chapter 14 glycolysis, gluconeogenesis, and the pentose phosphate pathway
TRANSCRIPT
Chapter 14
Glycolysis, Gluconeogenesis, and the Pentose Phosphate Pathway
Glucose
Roles of glucose Fuel (Glucose CO2 + H2O ; ∆G = ~ -2,840 kJ/mol) Precursor for other molecules
Utilization of glucose in animals and plant 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 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
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
Citric acid cycle Complete oxidation of acetyl-CoA CO2
Electron-transfer reactions in mitochondria e- transfer to O2 to generate H2O Generation of ATP
Fermentation : anaerobic conditions (hypoxia) Lactic acid fermentation
Reduction of pyruvate to lactate 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 Glc + 2 NAD+ 2 pyruvate + 2NADH + 2H+
G’1o = -146 kJ/mol 2ADP + 2Pi 2ATP + 2H2O
G’2o = 2(30.5) = 61.0 kJ/mol Glc + 2NAD+ + 2ADP + 2Pi 2 pyruvate + 2NADH + 2H+ + 2ATP + 2H2O
G’so = G’1o + G’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 G‡ & increase reaction specificity Many glycolytic enzymes are specific for Mg2+ complexed with
phosphate groups
Glycolysis : Step 1
1. Phosphorylation of Glc
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) Phosphohexose isomerase (phosphoglucose isomerase)
Reversible reaction (small G’o)
Glycolysis : Step 3
3. Phosphorylation of Fru 6-P to Fru 1,6-bisP 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 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 Dihydroxyacetone P glyceraldehyde 3-P Triose phosphate isomerase
Glycolysis : Step 6
6. Oxidation of glyceraldehyde 3-P to 1,3-bisphosphoglycerate 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 G’o of hydrolysis
Glyceraldehyde 3-P dehydrogenase
Glycolysis : Step 7
7. Phosphoryl transfer from 1,3-bisphosphoglycerate to ADP 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) Glyceraldehyde 3-P + ADP + Pi + NAD+
3-phosphoglycerate + ATP + NADH + H+
G’o = -12.5 kJ/mol Coupling through 1,3-bisphophoglycerate
(common intermediate) Removal of 1,3-bisphosphoglycerate in
step 7 strong negative G of step 6
Glycolysis : Step 8
8. 3-phosphoglycerate to 2-phosphoglycerate Phosphoglycerate mutase Mg2+
Two step reaction with 2,3-BPG intermediate
Glycolysis : Step 9
Dehydration of 2-phosphoglycerate to phosphoenolpyruvate (PEP) 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 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 (16) branch Phosphoglucomutase
Glc 1-P Glc 6-P
Bisphosphate intermediate
Digestion of Dietary Polysaccharides and Disaccharides
Digestion of starch and glycogen -amylase in saliva
Hydrolysis of starch to oligosaccharides
Pancreatic -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
Glyceraldehyde 3-P
Triose phosphate isomerase
Triose kinase
Galactose Glactokinase; Gal Glc 1-P
Galatosemia Defects in the enzymatic pathway
Mannose Hexokinase
Man + ATP Man 6-P + ADP
Phosphomannose isomerase Man 6-P Fru 6-P
Entry of Other monosaccharides into Glycolytic Pathway
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 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 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 -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
Swiss cheese Fermentation of milk by Propionibacterium freudenreichii Propionic acid & CO2 milk protein precipitation & holes
Other fermented food 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 G 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 carboxylase Mitochondrial enzyme with biotin coenzyme Activation of pyruvate by CO2 transfer oxaloacetate
Pyruvate + HCO3- + ATP oxaloacetate + ADP + Pi
From Pyruvate to PEP
Oxaloacetate + GTP PEP + CO2 + GDP
PEP carboxykinase Cytosolic and mitochondria enzyme
Overall reaction equation Pyruvate + ATP + GTP + HCO3
-
PEP + ADP + GDP + Pi + CO2, G’o = 0.9 kJ/mol
But,G = -25 kJ/mol
Alternative paths from pyruvate to PEP
From pyruvate Oxaloacetate + NADH + H+ 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 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
Nonoxidative phase
Recycling of Ribulose 5-P to Glc 6-P
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
Oxidative Pentose Phosphate Pathway
Nonoxidative Pentose Phosphate Pathway
6 Pentose phosphates
5 Hexose phosphates Reductive pentose phosphate pathway
Reversal of nonoxidative Pentose Phosphate Pathway
Photosynthetic assimilation of CO2 by plant
Nonoxidative Pentose Phosphate Pathway
Transketolase Transfer of a 2-C fragment from a ketose donor to an aldose acceptor Thiamine pyrophosphate (TPP) cofactor
Transaldolase 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