14 Glycolysis W. H. Freeman and Company

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1 14 Glycolysis 2013 W. H. Freeman and Company 1

2 Central Importance of Glucose Glucose is an excellent fuel. - Yields good amount of energy upon oxidation. - Can be efficiently stored in the polymeric form. - Many organisms and tissues can meet their energy needs on glucose only. Glucose is a versatile biochemical precursor. - Bacteria can use glucose to build carbon skeletons of: All the amino acids; Membrane lipids; Nucleotides in DNA and RNA; Cofactors. 2

3 Storage Pathways of Glucose Utilization - Can be stored in the polymeric form (starch, glycogen). Synthesis of Structural Polysaccharides - Cell walls of bacteria, fungi, and plants. Glycolysis - Generates energy via oxidation of glucose. - Short-term energy needs. Pentose Phosphate Pathway - Generates NADH via oxidation of glucose. - Detoxification and biosynthesis of lipids and nucleotides. 3

4 Week 10 Chapter 14 Glycolysis 14.1 Glycolysis 14.2 Feeder pathway for glycolysis 14.3 Fermentation 14.4 Gluconeogenesis 14.5 Pentose phosphate pathway 4

5 Glycolysis Glycolysis means glucose degradation. - Glyco- means sugar (example: glycoprotein, glycolipid). - -Lysis means breakdown (example: hydrolysis, proteolysis). Glucose converted to pyruvate in a series of enzymatic reactions. - Pyruvate can be further oxidized. - Pyruvate can be used as a precursor in biosynthesis. Some of energy captured by synthesis of ATP and NADH. 5

6 Glycolysis Has Two Phases Preparatory phase (5 steps). 1) glucose -> glucose 6-phosphate. 2) glucose 6-phosphate -> fructose 6-phosphate. 3) fructose 6-phosphate -> fructose 1,6-bisphosphate. 4) fructose 1,6-bisphosphate -> dihydroxyacetone phosphate + glyceraldehyde 3-phosphate. 5) dihydroxyacetone phosphate -> glyceraldehyde 3-phosphate. Payoff phase (5 steps). 6) glyceraldehyde 3-phosphate -> 1,3-bisphosphoglycerate. 7) 1,3-bisphosphoglycerate -> 3-phosphoglycerate. 8) 3-phosphoglycerate -> 2-phosphoglycerate. 9) 2-phosphoglycerate -> phosphoenolpyruvate. 10)phosphoenolpyruvate -> pyruvate. 6

7 Glycolysis: Preparatory Phase 1) Phosphorylation at C-6 OH. Catalyzed by kinase. ATP as phosphoryl group donor. 2) Isomerization. Catalyzed by isomerase. 3) Phosphorylation at C-1 OH. Catalyzed by kinase. ATP as phosphoryl group donor. 4) Ring opening (lysis). Catalyzed by aldolase. 5) Isomerization. Catalyzed by isomerase. Two molecules of ATP are invested. 7

8 Glycolysis: Payoff Phase 6) Oxidation and phosphorylation. Catalyzed by dehydrogenase. Inorganic phosphate as phosphoryl donor. 7) Phosphorylation of ADP. Catalyzed by kinase. 8) Isomerization. Catalyzed by mutase. 9) Dehydration. Catalyzed by enolase. 10)Phosphorylation of ADP. Catalyzed by kinase. Four molecules of ATP are produced. 8

9 Preparatory Phase Requires ATP 1) Phosphorylation at C-6 OH. ATP -> ADP. 2) Isomerization. Carbonyl from C-1 to C-2. 3) Phosphorylation at C-1 OH. ATP -> ADP. 4) C-C bond cleavage. 5) Isomerization. Carbonyl from C-2 to C-1. Two molecules of ATP invested. Hexose cleaved into two triose phosphates. 9

10 Step 1: Phosphorylation of Glucose Glucose is activated by phosphorylation. - Phosphorylation at C-6. - Catalyzed by hexokinase and ATP acts as phosphoryl donor. - Yields glucose 6-phosphate. Highly thermodynamically favorable. 10

11 Step 1: Phosphorylation of Glucose Why? - Trap glucose inside the cell. - Lower intracellular glucose level to allow further uptake. How? - Nucleophilic attack from glucose C-6 OH on ATP γ phosphate. - ATP-bound Mg 2+ shield negative charge on ATP. Hexokinase requires Mg 2+ for activity. 11

12 Binding Induces Conformational Change From Chapter 6 Enzymes When glucose is NOT present: - Active site residues NOT in position for catalysis. When glucose binds: - Binding induces conformational change. - ATP moves closer to glucose. Blocks water. Prevents attack from water. - Hexokinase changes to catalytically active form. 12

13 Step 2: Phosphohexose Isomerization Glucose is isomerized to fructose. - Isomerization: same atoms, but different arrangement. - An aldose (glucose) can isomerize to a ketose (fructose). - Catalyzed by isomerase. 13

14 Step 2: Phosphohexose Isomerization Why? - C-1 is easier to phosphorylate by kinase in step 3. - Allow symmetric cleavage by aldolase in step 4. How? - Ring opening and closing catalyzed by active site His. - Aldose -> ketose conversion catalyzed by active site Glu. 14

15 Mechanism of Phosphohexose Isomerase General acid-base catalysis. - C-2 deprotonated by active site Glu. - C-1 protonated by the same Glu residue. 15

16 Step 3: Second Phosphorylation Fructose 6-phosphate is further phosphorylated. - Phosphorylation at C-1. - Catalyzed by phosphofructokinase-1 (PFK-1). - Yields fructose 1,6-bisphosphate. Highly thermodynamically favorable. 16

17 Step 3: Second Phosphorylation Why? - Further activation of glucose. - Allow 1 phosphate group in each 3-carbon unit after cleavage. How? - Nucleophilic attack from fructose C-1 OH on ATP γ phosphate. - ATP-bound Mg 2+ shield negative charge on ATP. 17

18 Bis- or Di-, Tris- or Tri-phosphate? Two or three phosphate groups attached at different atoms. - Fructose 1,6-bisphosphate. - Inositol 1,4,5-trisphosphate. Two or three phosphate groups linked together. - Adenosine diphosphate. - Adenosine triphosphate. Adenosine Triphosphate 18

19 Step 4: Aldol Cleavage Fructose 1,6-bisphosphate is cleaved. - Catalyzed by aldolase. - Yields two different products. Dihydroxyacetone phosphate. Glyceraldehyde 3-phosphate. 19

20 Step 4: Aldol Cleavage Two products. - Both are the simplest triose phosphates. - One is ketose, and the other is aldose. 20

21 Step 5: Triose Phosphate Interconversion Dihydroxyacetone phosphate is converted to glyceraldehyde 3- phosphate. - Catalyzed by isomerase. - A ketose (dihydroxyacetone) can isomerize to an aldose (glyceraldehyde). 21

22 Step 5: Triose Phosphate Interconversion Why? - Only glyceraldehyde 3-phosphate is substrate of next enzyme. - Conversion allows glycolysis to proceed by one pathway. 22

23 Carbon From Glucose to Glyceraldehyde After triose phosphate isomerase reaction, - C-1, C-2 and C-3 of glucose are chemically indistinguishable from C-6, C-5 and C-4. 23

24 Payoff Phase Yields ATP and NADH 6) Oxidation. Aldehyde -> anhydride. 7) First ATP production. ADP -> ATP. 8) Phosphate shift. 3-P -> 2-P-glycerate. 9) Dehydration. 2-P-glycerate -> PEP. 10)Second ATP production. ADP -> ATP. Four ATP molecules produced. Triose phosphate converted to pyruvate. 24

25 Step 6: Oxidation of Triose Phosphate Glyceraldehyde 3-phosphate is oxidized. - Oxidized NOT to a free carboxyl group, but to a carboxylic acid anhydride with phosphoric acid. - Catalyzed by dehydrogenase. - Yields 1,3-bisphosphoglycerate. 25

26 Step 6: Oxidation of Triose Phosphate Why? - Generate a high-energy phosphate compound. - Oxidation of aldehyde with NAD + gives NADH. - Prepare to produce ATP. This anhydride has a very high standard free energy of hydrolysis (-49 kj/mol). 26

27 Step 7: First Production of ATP Phosphoryl group transfers from 1,3-bisphosphoglycerate to ADP. - Catalyzed by kinase (named for reverse reaction). - Yields 3-phosphoglycerate and ATP. Highly thermodynamically favorable. 27

28 Step 8: Migration of Phosphate Shift of phosphoryl group between C-3 and C-2 of glycerate. - Catalyzed by mutase. Moves a functional group from one position to another within same molecule. - Yields 2-phosphoglycerate. 28

29 Step 9: Dehydration Removal of a molecule of water. - Catalyzed by enolase. - Yields phosphoenolpyruvate. 29

30 Step 9: Dehydration Why? - Generate a high-energy phosphate compound. - Prepare to produce ATP. This phosphate compound has a very high standard free energy of hydrolysis (-62 kj/mol). 30

31 Step 10: Second Production of ATP Phosphoryl group transfers from phosphoenolpyruvate to ADP. - Catalyzed by kinase (named for reverse reaction). - Yields pyruvate and ATP. Highly thermodynamically favorable. 31

32 Summary of Glycolysis Glucose + 2 NAD ADP + 2 P i -> 2 Pyruvate + 2 NADH + 2 H ATP Used 1 glucose; 2 ATP. Made 2 pyruvates; 2 NADH; 4 ATP. Glycolysis is heavily regulated. - Produce ATP only when needed. 32

33 Carbon From Glucose to Pyruvate At the end of preparatory phase, - C-1, C-2 and C-3 (and C-6, C-5 and C-4) of glucose become C-3, C-2 and C-1 of glyceraldehyde. At the end of payoff phase, - C-1, C-2 and C-3 of glucose become C-3, C-2 and C-1 of pyruvate

34 Summary 14.1 Glycolysis In glycolysis, one molecule of glucose is oxidized to two molecules of pyruvate, with energy conserved as ATP and NADH. All 10 glycolytic enzymes are in cytosol. All 10 intermediates are phosphorylated compounds. In preparatory phase, ATP is invested. In payoff phase, energy is conserved in the form of one NADH and two ATP per triose phosphate oxidized. 34

35 Week 10 Chapter 14 Glycolysis 14.1 Glycolysis 14.2 Feeder pathway for glycolysis 14.3 Fermentation 14.4 Gluconeogenesis 14.5 Pentose phosphate pathway 35

36 Feeder Pathways for Glycolysis Storage polysaccharides glycogen and starch. - Amylase -> maltose. - Phosphorylase -> glucose 1-phosphate -> glucose 6-phosphate. Disaccharides such as maltose, sucrose, and lactose. - Maltose -> 2 glucose. - Sucrose -> glucose + fructose. - Lactose -> glucose + galactose. Other monosaccharides such as fructose and mannose. - Fructose -> fructose 6-phosphate. - Galactose -> galactose 1-phosphate -> glucose 1-phosphate. - Mannose -> mannose 6-phosphate -> fructose 6-phosphate. Glucose Glucose 6-phosphate Fructose 6-phosphate 36

37 Feeder Pathways for Glycolysis polysaccharides disaccharides monosaccharides Glycolysis (preparatory phase) 37

38 Hydrolysis of Polysaccharides Dietary polysaccharides glycogen and starch. - Hydrolysis reaction: attack by water molecule. - Salivary α-amylase and pancreatic α-amylase. - α-amylase hydrolyzes (α1->4) glycosidic linkage. Maltose and maltotriose (di- and trisaccharides of glucose). Oligosaccharides called dextrins. Fragments of amylopectin containing (α1->6) linkages. β-amylase. - Found in bacteria, fungi and plants (not in animals). - Catalyze hydrolysis of α1->4 glycosidic bond. - Break starch into maltose. - Result in sweet flavor of ripe fruit. - Optimal ph is

39 Phosphorolysis of Polysaccharides Endogenous polysaccharides glycogen and starch. - Phosphorolysis reaction: attack by inorganic phosphate. - Glycogen phosphorylase or starch phosphorylase, also attacks (α1->4) linkage. - Remove one residue at a time from nonreducing end. A polymer one unit shorter. Glucose 1-phosphate, converted to glucose 6- phosphate by mutase. 39

40 Hydrolysis of Disaccharides Disaccharides must be hydrolyzed to monosaccharides before entering cells. - Hydrolyzed by enzymes on surface of intestinal epithelial cells. Dextrin + n H2O -> n glucose (dextrinase). Maltose + H2O -> 2 glucose (maltase). Lactose + H2O -> glucose + galactose (lactase). Sucrose + H2O -> glucose + fructose (sucrase). Trehalose + H2O -> 2 glucose (trehalase). - Monosaccharides actively transported into cells, into blood, and to tissues. Lactose intolerance. - Lack of lactase activity -> lactose not digested and absorbed. - Lactose converted to toxic products by bacteria in large intestine. - Causes abdominal cramps and diarrhea. 40

41 Other Monosaccharides: Fructose Fructose is phosphorylated by hexokinase. - Present in free form in fruits and formed by hydrolysis of sucrose. Fructose + ATP -> fructose 6-phosphate + ADP (hexokinase). Fructose + ATP -> fructose 1-phosphate + ADP (liver hexokinase). Fructose 1-phosphate -> DHAP + glyceraldehyde (aldolase). Glyceraldehyde -> glyceraldehyde 3-phosphate (triose kinase). 41

42 Monosaccharides: Galactose & Mannose Galactose is phosphorylated and epimerized. - Formed by hydrolysis of lactose. Galactose + ATP galactose 1-phosphate + ADP. Galactose 1-phosphate glucose 1-phosphate. Mannose is phosphorylated and isomerized. - Formed by digestion of polysaccharides and glycoproteins. Mannose + ATP mannose 6-phosphate + ADP. Mannose 6-phosphate fructose 6-phosphate. 42

43 Summary 14.2 Feeder Pathway Ingested polysaccharides and disaccharides are converted to monosaccharides by hydrolytic enzymes. Monosaccharides enter epithelial cells, and are transported to other tissues. Endogenous glycogen and starch enter glycolysis in two steps: - Formation of glucose 1-phosphate. - Conversion to glucose 6-phosphate. Other hexoses enter glycolysis by phosphorylation and conversion to G6P, F6P, or F1P. 43

44 Glycolysis Review 44

45 Week 10 Chapter 14 Glycolysis 14.1 Glycolysis 14.2 Feeder pathway for glycolysis 14.3 Fermentation 14.4 Gluconeogenesis 14.5 Pentose phosphate pathway 45

46 Fates of Pyruvate Glycolysis is only the first stage of glucose degradation. Pyruvate is further metabolized via one of three routes. - Citric acid cycle. Aerobic condition. Complete oxidation to CO2 and H2O. - Lactic acid fermentation. Low oxygen condition (hypoxia). Reduced to lactate. - Ethanol fermentation. Hypoxic or anaerobic condition. Reduced to ethanol and CO2. 46

47 Anaerobic Glycolysis: Fermentation Under hypoxic conditions, NADH generated by glycolysis cannot be reoxidized to NAD + by O2. - No NAD + -> no electron acceptor for oxidation of glyceraldehyde 3-phosphate. - Glycolysis stops -> no ATP production. - NAD + must be regenerated in some other way. Fermentation. - Pyruvate reduced to lactate or ethanol. - NADH oxidized (NAD + regenerated) for further glycolysis. 47

48 Anaerobic Glycolysis: Fermentation Generation of energy (ATP) without consuming oxygen or NAD +. - Hypoxic (low-oxygen) or anaerobic (no-oxygen) condition. - NAD + reduced in glycolysis (step 6), and re-oxidized in pyruvate fermentation. No net change in oxidation state of carbon in glucose. - Glucose (C6H12O6) -> lactate (C3H6O3) or ethanol + CO2 (C2H6O + CO2 = C3H6O3). Used in production of food such as beer, yogurt, and soy sauce. 48

49 Lactic Acid Fermentation Occurs in erythrocytes and active skeletal muscles. - Erythrocytes have no mitochondria and cannot oxidize pyruvate to CO2. - Oxygen cannot be carried to muscle fast enough to oxidize pyruvate. Pyruvate reduced to lactate, and NAD + regenerated. - Catalyzed by lactate dehydrogenase (named for reverse reaction). - NADH oxidized to NAD + (pyruvate is electron acceptor). 49

50 Lactic Acid Fermentation Product lactate can be recycled. - Carried in blood to liver. - Converted to glucose slowly in liver. Product accumulation prevents continuous strenuous exercise. - During vigorous muscle contraction lactate builds up quickly (< 1 minute). - Acidification from ionization of lactic acid limits period of vigorous activity. Requires a recovery time. - High amount of oxygen consumption to fuel gluconeogenesis. - Restores muscle glycogen stores. 50

51 Ethanol Fermentation Occurs in brewer s and baker s yeast. Pyruvate reduced to ethanol and CO2 in two steps. - First step catalyzed by pyruvate decarboxylase (no redox). - Second step catalyzed by alcohol dehydrogenase (reduction). Humans do not have pyruvate decarboxylase. - We do express alcohol dehydrogenase for ethanol metabolism. 51

52 Ethanol Fermentation CO 2 produced in the first step is responsible for: - Carbonation in beer and champagne. - Dough rising when baking bread. Both enzymes require cofactors. - Pyruvate decarboxylase: Mg 2+ and thiamine pyrophosphate - Alcohol dehydrogenase: Zn 2+ and NAD +. 52

53 Applications of Fermentation Food production. - Yogurt. Carbohydrate in milk -> lactic acid -> drop in ph -> sour taste. Drop in ph -> milk proteins precipitate -> thick texture. - Swiss cheese. Carbohydrate in milk -> propionic acid -> drop in ph -> milk proteins precipitate. Bubbles of CO2 -> characteristic holes known as eyes. - Pickles, sausage, kimchi. Food preservation. - Drop in ph -> Microorganisms causing food spoilage cannot grow at low ph. Biofuel production. - Ethanol and more. 53

54 Summary 14.3 Fermentation NADH must be recycled to regenerate NAD +, which is required in payoff phase. Under aerobic conditions, electron pass from NADH to O2 in mitochondria. Under anaerobic or hypoxic conditions, NAD + is regenerated by transferring electrons from NADH to pyruvate, forming lactate (or ethanol and CO2). Food fermentation causes changes in ph, taste, and texture. Fermentation is also used in industry to produce valuable organic compounds. 54

55 Week 10 Chapter 14 Glycolysis 14.1 Glycolysis 14.2 Feeder pathway for glycolysis 14.3 Fermentation 14.4 Gluconeogenesis 14.5 Pentose phosphate pathway 55

56 Gluconeogenesis means new formation of sugar. - Gluco- means glucose. - Neo- means new. - Genesis means production. Pyruvate converted to glucose. - Lactate. What is Gluconeogenesis? - Glycerol. - Certain amino acids. 56

57 Glycolysis vs. Gluconeogenesis 7 are reactions in opposite directions. 3 are different reactions. Step 1 Step 3 Step 10 57

58 Glycolysis vs. Gluconeogenesis Reversible reactions are used by both pathways. - 7 of 10 reactions in gluconeogenesis are the reverse of glycolytic reactions. Irreversible reaction of glycolysis must be bypassed. - 3 reactions are highly thermodynamically favorable (irreversible in vivo). - Different enzymes in the different pathways. Gluconeogenesis occurs mainly in liver. Glycolysis occurs mainly in muscle and brain. Step 1 Irreversible Step 2 Reversible 58

59 Irreversible Reactions in Cells 3 reactions in glycolysis are essentially irreversible in vivo. - A large negative free energy change. - Cannot be used in gluconeogenesis. Glucose -> glucose 6-phosphate. Fructose 6-phosphate -> fructose 1,6-bisphosphate. Phosphoenolpyruvate -> pyruvate. - All the other 7 glycolytic reactions have a intracellular ΔG near 0. 59

60 Pyruvate to Phosphoenolpyruvate Requires two energy-consuming steps. - Pyruvate -> oxaloacetate. - Oxaloacetate -> phosphoenolpyruvate (PEP). 60

61 Step 1: Pyruvate to Oxaloacetate Pyruvate converted to oxaloacetate (carboxylation). - Pyruvate transported from cytosol to mitochondria. - Catalyzed by pyruvate carboxylase (mitochondrial enzyme). - 1 molecule of ATP consumed. Oxalic acid 草酸 Acetic acid 61

62 Step 2: Oxaloacetate to PEP Oxaloacetate converted to phosphoenolpyruvate (decarboxylation and phosphorylation). - Occurs in cytosol or mitochondria. - Catalyzed by phosphoenolpyruvate carboxykinase. - 1 molecule of GTP consumed. Pyruvate + ATP + GTP PEP + ADP + GDP Intracellular ΔG = -25 kj/mol. Irreversible inside the cell. 62

63 A Second Pyruvate to PEP Bypass Pyruvate as precursor. 1)Transported to mitochondria. 2)Converted to oxaloacetate. 3)Converted to malate. 4)Transported to cytosol. 5)Converted to oxaloacetate. 6)Converted to PEP. Lactate as precursor. 1)Converted to pyruvate. 2)Transported to mitochondria. 3)Converted to oxaloacetate. 4)Converted to PEP. 5)Transported to cytosol. Oxaloacetate NO transporter Malate Transporter present 63

64 Gluconeogenesis is Expensive Two additional bypass reactions. - Fructose 1,6-bisphosphate -> fructose 6-phosphate. Catalyzed by fructose 1,6-bisphosphatase. - Glucose 6-phosphate -> glucose. Catalyzed by glucose 6-phosphatase. Gluconeogenesis is energetically expensive. - Gluconeogenesis. 2 Pyruvate + 4 ATP + 2 GTP + 2 NADH -> glucose + 4 ADP + 2 GDP + 2 NAD +. - Glycolysis. Glucose + 2 ADP + 2 NAD + -> 2 pyruvate + 2 ATP + 2 NADH. 64

65 Summary 14.4 Gluconeogenesis Gluconeogenesis is a multistep process in which glucose is produced from pyruvate, lactate, or other compounds. Seven reversible reaction steps are catalyzed by same enzymes used in glycolysis. Three irreversible steps are bypassed by using different enzymes. - Pyruvate to PEP. - Fructose 1,6-bisphosphate to fructose 6-phosphate. - Glucose 6-phosphate to glucose. Formation of one molecule of glucose from pyruvate requires 4 ATP, 2 GTP, and 2 NADH. It is expensive. 65

66 Week 10 Chapter 14 Glycolysis 14.1 Glycolysis 14.2 Feeder pathway for glycolysis 14.3 Fermentation 14.4 Gluconeogenesis 14.5 Pentose phosphate pathway 66

67 Pentose Phosphate Pathway (PPP) Oxidative phase. - Hexose -> pentose. - CO2. - NADP + -> NADPH. Nonoxidative phase. - Pentose -> hexose. Rapidly dividing cells need ribose 5-phosphate to make RNA and DNA. Erythrocytes need NADPH to prevent oxidative damage. 67

68 Oxidative Phase -> Pentose + NADPH 1)Oxidation. Dehydrogenase. Alcohol -> aldehyde. NADP + -> NADPH. 2)Hydrolysis. Lactonase. Ester -> acid + alcohol. Ring opens. 3)Oxidation and decarboxylation. Dehydrogenase. 6-C acid -> 5-C ketone + CO2. NADP + -> NADPH. 4)Isomerization. lactone glucono-δ-lactone Ketose -> Aldose. Overall Reaction: Glucose 6-phosphate + 2 NADP + -> ribose 5-phosphate + CO2 + 2 NADPH. Net result: NADPH (reductant) and ribose (nucleotide precursor). 68

69 Nonoxidative Phase: Pentose -> Glucose Used in tissues that require more NADPH than pentose. - 6 pentoses (5-C) converted back to 5 hexoses (6-C). - More NADP + can be reduced to NADPH in oxidative phase. 69

70 Transketolase Transfers 2-Carbon Unit Transfer of a two-carbon fragment from a ketose to an aldose. - Example: 5-carbon + 5-carbon -> 7-carbon + 3-carbon. 70

71 Transaldolase Transfers 3-Carbon Unit Transfer of a three-carbon fragment. - 7-carbon + 3-carbon -> 4-carbon + 6-carbon. 71

72 NADPH Regulates PPP Glucose 6-phosphate has two routes. - Glycolysis for ATP. - Pentose phosphate pathway for NADPH + pentose. NADPH regulates partitioning between glycolysis and pentose phosphate pathway. - If NADPH is quickly used by cell -> increased flux of glucose 6-phosphate through PPP. - If NADPH accumulates -> PPP slows and glucose 6-phosphate used in glycolysis. 72

73 Summary 14.5 Pentose Phosphate Pathway In oxidative pentose phosphate pathway, oxidation and decarboxylation at C-1 of glucose 6-phosphate generate pentose phosphate (nucleotide synthesis) and CO2, reducing NADP + to NADPH (reducing power). The first phase (oxidative) of PPP converts hexose to pentose, and reduce NADP + to NADPH. The second phase (nonoxidative) converts pentose back to hexose. Transketolase and transaldolase catalyze interconversion of 3-, 4-, 5-, 6-, and 7-carbon sugars. Entry of G6P into glycolysis or PPP determined by relative concentration of NADP + and NADPH. 73

74 Last Two of the 20 Amino Acids Val, V Lys, Arg, Trp, W Leu, L Ile, I 74

75 Example Question Glucose labeled with 14 C in C-1 and C-6 gives rise in glycolysis to pyruvate labeled in: A) all three carbons. B) its carbonyl carbon. C) its carboxyl carbon. D) its methyl carbon. E) C and D

76 Example Question In an anaerobic muscle preparation, lactate formed from glucose labeled in C-3 and C-4 would be labeled in: A) all three carbon atoms. B) only the carbon atom carrying the OH. C) only the carboxyl carbon atom. D) only the methyl carbon atom. E) the methyl and carboxyl carbon atoms. 76

77 Example Question In an anaerobic muscle preparation, lactate formed from glucose labeled in C-2 would be labeled in: A) all three carbon atoms. B) only the carbon atom carrying the OH. C) only the carboxyl carbon atom. D) only the methyl carbon atom. E) the methyl and carboxyl carbon atoms. 77

78 Example Question If glucose labeled with 14 C in C-1 were fed to yeast carrying out the ethanol fermentation, where would the 14 C label be in the products? A) In C-1 of ethanol and CO2 B) In C-1 of ethanol only C) In C-2 (methyl group) of ethanol only D) In C-2 of ethanol and CO2 E) In CO2 only 78

79 Example Question Glucose, labeled with 14 C in different carbon atoms, enters the pentose phosphate pathway. The most rapid production of 14 CO2 will occur when the glucose is labeled in: A) C-1. B) C-3. C) C-4. D) C-5. E) C-6. 79

80 Example Question Which of the following reactions in glycolysis requires ATP as a substrate? A) Step 3: phosphorylation of fructose 6-phosphate by phosphofructokinase-1. B) Step 6: oxidation of glyceraldehyde-3-phosphate by dehydrogenase. C) Step 10: production of pyruvate by pyruvate kinase. D) Step 4: cleavage of fructose 1,6-bisphosphate by aldolase. E) Step 7: production of 3- phosphoglycerate by phosphoglycerate kinase. 80

81 Example Question Which of the following reactions in glycolysis produces ATP as a product? A) Step 1: phosphorylation of glucose by hexokinase. B) Step 6: oxidation of glyceraldehyde-3-phosphate by dehydrogenase. C) Step 7: production of 3- phosphoglycerate by phosphoglycerate kinase. D) Step 4: cleavage of fructose 1,6-bisphosphate by aldolase. E) Step 3: phosphorylation of fructose 6-phosphate by phosphofructokinase-1. 81

82 Example Question Which of the following reactions in glycolysis is a ketose to aldose conversion? A) Step 1: phosphorylation of glucose by hexokinase. B) Step 8: isomerization of 3-phosphoglycerate by phosphoglycerate mutase. C) Step 9: dehydration of 2-phosphoglycerate by enolase. D) Step 4: cleavage of fructose 1,6-bisphosphate by aldolase. E) Step 5: isomerization of dihydroxyacetone phosphate by triose phosphate isomerase. 82

83 Example Question The electron acceptor in the fermentation of glucose to lactate is: A) acetaldehyde. B) lactate. C) ethanol. D) NAD +. E) pyruvate. 83

84 Example Question The electron acceptor in the fermentation of glucose to ethanol is: A) acetaldehyde. B) acetate. C) ethanol. D) NAD +. E) pyruvate. 84

85 Example Question The conversion of 1 mol of fructose 1,6-bisphosphate to 2 mol of pyruvate by the glycolytic pathway results in a net formation of: A) 1 mol of NAD + and 2 mol of ATP. B) 1 mol of NADH and 1 mol of ATP. C) 2 mol of NAD + and 4 mol of ATP. D) 2 mol of NADH and 2 mol of ATP. E) 2 mol of NADH and 4 mol of ATP. 85

86 Example Question The oxidation of 3 mol of glucose by the pentose phosphate pathway may result in the production of: A) 2 mol of pentose, 4 mol of NADPH, and 8 mol of CO2. B) 3 mol of pentose, 4 mol of NADPH, and 3 mol of CO2. C) 3 mol of pentose, 6 mol of NADPH, and 3 mol of CO2. D) 4 mol of pentose, 3 mol of NADPH, and 3 mol of CO2. E) 4 mol of pentose, 6 mol of NADPH, and 6 mol of CO2. Overall Reaction: Glucose 6-phosphate + 2 NADP + -> ribose 5-phosphate + CO2 + 2 NADPH. 86