TCA Cycle

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TCA Cycle
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The 3-stage engine
glucose
Stage 2
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Topics TCA

• TCA cycle has a central role in energy metabolism
• Is fed by breakdown of carbohydrate lipid
  amino acids
• Carbon is released as CO2
• Individual reactions in the cycle
• Energy is generated as NADH, FADH2 and GTP
• Intermediates of TCA are used for biosynthesis of
  amino acids and other metabolites

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TCA cycle (with glycolysis) is the central
pathway of energy metabolism
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Different names, same process

• TCA cycle
• tricarboxylate acid cycle
• citric acid cycle
• Krebs cycle (for Sir Hans)

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(later)
Catabolism of amino acids, fats, and
carbohydrates in the three stages of cellular
respiration
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Carbon is released as CO2
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What is being accomplished chemically in TCA?
Cycle to burn C2 produce reducing equivalents
(energy) and CO2
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The overall cycle All 8 reactions

What are the net products? How do they transfer
metabolic energy?




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Individual reactions in the cycle
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Step 1 Formation of Citrate
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Citrate Synthase undergoes a dramatic
conformational change upon binding of the first
substrate, oxaloacetate
Induced Fit
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Steps 2a 2b Formation of Isocitrate via
cis-Aconitate

• dehydration of C3 to double bond
• rehydration at C2 (10 isocitrate) and C3 (90
  citrate)
• forward rxn driven by isocitrate utilization

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Iron-sulfur reaction center in aconitase
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Step 3 Oxidation of Isocitrate to
a-Ketoglutarate and CO2
1st carbon lost as CO2 1st production of NADH
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NADaccepts electrons in the form of hydride
(H-) transfer required for the oxidation of an
alcohol (isocitrate) to a ketone (a-KG) on
isocitrate deH-ase
and again at a-KG deH-ase and malate deH-ase
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Step 4 Oxidation of a-Ketoglutarate to
Succinyl-CoA and CO2

• Similar enzyme, identical mechanism to PyrDH
• E1, E2, E3
• Coenzymes TPP, lipoic acid, FAD, NAD and CoA

2nd NADH made 2nd CO2 lost
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Step 5 Conversion of Succinyl-CoA to Succinate
1 mole GTP (ATP) produced
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Step 6 Oxidation of Succinate to Fumarate
1 mole FADH2 produced
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FADcoenzyme in oxidizing succinate to fumarate
R
(H and H-)
on succinate deH-ase
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Step 7 Hydration of Fumarate to Malate
Stereospecific enzyme D-malate is not a substrate
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Step 8 Oxidation of Malate to Oxaloacetate
Reaction is driven by oxaloacetate utilization
(citrate synthase)
3rd NADH produced
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Regulation of metabolite flow from pyruvate
through the TCA cycle
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Energy is generated as NADH, FADH2 and GTP
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Products of one turn of the citric acid cycle
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12 ATP molecules produced from oxidation of 1
Acetyl CoA
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Role of the TCA cycle in anabolic (biosynthetic)
pathways
(NOT in humans)
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Example of an amino acid produced from a TCA
intermediate
Reaction is a transamination
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TCA intermediates used for biosynthesis are
replenished by side reactions
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Anaplerotic Reactions, such as that catalyzed by
Pyruvate Carboxylase, keep the TCA cylcle
replenished with intermediates used as
biosynthetic precursors. Acetyl CoA stimulates
Pyruvate Carboxylase allosterically.
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• Role of Biotin in Pyruvate Carboxylase.
• Carrier of CO2 activated by ATP,
• Covalently linked to a lysine on enzyme,
• Biotin, required in the diet, is bound by avidin,
  a protein in raw egg whites.

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• You should know
• Where the TCA cycle occurs (mitochondria)
• The reactions and intermediates of TCA cycle
• How carbohydrate carbons are converted to CO2
  (through AcCoA)
• How are NADH, FADH2 and GTP produced? How much?
• The regulated steps in the cycle
• The amino acids and other metabolites synthesized
  from TCA intermediates