Download 21.8 The Citric Acid Cycle

20.8 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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20.9 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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