Download 21.8 The Citric Acid Cycle

21.8 The Citric Acid Cycle
• The carbon atoms from the first two stages of
catabolism are carried into the third stage as
acetyl groups bonded to coenzyme A.
• Like the phosphoryl groups in ATP molecules, the
acetyl groups in acetyl-SCoA molecules are
readily removed in an energy-releasing hydrolysis
reaction.
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• Oxidation of two carbons to give CO2 and transfer
of energy to reduced coenzymes occurs in the
citric acid cycle, also known as the tricarboxylic
acid cycle (TCA) or Krebs cycle (after Hans Krebs,
who unraveled its complexities in 1937).
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The citric acid cycle is a closed
loop of reactions in which the
product of the final step which has
four carbon atoms, is the reactant
in the first step.
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• The net result of the citric acid cycle is:
– Production of four reduced coenzyme molecules,
3 NADH and 1 FADH2
– Conversion of an acetyl group to two CO2
molecules
– Production of one energy-rich molecule (GTP)
• ADP acts as an allosteric activator for the
enzyme for Step 3. NADH acts as an inhibitor
of the enzyme for Step 3.
• By such feedback mechanisms, the cycle is
activated when energy is needed and
inhibited when energy is in good supply.
• The eight steps of the citric acid cycle are
shown in greater detail on the next slide.
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21.9 The Electron-Transport Chain and
ATP Production
• At the conclusion of the citric acid cycle, the
reduced coenzymes formed in the cycle are ready
to donate their energy to making additional ATP
• Hydrogen and electrons from NADH and FADH2
enter the electron-transport chain at enzyme
complexes I and II, respectively.
• The enzyme for Step 6 of the citric acid cycle is
part of complex II. FADH2 produced there does
not leave complex II. Instead it is immediately
oxidized there by reaction with coenzyme Q.
• Following formation of the mobile coenzyme Q,
reductions occur when electrons are transferred.
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Coenzyme Q is also known as ubiquinone because of its widespread occurrence
and because its ring structure with the two ketone groups is a quinone.
• Electrons are passed from weaker to
increasingly stronger oxidizing agents, with
energy released at each transfer.
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Other important electron acceptors are various
cytochromes, which are proteins that contain
heme groups in which the iron cycles between
Fe+2 and Fe+3 and proteins with iron–sulfur
groups in which the iron also cycles between Fe+2
and Fe+3 .
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H+ ions are released for transport through the inner
membrane at complexes I, III, and IV. Some of these
ions come from the reduced coenzymes.
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• The H+ concentration difference creates a potential
energy difference across the two sides of the inner
membrane (like the energy difference between water
at the top and bottom of the waterfall).
• The maintenance of this concentration gradient
across the membrane is crucial—it is the mechanism
by which energy for ATP formation is made available.
• ATP synthase: The enzyme complex in the inner
mitochondrial membrane at which H+ ions cross the
membrane and ATP is synthesized from ADP.
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• Oxidative phosphorylation:
The synthesis of ATP from ADP
using energy released in the
electron transport chain.
• Each of the enzyme complexes
I–IV contains several electron
carriers.
• In the last step of the chain,
electrons combine with
oxygen that we breathe and
H+ ions from the surroundings
to produce water.
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21.10 Harmful Oxygen By-Products
and Antioxidant Vitamins
• More than 90% of the oxygen we breathe is used in
electron transport– ATP synthesis reactions.
• In these and other oxygen-consuming redox
reactions, the product may not be water, but one or
more of three highly reactive species.
• The superoxide ion, ·O2- , and the hydroxyl free
radical, ·OH, can grab an electron from a bond in
another molecule, which results in breaking that
bond. The third oxygen by-product is hydrogen
peroxide, H2O2 , a relatively strong oxidizing agent.
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Conditions that can enhance production of these
three reactive oxygen species are represented in the
drawing below. Some causes are environmental, such
as exposure to smog or radiation. Others are
physiological, including aging and inflammation.
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• Our protection against harmful oxygen species is
provided by superoxide dismutase and catalase,
which are among the fastest-acting enzymes.
• These and other enzymes are active inside cells
where oxygen by-products are constantly generated.
It is estimated that 1 in 50 of the harmful oxygen
species escapes destruction inside a cell.
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• Protection is also provided by the antioxidant
vitamins E, C, and A , all of which disarm free radicals
by bonding with them.
• Vitamin E is fat-soluble, and its major function is to
protect cell membranes from potential damage.
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