The Citric Acid Cycle

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The Citric Acid Cycle
Dr. Sooad Al-Daihan Biochemistry department
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Introduction

• Also called citric acid cycle or the Krebs cycle
• (after its discoverer, Hans Krebs).
• TCA cycle is a series of reactions catalyzed by
  different enzymes in which acetyl CoA is oxidized
  into CO2, H2O and energy.
• It occurs in the mitochondrial matrix
  aerobically.
• The enzymes involved in the TCA cycle are
  present in the mitochondrial matrix either free
  or attached to the inner surface of the
  mitochondrial membrane.

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• The citric acid cycle is the final common
  pathway for the oxidation of fuel molecules
• amino acids, fatty acids and carbohydrates.
• Most fuel molecules enter the cycle as acetyl
  coenzyme A.
• The function of the citric acid cycle is the
  harvesting of high-energy electrons from carbon
  fuels.

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• The citric acid cycle itself neither generates a
  large amount of ATP nor includes oxygen as a
  reactant.
• Instead, the citric acid cycle removes electrons
  from acetyl CoA and uses these electrons to form
  NADH and FADH2.
• The citric acid cycle includes a series of
  oxidation-reduction reactions that result in the
  oxidation of an acetyl group to two molecules of
  carbon dioxide.

The citric acid cycle oxidizes two-carbon units,
producing two molecules of CO2, one molecule of
GTP, and high-energy electrons in the form of
NADH and FADH2.
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The amphibolic nature of TCA cycle

• The citric acid cycle is the gateway to the
  aerobic metabolism of any molecule that can be
  transformed into an acetyl group.
• The cycle is also an important source of
  precursors, not only for the storage forms of
  fuels, but also for the building blocks of many
  other molecules such as amino acids, nucleotide
  bases, cholesterol, and porphyrin.

This pathway is utilized for both catabolic
reactions to generate energy anabolic reactions
to generate metabolic intermediates for
biosynthesis.
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Metabolic pathway

• In oxidative phosphorylation, electrons released
  in the reoxidation of NADH and FADH2 flow through
  a series of membrane proteins to generate a
  proton gradient across the membrane.
• The citric acid cycle, in conjunction with
  oxidative phosphorylation, provides the vast
  majority of energy used by aerobic cells in human
  beings, greater than 95.

• In TCA, the removal of high-energy electrons
  from carbon fuels.
• These electrons reduce O2 to generate a proton
  gradient .
• Which is used to synthesize ATP .

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The TCA Cycle Has Eight Steps
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• Step 1 Formation of Citrate
• - An irreversible reaction catalyzed by citrate
  synthase.
• -Inhibited by ATP , NADH, Citrate.
• Step 2 Formation of Isocitrate
• -A reversible reaction catalyzed by aconitase .
• Step 3 Oxidative decarboxylation of isocitrate
• -The enzyme isocitrate dehydrogenase catalyzes
  the irreversible oxidative decarboxylation of
  isocitrate to form a-ketoglutarate and CO2.
• -Stimulated by isocitrate, NAD, Mn2, ADP,
  Ca2.
• -Inhibited by NADH and ATP.

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• Step 4 Oxidative decarboxylation of
  a-ketoglutarate
• -In this irreversible reaction, a-ketoglutarate
  is converted to succinyl-CoA and CO2 by the
  action of the a-ketoglutarate dehydrogenase
  complex .
• -a-ketoglutarate dehydrogenase complex closely
  resembles the
• PDH complex in both structure and function.
• -NAD serves as electron acceptor and CoA as the
  carrier of the succinyl group.
• - Inhibited by NADH, ATP, Succinyl-CoA
• - Stimulated by Ca2

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• Step 5 Conversion of succinyl-CoA to succinate
• -Reversible reaction catayzed by succinyl-CoA
  synthetase (succinate thiokinase)
• -Results in the formation of GTP and CoA-SH
• -Nucleoside diphosphate kinase interconverts GTP
  and ATP by a readily reversible phosphoryl
  transfer reaction
• GTP ADP GDP ATP

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• Step 6 Oxidation of Succinate to Fumarate.
• Succinate is oxidized to fumarate by the
  flavoprotein succinate dehydrogenase
• Only TCA cycle enzyme contained within the
  mitochondrial membrane.
• Results in the formation of FADH2

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• Step 7 Hydration of fumarate to malate
• -The reversible hydration of fumarate to L-malate
  is catalyzed by fumarase .
• Step 8 Oxidation of malate to oxaloacetate
• -In the last reaction of the citric acid cycle,
  NAD-linked L-malate dehydrogenase catalyzes the
  oxidation of L-malate to oxaloacetate.

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Enzyme Control of the TCA Cycle
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Inhibitors of TCA Cycle

• Fluoroacetyl CoA
• -It inhibits aconitase enzyme
• -It combines with oxaloacetate giving rise to
  fluorocitrate .
• Malonic acid
• -Inhibits succinate dehydrogenase (competitive
  inhibition)
• Arsenate and Mercury
• -Inhibit Pyruvate dehydrogenase and
  a-ketoglutarate dehydrogenase complexs.
• - By reacting with sulphydral group of lipoic
  acid leading to accumulation of pyruvic lactic
  acid and a- ketoglutarate.

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Products of Krebs Cycle

• 2 CO2
• 3 NADH
• 1 ATP Per 1 Acetyl CoA
  (double for 1 glucose)
• 1 FADH2
• ATP Yield
• Each NADH yields 3 ATP
• Each FADH2 yields 2 ATP

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Summary of total energy yield of complete
oxidation of 1 glucose molecule
Step Coenzyme Yield ATP Yield Source of ATP
Glycolysis Stage 1 - 2 Phosphorylation of glucose and fructose uses 2 ATP
Glycolysis Stage 2 4 Substrate level phosphorylation
Glycolysis Stage 2 2 NADH 6 Oxidative phosphorylation
Pyruvate metabolism 2 NADH 6 Oxidative phosphorylation
TCA cycle 2 Substrate level phosphorylation
TCA cycle 6 NADH 18 Oxidative phosphorylation
TCA cycle 2 FADH2 4 Oxidative phosphorylation
Total Yield 38 ATP