Transcript Glycolysi

Chapter 14
Glycolysis, Gluconeogenesis, and
the Pentose Phosphate Pathway
Glucose
 Roles of glucose
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Fuel (Glucose  CO2 + H2O ; ∆G = ~ -2,840 kJ/mol)
Precursor for other molecules
 Utilization of glucose in animals and
plant
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Synthesis of structural polymers
Storage
 Glycogen, starch, or sucrose
Oxidation via glycolysis
 Pyruvate for ATP and metabolic
intermediate generations
Oxidation via pentose phosphate pathway
 Ribose 5-P for nucleic acid synthesis
 NADPH for reductive biosynthesis
 Generation of glucose
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Photosynthesis : from CO2
Gluconeogenesis (reversing glycolysis) :
from 3-C or 4-C precursors
14.1 Glycolysis
Glycolysis
Glucose
2 x Pyruvate
2 ATP & 2 NADH
Fermentation
the anaerobic degradation of glucose
ATP production
An Overview: Glycolysis
 Two phases of glycolysis (10 steps)
 Preparatory phase : 5 steps
 From Glc to 2 glyceraldehyde 3-P
 Consumption of 2 ATP molecules
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Payoff phase : 5 steps
 Generation of pyruvate
 Generation of 4 ATP from high-energy phosphate compounds
 1,3-bisphosphoglycerate, phosphoenylpyruvate
 Generation of 2 NADH
Preparatory Phase
Payoff Phase
Fates of Pyruvate
 Aerobic conditions
 Oxidative decarboxylation of pyruvate
 Generation of acetyl-CoA
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Citric acid cycle
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Electron-transfer reactions in mitochondria
 Complete oxidation of acetyl-CoA CO2
 e- transfer to O2 to generate H2O
 Generation of ATP
 Fermentation : anaerobic conditions (hypoxia)
 Lactic acid fermentation
 Reduction of pyruvate to lactate
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 NAD+ regeneration for glycolysis
Vigorously contracting muscle
Ethanol (alcohol) fermentation
 Conversion of pyruvate to EtOH and CO2
 Microorganisms (yeast)
Fate of Pyruvate
 Anabolic fates of pyruvate
 Source of C skeleton (Ala or
FA synthesis)
ATP & NADH formation coupled to
glycolysis
 Overall equation for glycolysis
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Glc + 2 NAD+  2 pyruvate + 2NADH + 2H+
 DG’1o = -146 kJ/mol
2ADP + 2Pi  2ATP + 2H2O
 DG’2o = 2(30.5) = 61.0 kJ/mol
Glc + 2NAD+ + 2ADP + 2Pi  2 pyruvate + 2NADH + 2H+ + 2ATP + 2H2O
 DG’so = DG’1o + DG’2o = -85 kJ/mol
60% efficiency in conversion of the released energy into ATP
 Importance of phosphorylated intermediates
 No export of phosphorylated compounds
 Conservation of metabolic energy in phosphate esters
 Binding energy of phosphate group
 Lower DG‡ & increase reaction specificity
 Many glycolytic enzymes are specific for Mg2+ complexed with
phosphate groups
Glycolysis : Step 1
 1. Phosphorylation of Glc
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Hexokinase
Substrates; D-glc & MgATP2-(ease nucleophilc attack by –OH of glc)
Induced fit
Soluble & cytosolic protein
Glycolysis : Step 2
 2. Glc 6-P  Fru 6-P (isomerization)
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Phosphohexose isomerase (phosphoglucose isomerase)
Reversible reaction (small DG’o)
Glycolysis : Step 3
 3. Phosphorylation of Fru 6-P to Fru 1,6-bisP
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Phosphofructokinase-1 (PFK-1)
Irreversible, committed step in glycolysis
Activation under low [ATP] or high [ADP and AMP]
Phosphoryl group donor
 ATP
 PPi : some bacteria and protist, all plants
Glycolysi : Step 4
 4. Cleavage of Fru 1,6-bisP
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Dihydroxyacetone P & glyceraldehyde 3-P
Aldolase (fructose 1,6-bisphosphate aldolase)
 Class I : animals and plant
 Class II : fungi and bacteria, Zn2+ at the active site
Reversible in cells because of lower concentrations of reactant
Class I Aldolase Reaction
Glycolysis : Step 5
 5. Interconversion of the triose phosphates
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Dihydroxyacetone P  glyceraldehyde 3-P
Triose phosphate isomerase
Glycolysis : Step 6
 6. Oxidation of glyceraldehyde 3-P to 1,3bisphosphoglycerate
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Glyceraldehyde 3-P dehydrogenase
NAD+ is the acceptor for hydride ion released from the aldehyde
group
Formation of acyl phosphate
 Carboxylic acid anhydride with phosphoric acid
 High DG’o of hydrolysis
Glyceraldehyde 3-P dehydrogenase
Glycolysis : Step 7
 7. Phosphoryl transfer from 1,3bisphosphoglycerate to ADP
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3-phosphoglycerase kinase
Substrate-level phosphorylation of ADP to
generate ATP
 c.f. Respiration-linked phosphorylation
 Coupling of step 6 (endergonic) and
step 7 (exergonic)
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Glyceraldehyde 3-P + ADP + Pi + NAD+ 
3-phosphoglycerate + ATP + NADH + H+
 DG’o = -12.5 kJ/mol
Coupling through 1,3-bisphophoglycerate
(common intermediate)
 Removal of 1,3-bisphosphoglycerate in
step 7  strong negative DG of step 6
Glycolysis : Step 8
 8. 3-phosphoglycerate to 2phosphoglycerate
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Phosphoglycerate mutase
Mg2+
Two step reaction with 2,3-BPG
intermediate
Glycolysis : Step 9
 Dehydration of 2-phosphoglycerate to
phosphoenolpyruvate (PEP)
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Enolase
Free energy for hydrolysis
 2-phosphoglycerate : -17.6 kJ/mol
 PEP : -61.9 kJ/mol
Glycolysis : Step 10
 Transfer of phosphoryl group
from PEP to ADP
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Pyruvate kinase
Substrate-level phosphorylation
Tautomerization from enol to keto
forms of pyruvate
Irreversible
 Important site for regulation
Overall Balance in Glycolysis
Glucose + 2ATP + 2NAD+ + 4ADP + Pi
2Pyruvate + 2ADP + 2NADH + 2H+ + 4ATP + 2H2O
Multienzyme complex
Substrate channeling
Tight regulation
Rate of glycolysis: anaerobic condition (2ATP)
aerobic condition (30-32)
ATP consumption
NADH regeneration
Allosteric regulation of enzymes; Hexokinase, PFK-1, pyruvate kinase
Hormone regulations; glucagon, insulin, epinephrine
Changes in gene expression for the enzymes
14.2 Feeder Pathways for Glycolysis
Entry of Carbohydrates into Glycolysis
Degradation of Glycogen and Starch
by Phosphorolysis
 Glycogen phosphorylase
 (Glc)n + Pi  Glc 1-P + (Glc)n-1
 Debranching enzyme
 Breakdown of (a16) branch
 Phosphoglucomutase
 Glc 1-P  Glc 6-P
 Bisphosphate intermediate
Digestion of Dietary Polysaccharides
and Disaccharides
 Digestion of starch and glycogen
 a-amylase in saliva
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 Hydrolysis of starch to oligosaccharides
Pancreatic a-amylase
  maltose and maltotriose, limit dextrin
 Hydrolysis of intestinal dextrins and disaccharides
 Dextrinase
 Maltase
 Lactase
 Sucrase
 Trehalase
 Transport of monosaccharide into the epithelial cells
 c.f. lactase intolerance
 Lacking lactase activity in the intestine
 Converted to toxic product by bacteria
 Increase in osmolarity  increase in water retention in the
intestine
Entry of Other monosaccharides into
Glycolytic Pathway
 Fructose
 In muscle and kidney
 Hexokinase
 Fru + ATP  Fru 6-P + ADP
 In liver
 Fructokinase
 Fru + ATP  Fru 1-P + ADP
 Fructose 1-P aldolase
Triose phosphate
isomerase
Glyceraldehyde 3-P
Triose kinase
Entry of Other monosaccharides into
Glycolytic Pathway
 Galactose
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Glactokinase; Gal  Glc 1-P
Galatosemia
 Defects in the enzymatic pathway
 Mannose
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Hexokinase
 Man + ATP  Man 6-P + ADP
Phosphomannose isomerase
 Man 6-P  Fru 6-P
14.3 Fates of Pyruvate under Anaerobic
Conditions: Fermentation
Pyruvate fates
Hypoxic conditions
- Rigorously contracting muscle
- Submerged plant tissues
- Solid tumors
- Lactic acid bacteria
Failure to regenerate NAD+
Fermentation is the way of
NAD+ regeneration
Lactic Acid Fermentation
 Lactate dehydrogenase
 Regeneration of NAD+
 Reduction of pyruvate to lactate
 Fermentation
 No oxygen consumption
 No net change in NAD+ or
NADH concentrations
 Extraction of 2 ATP
Ethanol Fermentation
 Two step process
 Pyruvate decarboxylase
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Irreversible decarboxylation of pyruvate
Brewer’s and baker’s yeast & organisms
doing ethanol fermentation
 CO2 for brewing or baking
Mg2+ & thiamine pyrophosphate (TPP)
 Alcohol dehydrogenase
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Acetaldehyde + NADH + H+  EtOH + NAD+
Human alcohol dehydrogenase
 Used for ethanol metabolism in liver
Thiamine Phyrophosphate (TPP) as
Active Aldehyde Group Carrier
 TPP
 Vitamin B1 derivative
 Cleavage of bonds adjacent to a carbonyl group
 Decarboxylation of a-keto acid
 Rearrangement of an activated acetaldehyde group
Role of Thiamine Pyrophosphate (TPP)
in pyruvate decarboxylation
 TPP
 Nucleophilic carbanion of C-2 in
thiazolium ring
 Thiazolium ring acts as “e- sink”
Fermentation in Industry
 Food
 Yogurt
 Fermentation of carbohydrate in milk by Lactobacillus bulgaricus
 Lactate  low pH & precipitation of milk proteins
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Swiss cheese
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Other fermented food
 Fermentation of milk by Propionibacterium freudenreichii
 Propionic acid & CO2  milk protein precipitation & holes
 Kimchi, soy sauce
 Low pH prevents growth of microorganisms
 Industrial fermentation
 Fermentation of readily available carbohydrate (e.g. corn
starch) to make more valuable products
 Ethanol, isopropanol, butanol, butanediol
 Formic, acetic, propionic, butyric, succinic acids
14.4 Gluconeogenesis
Gluconeogenesis
 Pyruvate & related 3-/ 4-C compounds  glucose
 Net reaction
 2 pyruvate + 4ATP + 2GTP + 2NADH + 2H+ + 4H2O  Glc +
4ADP + 2GDP + 6Pi +2NAD+
 In animals
 Glc generation from lactate, pyruvate, glycerol, and amino acids
 Mostly in liver
 Cori cycle ;
Lactate produced in muscle
 converted to glc in liver  glycogen storage or back to muscle
 In plant seedlings
 Stored fats & proteins  disaccharide sucrose
 In microorganisms
 Glc generation from acetate, lactate, and propionate in the medium
Gluconeogenesis
Glycolysis vs. Gluconeogenesis
 7 shared enzymatic reactions
 3 bypass reactions; irreversible steps requiring unique enzymes
 Large negative DG in glycolysis
 Hexokinase vs. glc 6-phosphatase
 Phosphofructokinase-1 vs. fructose 1,6-bisphosphatase
 Pyruvate kinase vs. pyruvate carboxylase + PEP carboxykinase
From Pyruvate to PEP
Pyruvate + HCO3- + ATP  oxaloacetate + ADP + Pi
 Pyruvate carboxylase
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Mitochondrial enzyme with biotin coenzyme
Activation of pyruvate by CO2 transfer  oxaloacetate
From Pyruvate to PEP
Oxaloacetate + GTP  PEP + CO2 + GDP
 PEP carboxykinase
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Cytosolic and mitochondria enzyme
 Overall reaction equation
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Pyruvate + ATP + GTP + HCO3PEP + ADP + GDP + Pi + CO2, DG’o = 0.9 kJ/mol
But, DG = -25 kJ/mol
Alternative paths from pyruvate to PEP
 From pyruvate
 Oxaloacetate + NADH + H+ 
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malate + NAD+
(mitochondria)
Malate + NAD+  oxaloacetate + NADH + H+
(cytosol)
[NADH]/[NAD+] in cytosol : 105 times lower
than in mitochondria
Way to provide NADH for gluconeogenesis
in cytosol
 From lactate
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NADH generation by oxidation of lactate
No need to generate malate intermediate
14.5 Pentose Phosphate Pathway of Glucose
Oxidation
Pentose Phosphate Pathway
 Oxidative phase; NADPH & Ribose 5-P
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Pentose ribose 5-phosphate
 Synthesis of RNA/DNA, ATP, NADH,
FADH2, coenzyme A in rapidly dividing
cells (bone marrow, skin etc)
NADPH
 Reductive biosynthesis
- Fatty acid (liver, adipose, lactating
mammary gland)
- Steroid hormones & cholesterol (liver,
adrenal glands, gonads)
 Defense from oxygen radical damages
- High ratio of NADPH/NADP+  a
reducing atmosphere  preventing
oxidative damages of macromolecules
 Nonoxidative phase
 Recycling of Ribulose 5-P to Glc 6-P
Oxidative Pentose Phosphate
Pathway
Nonoxidative Pentose Phosphate
Pathway
 6 Pentose phosphates 
5 Hexose phosphates
 Reductive pentose phosphate pathway
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Reversal of nonoxidative Pentose Phosphate
Pathway
Photosynthetic assimilation of CO2 by plant
Nonoxidative Pentose Phosphate
Pathway
 Transketolase
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Transfer of a 2-C fragment from a ketose donor to an aldose acceptor
Thiamine pyrophosphate (TPP) cofactor
 Transaldolase
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Transfer of a 3-C fragment
Lys : Schiff base with the carbonyl group of ketose
Stabilization of carbanion intermdeidate
Nonoxidative Pentose Phosphate
Pathway
Regulation of Pentose phosphate
Pathway