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Chapter 13 - The Citric Acid Cycle
• The citric acid cycle is involved in the aerobic catabolism of
carbohydrates, lipids and amino acids
• Intermediates of the cycle are starting points for many
biosynthetic reactions
• Enzymes of the cycle are in the mitochondria of eukaryotes
• Energy of the oxidation reactions is largely conserved as
reducing power (stored electrons)
• Coenzymes reduced:
NAD+
NADH
FAD
FADH2
Ubiquinone (Q)
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Reduced Ubiquinone (QH2)
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Transport of Pyruvate from the cytosol
into the Mitochondria
• Pyruvate translocase transports pyruvate into the
mitochondria in symport with H+
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Pyruvate dehydrogenase
complex
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Conversion of Pyruvate to Acetyl CoA
• Pyruvate dehydrogenase complex is a multienzyme
complex containing:
3 enzymes + 5 coenzymes + other proteins
E1 = pyruvate dehydrogenase
E2 = dihydrolipoamide acetyltransferase
E3 = dihydrolipoamide dehydrogenase
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Components of the PDH Complex
in mammals and E. coli
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Fig 13.1 Reactions of the PDH complex
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Fig 13.1 Reactions of the PDH complex
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Fig 13.1 Reactions of the PDH complex
Acetylated
lipoamide
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Fig 13.1 Reactions of the PDH complex
TCA
cycle
Reduced
lipoamide
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Fig 13.1 Reactions of the PDH complex
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Oxidized
lipoamide
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Fig 13.1 Reactions of the PDH complex
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Oxidized
lipoamide
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Fig 13.1 Reactions of the PDH complex
Acetylated
lipoamide
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Fig 13.1 Reactions of the PDH complex
TCA
cycle
Reduced
lipoamide
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Fig 13.1 Reactions of the PDH complex
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Oxidized
lipoamide
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The Citric Acid Cycle Oxidizes AcetylCoA
• Table 13.1
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Summary of the citric acid cycle
• For each acetyl CoA which enters the cycle:
(1) Two molecules of CO2 are released
(2) Coenzymes NAD+ and Q are reduced
to NADH and QH2
(3) One GDP (or ADP) is phosphorylated
(4) The initial acceptor molecule
(oxaloacetate) is reformed
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Fig 13.3
• Citric acid cycle
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Fig 13.3
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Fig 13.3
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6. The Succinate Dehydrogenase (SDH) Complex
• Located on the inner mitochondrial membrane, in contrast to
other enzymes of the TCA cycle which are dissolved in the
mitochondrial matrix
• Complex of polypeptides, FAD and iron-sulfur clusters
• Electrons are transferred from succinate to FAD, forming FADH2,
then to ubiquinone (Q), a lipid-soluble mobile carrier of electrons
• Reduced ubiquinone (QH2) is released as a mobile product
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Fig 12.4
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• Fates of carbon
atoms in the cycle
• 6C5C4C
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Energy conservation by the cycle
• Energy is conserved
in the reduced
coenzymes NADH,
QH2 and one GTP
• NADH, QH2 can be
oxidized to produce
ATP by oxidative
phosphorylation
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Reduced Coenzymes Fuel the Production of ATP
• Each acetyl CoA entering the cycle nets:
(1) 3 NADH
(2) 1 QH2
(3) 1 GTP (or 1 ATP)
• Oxidation of each NADH yields 2.5 ATP
• Oxidation of each QH2 yields 1.5 ATP
• Complete oxidation of 1 acetyl CoA = 10 ATP
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Fig 13.10 Glucose degradation via glycolysis,
citric acid cycle, and oxidative phosphorylation
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Regulation of the Citric Acid Cycle
• The citric acid cycle is controlled by:
(1) Allosteric modulators
(2) Covalent modification of cycle enzymes
(3) Supply of acetyl CoA
(4) Regulation of pyruvate dehydrogenase
complex controls acetyl CoA supply
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Fig 13.11 Regulation of the pyruvate
dehydrogenase complex
• Increased levels of acetyl CoA and NADH inhibit E2, E3
• Increased levels of CoA and NAD+ activate E2, E3
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Fig 13.12 Regulation of mammalian PDH
complex by covalent modification
• Phosphorylation/dephosphorylation of E1
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Regulation of isocitrate dehydrogenase
Mammalian ICDH
• Activated by calcium (Ca2+) and ADP
• Inhibited by NADH
(-)
NAD+
+
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+
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NADH
Regulation of the
citric acid cycle
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Entry and Exit of Metabolites
• Intermediates of the citric acid cycle are
precursors for carbohydrates, lipids, amino
acids, nucleotides and porphyrins
• Reactions feeding into the cycle replenish the
pool of cycle intermediates
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Fig 13.13
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1. Citrate Synthase
• Citrate formed from acetyl CoA and oxaloacetate
• Only cycle reaction with C-C bond formation
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2. Aconitase
• Elimination of H2O from citrate to form C=C
bond of cis-aconitate
• Stereospecific addition of H2O to cis-aconitate
to form 2R,3S-Isocitrate
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3. Isocitrate Dehydrogenase
• Oxidative decarboxylation of isocitrate to
a-ketoglutarate (a metabolically irreversible reaction)
• One of four oxidation-reduction reactions of the cycle
• Hydride ion from the C-2 of isocitrate is transferred to
NAD+ to form NADH
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4. The a-Ketoglutarate Dehydrogenase Complex
• Similar to pyruvate dehydrogenase complex
E1 - a-ketoglutarate dehydrogenase (with TPP)
E2 - succinyltransferase (with flexible lipoamide prosthetic group)
E3 - dihydrolipoamide dehydrogenase (with FAD)
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5. Succinyl-CoA Synthetase
• Free energy in thioester bond of succinyl CoA
is conserved as GTP (or ATP in plants and
some bacteria)
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6. The Succinate Dehydrogenase (SDH) Complex
• Located on the inner mitochondrial membrane, in contrast to
other enzymes of the TCA cycle which are dissolved in the
mitochondrial matrix
• Complex of polypeptides, FAD and iron-sulfur clusters
• Electrons are transferred from succinate to FADH2, then to
ubiquinone (Q), a lipid-soluble mobile carrier of electrons
• Reduced ubiquinone (QH2) is released as a mobile product
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7. Fumarase
• Addition of water to the double bond of
fumarate to form malate
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8. Malate Dehydrogenase
• Oxidation of malate to oxaloacetate, with
transfer of electrons to NAD+ to form NADH
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The Glyoxylate Cycle
• Pathway for the formation of glucose from noncarbohydrate
precursors in plants, bacteria and yeast (not animals)
• Glyoxylate cycle leads from 2-carbon compounds to glucose
• In animals, acetyl CoA is not a carbon source for the net
formation of glucose (2 carbons of acetyl CoA enter cycle, 2 are
released as 2 CO2)
• Allows for the formation of glucose from acetyl CoA
• Ethanol or acetate can be metabolized to acetyl CoA and then to
glucose via the glyoxylate cycle
• Stored seed oils in plants are converted to carbohydrates during
germination
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Fig 13.14
The Glyoxylate Cycle
bypasses the two
decarboxylation steps
of the citric acid cycle,
conserving the carbon
atoms as glyoxylate for
synthesis of glucose.
Germinating seeds use
this pathway to
synthesize sugar
(glucose) from oil
(triacylglycerols).
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Glyoxylate cycle in germinating castor beans
• Conversion of acetyl CoA to glucose requires the transfer of
metabolites among three metabolic compartments
(1) The glyoxysome (2) The cytosol (3) The mitochondria
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Fig 13.14 Isocitrate lyase: first bypass
enzyme of glyoxylate cycle
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Malate synthase: second bypass
enzyme of glyoxylate cycle
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Bypass reactions of glyoxylate cycle
Citric Acid
Cycle
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