Download Lec 3: Carbohydrate metabolism

Lec 3: Carbohydrate metabolism
Pyruvate & where it goes…. Gluconeogenesis – synthesis of glucose from pyruvate
Reference material
Biochemistry 4th edition, Mathews, Van Holde, Appling, Anthony‐Cahill. Pearson ISBN:978‐0‐13‐800464‐4
Lehninger Principles of Biochemistry 4th edition, David L. Nelson, Michael M. Cox. W. H. Freeman ISBN:978‐0716743392
Overview of Glycolysis
Embden‐Meyerhof‐Parnas (EMP) pathway
Glyceraldehyde‐3‐phosphate
dehydrogenase
hexokinase
Phosphoglucoisomerase
Phosphofructokinase
Phosphoglycerate kinase
Phosphoglycerate mutase
enolase
aldolase
Pyruvate kinase
Triose isomerase
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Thermodynamics of
glycolysis
With exception of PGK step (Step 7), all the other steps associated with ATP consumption or generation are regulated steps in the pathway. Why?
These reactions have large decrease in ΔG, which makes them irreversible steps in vivo.
Recall that irreversible steps are the place for metabolic control!! Metabolic fate of pyruvate
•
Pyruvate can return upstream central metabolism through gluconeogenesis.
Intermediates of glycolysis/ gluconeogenesis are precursors to many amino acids, nucleotides, etc.
•
Pyruvate can form Oxaloacetate (OAA), the most important anaplerotic
reaction (reactions that replenish intermediates of TCA cycle). Anaplerotic reactions are important as intermediates of TCA cycle are used to make many amino acids
•
Pyruvate can go to acetyl‐CoA, which
enters TCA cycle.
•
Pyruvate can be used in fermentation, producing lactate and/or ethanol, which have significant importance.
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Fermentation
•
When O2 is present, respiration takes place. NADH generated passes its electrons onto
O2, forming water.
•
NAD+ is regenerated and glycolysis continues
•
However, in the lack of O2 (such as anaerobic microorganisms and intensively exercised
muscle), NADH still needs to dump its electron on somewhere in order to regenerate NAD+ for glycolysis to continue.
•
Pyruvate can be used as a electron sink when O2 is not available.
ATP
Glucose
NAD+
O2
respiration
ATP
H2O
NADH
Pyruvate
Glycolysis stops due to No NAD+
No glycolysis = No ATP
No ATP = Cell dies
Lactate or ethanol
NAD+ is replenished
Glycolysis can continue to generate ATP
Pyruvate as electron acceptor
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Lactate dehydrogenase
A hydride from NADH is transferred directly to pyruvate’s carbonyl carbon
• Even though thermodynamics favors
formation of lactate, this reaction is
reversible as cells can utilize lactate
(through oxidation back into
pyruvate)
Cori cycle: how lactate is recycled in human body
The Cori cycle:
• Lactate produced in glycolysis
during muscle exertion is
transported to the liver, for
resynthesis of glucose by
gluconeogenesis.
• Transport of glucose back to
muscle for synthesis of
glycogen, and its reutilization
in glycolysis, completes the
cycle.
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Ethanol formation • Ethanol production from pyruvate requires two enzymes: Pyruvate decarboxylase
& alcohol dehydrogenase. Most common alcohol producing microbes are
Saccharomyces serevisiae (baker’s yeast) and Zymomonas mobilis (bacteria)
• Humans have alcohol dehydrogenase, primarily for metabolizing ethanol. Ethanol
oxidation to acetaldehyde, which is then converted to acetyl‐CoA and enters TCA
cycle.
Pyruvate decarboxylase
Decarboxylation of α‐keto acid does not happen without catalysis
*Note: decarboxylation of β‐keto
acid occurs spontaneously
•
The first reaction requires thiamine pyrophosphate
as a coenzyme.
•
The vitamin is structurally complex, but its conversion
to the coenzyme form, thiamine pyrophosphate, or
TPP, involves simply an ATP-dependent
pyrophosphorylation.
•
Thiamine pyrophosphate is the coenzyme for all
decarboxylations of -keto acids.
•
The thiazole ring of TPP is the functional part of
the coenzyme, allowing it to bind and transfer
activated aldehydes.
unstable carbanion intermediate
(Vitamin B1)
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Pyruvate decarboxylase
Aerobic vs. anaerobic
•
•
•
Generating from glycolysis is much faster than from respiration, however with much
lower ATP yield.
Muscle cells utilizes this fast ATP generation when undergoing intense exercise
Cancer cells achieve high growth rate through metabolizing glucose and producing
lactate even when O2 is present.
Aerobic
(slow, good energy yield)
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Anaerobic
(fast, low energy yield)
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Different color in muscle can reflect different levels of aerobic vs. anaerobic metabolism
Lots of heme‐containing mitochondria, used in aerobic metabolism
Fewer mitochondria; heavy reliance on anaerobic metabolism
Why is gluconeogenesis important?
•
Many organisms and many cell types require a constant supply of
glucose (ex: neurons, red blood cells). •
Brain, skeletal muscle, kidney medulla, erythrocytes, and testes
use glucose as their sole or primary energy source, but they lack the enzymatic machinery to synthesize it. Liver and kidney cortex
are the primary gluconeogenic tissues.
•
In humans, glucose can be synthesized from pyruvate (or lactate, or oxaloacetate, or certain amino acids) through this pathway (mainly occurring in the liver). Particularly important for
converting human diet intake to glucose
•
Gluconeogenesis provides flexibility in metabolism. Ability to
interconvert different types of molecules is important!
•
Nature probably evolved gluconeogenesis before glycolysis.
Before organic carbons were abundant, first organisms were evolved to utilize CO and CO2 and store these resources in
carbohydrates (such as cell walls).
Emergence of life
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Gluconeogenesis – biosynthesis of glucose
Consider the exact reverse of glycolysis:
2 Pyruvate + 2 ATP + 2 NADH Glucose + 2 ADP + 2 NAD+
∆G0’ = +83.1 kJ/mol
This pathway would be very thermodynamically unfavorable
To make glucose from pyruvate more favorably….
Glucose + 4 ADP + 2 GDP + 2 NAD+
2 Pyruvate + 4 ATP + 2 GTP + 2 NADH ∆G0’ = –32.7 kJ/mol
Consume additional “4” ATP
In terms of energy GTP = ATP “ATP”
P P P
Ribose
“GTP”
P P P
Ribose
Gluconeogenesis
• Most steps are shared between glycolysis
and gluconeogenesis except the 3 regulated
reactions.
• Two phosphatases (enzymes removing
phophoryl groups with H2O) are used to
remove the phophoryl groups added by
hexokinase and PFK.
ΔG’o = ‐ 13.8 kJ/mol
ΔG’o = ‐ 16.3 kJ/mol
• Two ATP dependent reactions are used to
convert pyruvate to PEP.
• Pyruvate carboxylase
• PEP carboxykinase (PEPCK)
• In Escherichia coli, a single enzyme
called PEP synthase is used to convert
pyruvate to PEP using ATP  AMP (which has higher energy output than
ATP  ADP)
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Gluconeogenesis – Reaction 1: Pyruvate carboxylase
• Pyruvate carboxylase contains a biotin prosthetic group
• Biotin is a “CO2” carrier (just like NAD is H:‐ carrier)
Biotin
(Vitamin B7)
• ΔG’o = ‐ 2.1 kJ/mol
Pyruvate carbanion Formed from deprotonation of α‐hydrogen
*Recall that HCO3‐ is from: HCO3‐
(equilibrium with CO2)
H2CO3
Oxaloacetate
CO2 + H2O
Gluconeogenesis – Reaction 2: PEP carboxykinase
• In this step, both decarboxylation & GTP hydrolysis are used to provide
energy for making PEP (which has very high energy).
• ΔG’o = + 2.9 kJ/mol
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Gluconeogenesis – Reaction 3 & 4: Phosphatases
ΔG’o = ‐ 13.8 kJ/mol
ΔG’o = ‐ 16.3 kJ/mol
Regulations between glycolysis and gluconeogenesis
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Conditions that promote glycolysis inhibit gluconeogenesis, and vice versa.
•
Pay particular attention to major activators and inhibitors of these reactions. 10
Specific example: PFK & FBPase control with external (extracellular) signal
•
Humans have multiple organs
and cell types that need to communicate with each other!
•
Signal transductions are needed
to relay messages between one cell to another.
Here, the same enzyme can do both PFK2 & FBPase2 reactions
Because it has two domains (2 enzyme functions)
Protein phosphorylation determines which function dominates
Example of signal from outside of cell controls glycolysis & gluconeogenesis on/off
(opposite to insulin)
“PFK‐2”
“FBPase‐2”
(by protein kinase A)
ATP
cAMP dependent Protein kinase
H2O
Pi
Protein phosphatase
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