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Transcript of Glucose Metabolism Pratt and Cornely, Chapter 13.
![Page 1: Glucose Metabolism Pratt and Cornely, Chapter 13.](https://reader035.fdocuments.us/reader035/viewer/2022062421/56649db05503460f94a9e7ed/html5/thumbnails/1.jpg)
Glucose Metabolism
Pratt and Cornely, Chapter 13
![Page 2: Glucose Metabolism Pratt and Cornely, Chapter 13.](https://reader035.fdocuments.us/reader035/viewer/2022062421/56649db05503460f94a9e7ed/html5/thumbnails/2.jpg)
Glycolysis Expectations
• Memorize/learn Figure 13.2• Know overall reaction and stages• Explain chemical logic of each step• Enzyme mechanisms presented in book
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Glycolysis
• Ten enzymes that take glucose to pyruvate
• Cytosol• ATP and NADH
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Reactions and Enzymes of Glycolysis
• Hexose and triose phases
• Energy input and payoff phases
ATP ATP
ADP ADP2x
Pi + NAD+
NADH
2x
ADP ADP
ATP ATP
2x 2x
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Energy Input
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Energy Payoff
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Know...
• Substrates • Co-substrates • Products • Enzyme names
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1. Hexokinase
• Previous concepts: Induced fit, kinase• Energy use/production? • Chemical logic?
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Problem 3• (Notice miswording) The DGo’ value for
hexokinase is -16.7 kJ/mol, and the DG value under cellular conditions is similar.– What is the ratio of G-6-P to glucose under standard
conditions at equilibrium if the ratio of ATP:ADP is 10:1?
– How high would the ratio of G-6-P to glucose have to be to reverse the hexokinase reaction by mass action?
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2. Phosphoglucose Isomerase• Previous concepts: Isomerization• Energy use/production? CONCEPT: Near-equilibrium• Chemical logic?• Stereochemistry—reverse does not produce mannose!
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3. PFK-1• Previous concepts: Allosteric inhibition• Energy use/production? • Chemical logic?• First committed step of glycolysis
– Why?– regulation
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Regulation
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4. Aldolase• Previous concepts: Standard free energy is +23kJ, but it
is a near equilibrium reaction• Energy use/production? • Chemical logic?• Beginning of triose stage
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Aldolase Mechanism
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5. Triose Phosphate Isomerase• Previous concepts: Catalytic perfection• Energy use/production? • Chemical logic?• Most similar to which previous reaction?
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6. Glyceraldehyde-3-P DH• Previous concepts: Redox and
dehydrogenase• Energy use/production? • Chemical logic?
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GAPDH Mechanism
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7. Phosphoglycerate Kinase • Previous concepts: High energy bond• Energy use/production?
– Substrate level phosphorylation• Chemical logic?• Coupled to reaction 6
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Coupled Reactions
• GAPDH = 6.7 kJ/mol• PG Kinase = -18.8 kJ/mol• Overall:
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8. Phosphoglycerate Mutase• Previous concepts: Covalent catalysis• Energy use/production? • Chemical logic?• Mutase—isomerization with P transfer
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Mechanism
• Not a simple transfer• What happens if the bisphosphate escapes?
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9. Enolase• Concept: Phosphoryl group transfer potential• Energy use/production? • Chemical logic?
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10. Pyruvate Kinase• Energy use/production? • Chemical logic?• Regulation: F-1,6-BP can act as a feed-
forward activator to ensure fast glycolysis
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Overall Energetics
• Standard Free energies are up and down
• Free energies under cellular conditions are downhill – Three irreversible
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Fate of Pyruvate
Aerobic Energy
Anaerobic inmicroorganisms
Anaerobic inhigher organisms
Gluconeogenesis
Amino acidand nitrogenmetabolism
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The Problem of Anaerobic Metabolism
• With oxygen, the NADH produced in glycolysis is re-oxidized back to NAD+
• NAD+/NADH is a co-substrate which means…• If there is no oxygen, glycolysis will stop
because…• The solution to the problem is to…
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The solution in Yeast• Pyruvate is decarboxylated
(cofactor?) to acetaldehyde• Acetaldehyde transformed to
ethanol – What type of reaction?– What cofactor?
• NAD+ is regenerated to be reused in GAPDH
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The Solution in Us
• Lactate formation
• Balanced equation
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We don’t operate anaerobically...
• Most energy still trapped in lactate
• Back to pyruvate, then acetyl-CoA
• Citric acid cycle
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Other sugars enter glycolysis
High fructose diet puts sugars through glycolysis while avoiding major regulation step
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Glucose Metabolism Overview
• Keep the main pathway purposes distinct
• But learn details of chemistry and regulation based on similarities
O
HO
HO
OH (P)
OH
OH
DHAP
Pyruvate
Gluconeogenesis
Lactate
Amino Acids
Glycerol(Triacylglycerides)
Glycogen
Glycogen Degradation
Glycogen
Glycogen Synthesis
Ribose,NADPH
ATP
DHAP
Pyruvate
Pentose Phosphate Pathway
Energy Production
Starch
Diet
![Page 33: Glucose Metabolism Pratt and Cornely, Chapter 13.](https://reader035.fdocuments.us/reader035/viewer/2022062421/56649db05503460f94a9e7ed/html5/thumbnails/33.jpg)
Glucose Metabolism Overview
• Gluconeogenesis• Glycogen
metabolism• Pentose
Phosphate Pathway
O
HO
HO
OH (P)
OH
OH
DHAP
Pyruvate
Gluconeogenesis
Lactate
Amino Acids
Glycerol(Triacylglycerides)
Glycogen
Glycogen Degradation
Glycogen
Glycogen Synthesis
Ribose,NADPH
ATP
DHAP
Pyruvate
Pentose Phosphate Pathway
Energy Production
![Page 34: Glucose Metabolism Pratt and Cornely, Chapter 13.](https://reader035.fdocuments.us/reader035/viewer/2022062421/56649db05503460f94a9e7ed/html5/thumbnails/34.jpg)
Precursors for Gluconeogenesis
• Names of compounds?
• Type of reaction?• Type of enzyme?• Cofactor(s)?• More on lactate
processing later…
OH
OH
OH
OPO3
O
OH
O
O
O
NH2
O
O
O
O
O
OH
O
O
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Chemistry of Gluconeogenesis
• Chemically opposite of glycolysis (mainly)• Energetically costly—no perpetual motion
machine!• Points of regulation
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Glycolysis• Step 1: costs 1 ATP• Step 3: costs 1 ATP• Step 7: makes 2 ATP• Step 10: makes 2
ATP
• Gluconeogenesis• Step 10: no change• Step 8: no change• Step 3: costs 2 ATP• Step 1: costs 4 ATP
equivalents
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Step 1a
• Pyruvate Carboxylase– Biotin– Costs ATP to make driving force for next reaction– First step in biosynthesis of glucose and many
other molecules• Related to which amino acid?
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Mechanism
• Mixed anhydride• Coupled through
biotin coenzyme
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Step 1b
• PEP carboxykinase– ATP cost to restore PEP– CO2 loss drives rxn
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Step 8• Fructose-1,6-bisphosphatase• No additional energy input• Phosphate ester hydrolysis is spontaneous
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Step 10
• Glucose 6-phosphatase– Liver (and others)– Not in muscle
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Problem 34
• A liver biopsy of a four-year old boy indicated that the F-1,6-Bpase enzyme activity was 20% normal. The patient’s blood glucose levels were normal at the beginning of a fast, but then decreased suddenly. Pyruvate and alanine concentrations were also elevated, as was the glyceraldehyde/DHAP ratio. Explain the reason for these symptoms.
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Key Regulation• At the committed step in glucogenic cells• Principle of Reciprocal regulation• Local regulation vs Hormone regulation
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Key Regulation
• Local regulation– AMP/ATP (energy charge)– Citrate (feedback)
• Hormone regulation– Fructose-2,6-bisphosphate
• Gluconeogenesis is inhibited• Glycolysis is stimulated
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Problem 39
• Brazilin, a compound found in aqueous extracts of sappan wood, has been used to treat diabetics in Korea. It increases the activity of the enzyme that products F-2,6-BP and stimulates the activity of pyruvate kinase. What is the effect of adding brazilin to liver cells in culture? Why would brazilin be an effective treatment for diabetes?
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Glucose Metabolism Overview
• Gluconeogenesis• Glycogen
metabolism• Pentose
Phosphate Pathway
O
HO
HO
OH (P)
OH
OH
DHAP
Pyruvate
Gluconeogenesis
Lactate
Amino Acids
Glycerol(Triacylglycerides)
Glycogen
Glycogen Degradation
Glycogen
Glycogen Synthesis
Ribose,NADPH
ATP
DHAP
Pyruvate
Pentose Phosphate Pathway
Energy Production
![Page 47: Glucose Metabolism Pratt and Cornely, Chapter 13.](https://reader035.fdocuments.us/reader035/viewer/2022062421/56649db05503460f94a9e7ed/html5/thumbnails/47.jpg)
Glycogen
• Storage molecule• Primer necessary• Very large!• Multiple ends allow
for quick synthesis and degradation
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Chemistry of Synthesis
• Step 1
• Near equilibrium• The link to glucose-6-phophate, our central
molecule
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Chemistry of Synthesis• Step 2• Count high energy
bonds• Pyrophosphatase
– Common motiff• UDP-glucose:
activated for incorporation
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Chemistry of Synthesis• Step 3• Glycogen
synthase• Growing end is
non-reducing• UDP released
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Energetics of Synthesis
• Total cost: one ATP equivalent from G-6-p
O
HO
HO
O
OH
OH
P
O
O O
O
HO
HO
O
OH
OH
P-P-Uridine
O
HO
OH
OH
OH
O
O
OH
HO
O
HO
Glucose-6-P
UDP
UTP
2 Pi
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Chemistry of Degradation
• Glycogen phosphorylase
• Key Regulation site• Inorganic phosphate
as a nucleophile• Remake G-1-P with
no ATP cost
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Overall Energetics
O
HO
HO
O
OH
OH
P
O
O O
O
HO
HO
O
OH
OH
P-P-Uridine
O
HO
OH
OH
OH
O
O
OH
HO
O
HO
Glucose-6-P
UDP
UTP
2 Pi
Pi
![Page 54: Glucose Metabolism Pratt and Cornely, Chapter 13.](https://reader035.fdocuments.us/reader035/viewer/2022062421/56649db05503460f94a9e7ed/html5/thumbnails/54.jpg)
Key Enzymes
O
HO
HO
O
OH
OH
P
O
O O
O
HO
HO
O
OH
OH
P-P-Uridine
O
HO
OH
OH
OH
O
O
OH
HO
O
HO
Glucose-6-P
UDP
UTP
2 Pi
Pi
Glycogen Synthase
Glycogen Phosphorylase
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Glycogen Storage Diseases
Many disrupt glycogen breakdown in muscle and/or liver (hypoglycemia, enlarged liver, muscle cramps...)
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Glucose Metabolism Overview
• Gluconeogenesis• Glycogen
metabolism• Pentose
Phosphate Pathway
O
HO
HO
OH (P)
OH
OH
DHAP
Pyruvate
Gluconeogenesis
Lactate
Amino Acids
Glycerol(Triacylglycerides)
Glycogen
Glycogen Degradation
Glycogen
Glycogen Synthesis
Ribose,NADPH
ATP
DHAP
Pyruvate
Pentose Phosphate Pathway
Energy Production
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Pentose Phosphate Pathway
• Dual Purpose– Synthesis of “reducing potential”– Synthesis of 5-carbon sugars
• At cost of one carbon worth of carbohydrate• Net reaction:
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Complex, 2-Stage Process
• Oxidative Stage– Generates reducing
power and ribose• Non-oxidative stage
– Regenerates 3- and 6-carbon sugars from 5 carbon sugars
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Oxidative Stage Step 1:
• G-6-P DH• Lactone formation
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Oxidative Stage Step 2:
• Also a spontaneous hydrolysis• Practice mechanism, carbohydrate orientation
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Oxidative Stage Step 3:
• Oxidative decarboxylation• We will see this process again
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Biosynthesis of Ribose
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Non-oxidative Stage• To understand purpose, realize that we
generally need to make much more NADPH than ribose
• Problem: stuck with C5, but need C6 and C3• Solution: “Shunt” C5 back to C6 through near-
equilibrium reactions
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PPP Reactions
• Epimerase• Isomerase• Transketolase• Transaldolase
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Transketolase• Use cofactor (B1) to overcome chemical problem
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Mechanism
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Different Modes for Different Purposes
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Problem 58
• A given metabolite may follow more than one metabolic pathway. List all possible fates of glucose-6-P in (a) a liver cell and (b) a muscle cell.
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Summaryof glucosemetabolism