The citric acid cycle

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Chapter 16 The citric acid cycle
You will be tested on the entire chapter
Citric acid cycle also known as the
tricarboxylic acid (TCA) cycle or the Krebs
cycle
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Catabolism of proteins, fats, carbohydrates in
the 3 stages of cellular respiration
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(No Transcript)
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After Glycolysis
What next
Glucose 2 ADP 2 Pi 2 NAD
Glycolysis
2 Pyruvate 2 ATP 2 NADH 2 H 2 H2O

• Cell needs to regenerate the NAD that was
  utilized in glycolysis
• NAD regeneration can be accomplished by
  aerobic or anaerobic metabolism

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Three possible fates of pyruvate
Aerobic Metabolism
Pyruvate
NADH H
Anaerobic metabolism
Lactate dehydrogenase
Acetaldehyde
NAD
Acetyl CoA
NADH H
Alcohol dehydrogenase
L-lactate
NAD
Homolactic fermentation
TCA cycle
Ethanol
Alcoholic fermentation
Electron transport
Adapted from Chemistry 153A Course Compendium by
M. A. Bates
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Aerobic Metabolism
Pyruvate dehydrogenase complex
A cluster of 3 enzymes located in the -
mitochondria of eukaryotic cells - cytosol of
prokaryotes A classic well studied example of a
multienzyme complex
Non-covalent stable aggregation of enzymes
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Overall reaction catalyzed by the pyruvate
dehydrogenase complex
Oxidative decarboxylation
Irreversible reaction Requires sequential action
of - 3 different enzymes 5 different
coenzymes
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• Different Coenzymes or Prosthetic Groups
  Required for Pyruvate Dehydrogenase Complex are
• Thiamine pyrophosphate (TPP)
• Flavin adenine dinucleotide (FAD)
• Coenzyme A (CoA)
• Nicotinamide adenine dinucleotide (NAD)
• Lipoate

Four different vitamins required in human
nutrition are vital components of this system
Thiamine (in TPP) Riboflavin (in FAD) Niacin
(in NAD) Pantothenate (in CoA)
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• Thiamine pyrophosphate (TPP)

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2. Flavin adenine dinucleotide (FAD)
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3. Coenzyme A (CoA)
Critical to its role as an acyl carrier
(vitamin B5)
Hydroxyl group of pantothenic acid is joined to
ADP moeity
Modified ADP
Carboxylic group attached to b-mercaptoethylamine
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4. Nicotinamide adenine dinucleotide (NAD)
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5. Lipoate
Lipoate has 2 thiol groups
Lipoyllysyl moiety is the prosthetic group of
dihydrolipoyl transacetylase (E2)
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Pyruvate Dehydrogenase Complex

• Consists of 3 distinct enzymes
• 1) Pyruvate dehydrogenase (E1)
• 2) Dihydrolipoyl transacetylase (E2)
• 3) Dihydrolipoyl dehydrogenase (E3)
• Form non-covalent stable aggregation of enzymes
• Each present in multiple copies
• The of copies of each enzyme the actual size
• of the complex varies among organisms

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Pyruvate Dehydrogenase Complex from E. coli

• E2 forms the core of the cluster to which
• other enzymes are attached
• E2 24 copies Contains bound lipoate
• E1 24 copies Contains bound TPP
• E3 12 copies Contains bound FAD

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Reaction sequence of the pyruvate dehydrogenase
complex
1) Pyruvate TPP-E1
CO2
Hydroxyethyl-TPP-E1 2) Hydroxyethyl-TPP-E1
Lipoamide-E2 TPP-E1
Acetyl-dihydrolipoamide-E2 3)
Acetyl-dihydrolipoamide-E2 CoA
Acetyl-CoA Dihydrolipoamide-E2 4)
Dihydrolipoamide-E2 FAD-E3 cysS-Scys
Lipoamide-E2 FADH2-E3-cysS-cysS 5)
FADH2-E3-cysS-cysS NAD
FAD-E3-S-S NADH H
Overall Reaction
Pyruvate NAD CoA
Acetyl-CoA NADH H CO2
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Enzyme Reaction Catalyzed E1 Reaction 1
2 E2 Reaction 3 E3 Reaction 4 5
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• Aerobic oxidation of glucose that includes
• the oxidation of pyruvate provides
• energy in the brain
• Thiamine (vitamin B1) deficiency
• - Unable to oxidize pyruvate normally
• Beriberi, a disease that is characterized by
• loss of neural function is due to thiamine
• deficiency

Habitual consumption of large amounts of alcohol
can lead to thiamine deficiency
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Beriberi - is a disease caused by a deficiency
of thiamine (vitamin B1) - that affects the
muscles, heart, nerves, and digestive system -
literally means "I can't, I can't" in Singhalese,
which reflects the crippling effect it has on its
victims - is common in parts of southeast Asia,
where white rice is the main food
http//www.nlm.nih.gov/medlineplus/all_healthtopic
s.htmlP
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Reactions of the citric acid cycle
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4
4
6
4
5
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Reactions of the citric acid cycle
Steps 1, 3, 4 irreversible
4 out of 8 steps are oxidations
Energy is conserved by electron transfer to FAD
or NAD, forming FADH2 or NADH H
All of these reactions take place in mitochondria
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Citric acid cycle Practice sheet 1
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Citric acid cycle Practice sheet 2
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1) Formation of citrate
Condensation of acetyl CoA with oxaloacetate
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2
S-CoA
intermediate
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Citrate synthase
Oxaloacetate
A stable analog of acetyl-CoA bound
Open form of enzyme
Closed form with oxaloacetate
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2) Formation of Isocitrate via cis-Aconitate
The equilibrium mixture at pH 7.4 25 oC
contains less than 10 isocitrate In the
cell the reaction is pulled to the right because
isocitrate is consumed rapidly, lowering its
steady state concentration
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Iron-sulfur center in aconitase
4
3
1
2
Basic residue (keeps the citrate in active site)
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Aconitase
Fe
S
Fe
S
4Fe-4S cluster