to  Traits of Metabolism Gluconeogenesis  Focus on comparing Gluconeogenesis to Glycolysis • The synthesis of glucose from non-carbohydrate precursors (such as pyruvate and lactate) is called gluconeogenesis. 1. Why is this metabolic pathway important? because the ______depends on glucose as its primary fuel and ______use only glucose as a fuel

Red Blood Cells do not have mitochondria, so they do not do aerobic metabolism. All ATP is made only from glycolysis. • The daily glucose requirement of the brain in a typical adult human being is about 120 g, which accounts for most of the 160 g of glucose needed daily by the whole body. • The amount of glucose present in body fluids is about 20 g, and that readily available from , a storage form of glucose, is approximately 190 g. •Thus, the direct glucose reserves are sufficient to meet glucose needs for about a day. During a longer period of not eating, glucose must be formed from non-carbohydrate sources, through gluconeogenesis.

http://higheredbcs.wiley.com/legacy/college/boyer/0471661791/ animations/animations.htm Glycolysis and Hexokinase 1 Gluconeogenesis 2 • Under typical cellular conditions, most of the decrease in free energy in Phosphofructo 3 kinase glycolysis takes place in the 4 three essentially irreversible steps catalyzed by: (1) hexokinase 5 (3) phosphofructokinase 6 (10) pyruvate kinase • In gluconeogenesis, these 7 three irreversible steps are by-passed by other reactions, 8 in order to overcome the energetic barrier. 9 • Gluconeogenesis is energetically costly. Pyruvate 10 kinase http://higheredbcs.wiley.com/legacy/college/boyer/ 0471661791/animations/animations.htm Text Fig 16.2  Gluconeogenesis  Energetics Gluconeogenesis is not the Reverse of Glycolysis DG’ for the formation of pyruvate from glucose (in glycolysis) is about -84 kJ/mol 1

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10

Unique Reactions of Gluconeogenesis: In gluconeogenesis, the three irreversible steps above are by-passed by other unique reactions, in order to overcome the energetic barrier.

Replace step 10 (with 2 steps)

Replace step 3:

Replace step 1: Energetics of Gluconeogenesis in glycolysis (step 3 - kinase) DGo’ = -14.2 kJ/mol in gluconeogenesis DGo’ = -16.3 kJ/mol (step 8 - phosphatase) Exergonic hydrolysis catalyzed by Fructose 1,6-bisphosphatase

Both kinase and phosphatase reactions are exergonic. Kinase uses ATP for phosphate source and for energy source . Phosphatases hydrolyze phosphorylated alcohols (ROP), which have some What is the phosphoryl transfer potential small amount of free of Pi (inorganic phosphate? energy of hydrolysis. Gluconeogenesis - NOT a reversal of glycolysis The three exergonic (irreversible) steps of glycolysis in cell: (1) Hexokinase phosphorylate (2) Phosphofructokinase (3) Pyruvate kinase Substrate level phosphorylation are replaced by other favorable reactions: dephosphorylate (1) Glucose-6-phosphatase (2) Fructose-1,6-bisphosphatase

Pyruvate  Oxaloacetate  PEP (3a) (3b) Phosphoenolpyruvate carboxykinase (reactions 3a & 3b) know overview only, NO details Gluconeogenesis vs. Reversal of Glycolysis The stoichiometry of gluconeogenesis is:

Gluconeogenesis costs 6 ATP equivalents

In contrast, the stoichiometry for the reversal of glycolysis is:

Glycolysis produces 2 ATP DGo’ = - 84 kJ/mol

Overall, both pathways are highly exergonic (spontaneous)!

Recall that one ATP equivalent changes the equilibrium constant by a factor of about 108. Hence, the input of four additional high-energy bonds in Gluconeogenesis changes the equilibrium by a factor of about 1032. This is a clear example of the coupling of reactions: ATP hydrolysis is used to power an energetically unfavorable reaction. Non-carbohydrate Precursors

of Gluconeogenesis

• Lactate – formed by in active when rate of glycolysis exceed rate of oxidative metabolism (because of insufficient levels of oxygen). • Amino acids – from proteins in the diet and during starvation from breakdown of proteins in skeletal muscle. • – from hydrolysis of triacylglycerols in fat cells. Glycerol can enter the glycolytic or gluconeogenic pathways through DHAP:

Phosphorylate at C3 Oxidize at C2

- is used Comparison of Glycolysis & Gluconeogenesis + is made Glycolysis makes 2 ATP Gluconeogenesis costs 6 ATP

(1) Hexokinase (- ATP) (11) Glucose-6-phosphatase

(9) Fructose-1,6-bisphosphatase (3) Phosphofructokinase (- ATP) (1) Pyruvate carboxylase (- ATP) and (10) Pyruvate kinase (+ ATP) (2) Phosphoenolpyruvate carboxykinase (- GTP)

(7) (+ATP) (5) Phosphoglycerate kinase (-ATP) happens twice In most tissues gluconeogenesis stops after formation of glucose-6-phosphate, which can be stored as glycogen. One advantage to ending gluconeogenesis at glucose 6-phosphate is that, unlike free glucose, the molecule cannot diffuse out of the cell. To keep glucose inside the cell, the generation of free glucose is controlled in two ways: 1. responsible for the conversion of glucose 6-phosphate into glucose, glucose 6-phosphatase, is regulated. 2. Enzyme is present only in tissues whose metabolic duty is to maintain blood-glucose homeostasis. Tissues that release glucose into the blood are the and to lesser extent the kidney. Gluconeogenesis Glycolysis and Gluconeogenesis are coordinated, in a tissue- specific fashion, to ensure that the glucose-dependent energy needs of ALL cells are met. In a particular cell, when one pathway is upregulated, the other is downregulated. Reciprocal regulation

• The main site of gluconegenesis is Cori Cycle (Figure 16.33) in the liver, with a small amount occurring in the kidneys. • Gluconeogenesis in these two organs helps maintain glucose levels in the blood so that the brain, red blood cells and muscles can extract sufficient glucose from blood to satisfy their energy needs.

http://www.wiley.com/college/fob/quiz/quiz21/21-5.html The Cori Cycle

CORI CYCLE - Lactate formed by active muscle travels through the blood and is converted into glucose by the liver. This cycle shifts part of the metabolic burden of active muscle to the liver.

Cori Cycle (Figure 16.33) Lactate Dehydrogenase (LDH) Isozymes:

H4, H3M1, H2M2, H1M3, M4

H = heart (higher affinity for lactate) takes in lactate, oxidizes it to pyruvate, then uses pyruvate via aerobic metabolism (, electron transport chain)

M = skeletal muscle and liver, have lower affinity for lactate

In the liver, lactate is oxidized to pyruvate by lactate dehydrogenase. This pyruvate undergoes gluconeogenesis to produce free glucose, which is released into the blood. The liver restores the level of glucose necessary for active muscle cells to continue anaerobic glycolysis for immediate energy needs. and the Liver Workbook, Chapter 16 Self-Test, Q27: 1. Which of the following statements about the Cori cycle and its physiologic consequences are true?

A) It involves the synthesis of glucose in muscle. B) It involves release of lactate by muscle. C) It involves lactate synthesis in the liver. Anaerobic glycolysis in D) It involves ATP synthesis in muscle. muscle E) It involves release of glucose by the liver.

2. How much energy is extracted when one glucose is converted to 2 pyruvate?

3. What is the energy cost of producing one glucose from 2 pyruvate? Gluconeogenesis in liver Other Metabolic Pathways Associated with Glycolysis • The entry point of other important dietary Glycogen Energy Storage sugars into glycolysis varies with the monosaccharide and Pentose Phosphate Pathway the organ. PFK Biosynthesis • Common entry points NADPH are: Glucose-6-P and Growth Fructose-6-P ribose-5-phosphate DHAP & GAP

GLYCOGEN FRUCTOSE hexokinase DHAP O Phospho- Isomeri- Phospho- O O H O rylation H zation rylation CH2 O P O- C C CH2OH CH2 O P O- O O- H OH H OH O O O- CH OH HO H HO H HO H PFK HO H 2 O H H OH hexo- H OH H OH H OH C H OH kinase H OH O H OH O H OH O H OH O CH OH CH O P O- 2 CH2 O P O- 2 CH2 O P O- CH2 O P O- O- O- O- O- Glucose G6P F6P FBP GAP Multiple Fates of Glucose-6-Phosphate (G6P) from Introduction to Chapter 20 (p. 589 – 590)

1. Used as fuel in anaerobic or Figure 21.3 aerobic glycolysis 2. Converted to free gluocose by liver and released into the blood to raise blood sugar when too low 3. Processed by Pentose Phosphate Pathway, which produces: • 5C Ribose (for DNA/RNA synthesis) and • NADPH (for reductive biosynthesis of biomolecules)

First stage of PPP:

http://www.wiley.com/college/fob/quiz/quiz15/15-1.html Pentose Phosphate Pathway – Needed for Biosynthesis Anabolism is REDUCTIVE so biosynthesis and growth require the reduced cofactor ______, which is produced by the pentose phosphate pathway. • In PPP glucose-6-phosphate is converted into ribose-5-phosphate (needed for nucleotide biosynthesis to form RNA & DNA), with production of two NADPH for metabolic biosynthesis. • Excess Ribose-5-P formed is completely converted into the glycolytic intermediates fructose-6-P and glyceraldehyde-3-P. 2 NADP+ 2 NADPH

H O + CO2

H OH H OH H OH

CH2OH Ribose-5-phosphate PHASE 2 (nonixidative) 2 Fructose-6-P + glyceraldehyde-3-P (which feed into glycolysis) Pentose Phosphate Pathway • The pentose phosphate pathway (occurs in cytosol) meets the need of all organisms for a source of NADPH to use in reductive biosynthesis. Synthetic pathways requiring NADPH: Detoxification processes need NADPH: • Fatty acid biosynthesis • Reduction of oxidized glutathione • Cholesterol biosynthesis • Cytochrome P450 monooxygenases • Neurotransmitter biosynthesis • Nucleotide biosynthesis (and NAD+, FAD, acetyl CoA)

Liver exports glucose NADPH & ribose-5-P Glycogen (energy storage, when energy not needed) Glucose-6-P

Glycolysis: Gluconeogenesis: • convert glucose into pyruvate • converts pyruvate into glucose • produce 2 ATP & 2 NADH • utilizes 4 ATP, 2 GTP & 2 NADH

Pyruvate Metabolism and the Liver

Workbook, Chapter 20 Self-Test, Q13: Liver synthesizes fatty acids and lipids for export to other tissues. Would you expect the Pentose Phosphate Pathway (PPP) to have a low or a high activity in this organ? Explain. HIGH Biosynthesis of fatty acids requires reducing equivalents in the form of NADPH. In all organs that carry out reductive biosynthesis, the PPP supplies a large portion of the required NADPH.

What types of reactions do the following enzymes catalyze? (A) phosphatase – hydrolyzes phosphate from phosphorylated alcohol so transfers phosphate to H2O (B) kinase – transfers phosphate from ATP to alcohol

(C) ATPase – hydrolyzes ATP and uses energy released to create a concentration gradient or drive a DG + reaction Carbohydrate Metabolism and the Liver Workbook, Chapter 16 Self-Test, Q27: 1. Which of the following statements about the Cori cycle and its physiologic consequences are true? Ans: B, D, E A) It involves the synthesis of glucose in muscle. B) It involves release of lactate by muscle. C) It involves lactate synthesis in the liver. Anaerobic glycolysis in D) It involves ATP synthesis in muscle. muscle E) It involves release of glucose by the liver.

2. How much energy is extracted when one glucose is converted to 2 pyruvate? 2 ATP + 2 NADH 3. What is the energy cost of producing one glucose from 2 pyruvate? Gluconeogenesis 4 ATP + 2 GTP + 2 NADH in liver