Transcript PP

Glucose Metabolism
Pratt and Cornely, Chapter 13
Glycolysis Expectations
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Memorize/learn Figure 13.2
Know overall reaction and stages
Explain chemical logic of each step
Enzyme mechanisms presented in book
Glycolysis
• Ten enzymes that take
glucose to pyruvate
• Cytosol
• ATP and NADH
Reactions and Enzymes of Glycolysis
ATP
ATP
Pi + NAD+
ADP
2x
ADP
ADP
NADH
ADP
2x
2x
ATP
2x
ATP
• Hexose and triose
phases
• Energy input and
payoff phases
Energy Input
Energy Payoff
Know...
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Substrates
Co-substrates
Products
Enzyme names
1. Hexokinase
• Previous concepts: Induced fit, kinase
• Energy use/production?
• Chemical logic?
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?
2. Phosphoglucose Isomerase
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Previous concepts: Isomerization
Energy use/production? CONCEPT: Near-equilibrium
Chemical logic?
Stereochemistry—reverse does not produce mannose!
3. PFK-1
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Previous concepts: Allosteric inhibition
Energy use/production?
Chemical logic?
First committed step of glycolysis
– Why?
– regulation
Regulation
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
Aldolase
Mechanism
5. Triose Phosphate Isomerase
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Previous concepts: Catalytic perfection
Energy use/production?
Chemical logic?
Most similar to which previous reaction?
6. Glyceraldehyde-3-P DH
• Previous concepts: Redox and
dehydrogenase
• Energy use/production?
• Chemical logic?
GAPDH Mechanism
7. Phosphoglycerate Kinase
• Previous concepts: High energy bond
• Energy use/production?
– Substrate level phosphorylation
• Chemical logic?
• Coupled to reaction 6
Coupled Reactions
• GAPDH = 6.7 kJ/mol
• PG Kinase = -18.8 kJ/mol
• Overall:
8. Phosphoglycerate Mutase
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Previous concepts: Covalent catalysis
Energy use/production?
Chemical logic?
Mutase—isomerization with P transfer
Mechanism
• Not a simple transfer
• What happens if the bisphosphate escapes?
9. Enolase
• Concept: Phosphoryl group transfer potential
• Energy use/production?
• Chemical logic?
10. Pyruvate Kinase
• Energy use/production?
• Chemical logic?
• Regulation: F-1,6-BP can act as a feedforward activator to ensure fast glycolysis
Overall Energetics
• Standard Free
energies are up and
down
• Free energies under
cellular conditions
are downhill
– Three irreversible
Fate of Pyruvate
Amino acid
and nitrogen
metabolism
Gluconeogenesis
Aerobic
Energy
Anaerobic in
higher organisms
Anaerobic in
microorganisms
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…
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
The Solution in Us
• Lactate formation
• Balanced equation
We don’t operate anaerobically...
• Most energy still
trapped in lactate
• Back to pyruvate,
then acetyl-CoA
• Citric acid cycle
Other sugars enter glycolysis
High fructose diet
puts sugars through
glycolysis while
avoiding major
regulation step
Glucose Metabolism
Overview
• Keep the main
pathway
purposes distinct
• But learn details
of chemistry and
regulation based
on similarities
Glucose Metabolism
Overview
• Gluconeogenesis
• Glycogen
metabolism
• Pentose
Phosphate
Pathway
Precursors for Gluconeogenesis
• Names of
compounds?
• Type of reaction?
• Type of enzyme?
• Cofactor(s)?
• More on lactate
processing later…
Chemistry of Gluconeogenesis
• Chemically opposite of glycolysis (mainly)
• Energetically costly—no perpetual motion
machine!
• Points of regulation
Glycolysis
• Gluconeogenesis
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Step 1: costs 1 ATP
Step 3: costs 1 ATP
Step 7: makes 2 ATP
Step 10: makes 2
ATP
Step 10: no change
Step 8: no change
Step 3: costs 2 ATP
Step 1: costs 4 ATP
equivalents
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?
Mechanism
• Mixed anhydride
• Coupled through
biotin coenzyme
Step 1b
• PEP carboxykinase
– ATP cost to restore PEP
– CO2 loss drives rxn
Step 8
• Fructose-1,6-bisphosphatase
• No additional energy input
• Phosphate ester hydrolysis is spontaneous
Step 10
• Glucose 6-phosphatase
– Liver (and others)
– Not in muscle
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.
Key Regulation
• At the committed step in glucogenic cells
• Principle of Reciprocal regulation
• Local regulation vs Hormone regulation
Key Regulation
• Local regulation
– AMP/ATP (energy charge)
– Citrate (feedback)
• Hormone regulation
– Fructose-2,6-bisphosphate
• Gluconeogenesis is inhibited
• Glycolysis is stimulated
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?
Glucose Metabolism
Overview
• Gluconeogenesis
• Glycogen
metabolism
• Pentose
Phosphate
Pathway
Glycogen
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Storage molecule
Primer necessary
Very large!
Multiple ends allow
for quick synthesis
and degradation
Chemistry of Synthesis
• Step 1
• Near equilibrium
• The link to glucose-6-phophate, our central
molecule
Chemistry of Synthesis
• Step 2
• Count high energy
bonds
• Pyrophosphatase
– Common motiff
• UDP-glucose:
activated for
incorporation
Chemistry of Synthesis
• Step 3
• Glycogen
synthase
• Growing end is
non-reducing
• UDP released
Energetics of Synthesis
• Total cost: one ATP equivalent from G-6-p
Chemistry of Degradation
• Glycogen
phosphorylase
• Key Regulation site
• Inorganic phosphate
as a nucleophile
• Remake G-1-P with
no ATP cost
Overall Energetics
Key Enzymes
Glycogen Storage Diseases
Many disrupt glycogen breakdown in muscle and/or liver
(hypoglycemia, enlarged liver, muscle cramps...)
Glucose Metabolism
Overview
• Gluconeogenesis
• Glycogen
metabolism
• Pentose
Phosphate
Pathway
Pentose Phosphate Pathway
• Dual Purpose
– Synthesis of “reducing potential”
– Synthesis of 5-carbon sugars
• At cost of one carbon worth of carbohydrate
• Net reaction:
Complex, 2-Stage Process
• Oxidative Stage
– Generates
reducing power
and ribose
• Non-oxidative
stage
– Regenerates 3- and
6-carbon sugars
from 5 carbon
sugars
Oxidative Stage Step 1:
• G-6-P DH
• Lactone formation
Oxidative Stage Step 2:
• Also a spontaneous hydrolysis
• Practice mechanism, carbohydrate orientation
Oxidative Stage Step 3:
• Oxidative decarboxylation
• We will see this process again
Biosynthesis of Ribose
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
PPP Reactions
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Epimerase
Isomerase
Transketolase
Transaldolase
Transketolase
• Use cofactor (B1) to overcome chemical problem
Mechanism
Different Modes for Different Purposes
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.
Summary
of glucose
metabolism