Chapter 19 Bioenergetics; How the Body Converts Food to Energy

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Chapter 19 Bioenergetics How the BodyConverts
Food to Energy
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Metabolism

• Metabolism The sum of all chemical reactions
  involved in maintaining the dynamic state of a
  cell or organism.
• Pathway A series of biochemical reactions.
• Catabolism The process of breaking down large
  nutrient molecules into smaller molecules with
  the concurrent production of energy.
• Anabolism The process of synthesizing larger
  molecules from smaller ones.

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Metabolism

• Metabolism is the sum of catabolism and
  anabolism.

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Metabolism

• Figure 19 .1 Simplified schematic diagram of
  the common metabolic pathway, an imaginary funnel
  representing what happens in the cell.

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Cells and Mitochondria

• Animal cells have many components, each with
  specific functions some components along with
  one or more of their functions are
• Nucleus Where replication of DNA takes place.
• Lysosomes Remove damaged cellular components and
  some unwanted foreign materials.
• Golgi bodies Package and process proteins for
  secretion and delivery to other cellular
  components.
• Mitochondria Organelles in which the common
  catabolic pathway takes place in higher
  organisms the purpose of this catabolic pathway
  is to convert the energy stored in food
  molecules into energy stored in molecules of ATP.

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The Common Metabolic Pathway

• The two parts to the common catabolic pathway
• The citric acid cycle, also called the
  tricarboxylic acid (TCA) or Krebs cycle.
• Electron transport chain and phosphorylation,
  together called oxidative phosphorylation.
• Four principal compounds participating in the
  common catabolic pathway are
• AMP, ADP, and ATP agents for the storage and
  transfer of phosphate groups.
• NAD/NADH agents for the transfer of electrons
  in biological oxidation-reduction reactions.
• FAD/FADH2 agents for the transfer of electrons
  in biological oxidation-reduction reactions.
• Coenzyme A abbreviated CoA or CoA-SH An agent
  for the transfer of acetyl groups.

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Adenosine Triphosphate (ATP)

• ATP is the most important compound involved in
  the transfer of phosphate groups.
• ATP contains two phosphoric anhydride bonds and
  one phosphoric ester bond.

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Adenosine Triphosphate (ATP)

• Hydrolysis of the terminal phosphate (anhydride)
  of ATP gives ADP, dihydrogen phosphate ion, and
  energy.
• Hydrolysis of a phosphoric anhydride liberates
  more energy than the hydrolysis of a phosphoric
  ester.
• We say that ATP and ADP each contain high-energy
  phosphoric anhydride bonds.
• ATP is a universal carrier of phosphate groups.
• ATP is also a common currency for the storage and
  transfer of energy.

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NAD/NADH

• Nicotinamide adenine dinucleotide (NAD) is a
  biological oxidizing agent.

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NAD/NADH

• NAD is a two-electron oxidizing agent, and is
  reduced to NADH.
• NADH is a two-electron reducing agent, and is
  oxidized to NAD. The structures shown here are
  the nicotinamide portions of NAD and NADH.
• NADH is an electron and hydrogen ion
  transporting molecule.

NAD
NADH
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FAD/FADH2

• Flavin adenine dinucleotide (FAD) is also a
  biological oxidizing agent.

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FAD/FADH2

• FAD is a two-electron oxidizing agent, and is
  reduced to FADH2.
• FADH2 is a two-electron reducing agent, and is
  oxidized to FAD.
• Only the flavin moiety is shown in the structures
  below.

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Coenzyme A

• Coenzyme A (CoA) is an acetyl-carrying group.
• Like NAD and FAD, coenzyme A contains a unit of
  ADP.
• CoA is often written CoA-SH to emphasize the fact
  that it contains a sulfhydryl group.
• The vitamin part of coenzyme A is pantothenic
  acid.
• The acetyl group of acetyl CoA is bound as a
  high-energy thioester.

Acetyl Coenzyme A
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Coenzyme A

• Figure 19.7 The structure of coenzyme A The
  business end is the -SH (sulfhydryl) group at the
  left end.

Phosphorylated ADP
Mercaptoethylamine
Pantothenic acid
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Citric Acid Cycle
Krebs Cycle
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Citric Acid Cycle

• Figure 19.9 A simplified view of the citric acid
  cycle showing only the carbon balance. The fuel
  is the two-carbon acetyl group of acetyl CoA.
  With each turn of the cycle two carbons are
  released as CO2.

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Citric Acid Cycle

• Step 1 The condensation of acetyl CoA with
  oxaloacetate
• The high-energy thioester of acetyl CoA is
  hydrolyzed.
• This hydrolysis provides the energy to drive Step
  1.
• Citrate synthase, an allosteric enzyme, is
  inhibited by NADH, ATP, and succinyl-CoA.

Oxaloacetate
Acetyl CoA
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Citric Acid Cycle

• Step 2 Dehydration and rehydration, catalyzed by
  aconitase, gives isocitrate.
• Citrate and aconitate are achiral neither has a
  stereocenter.
• Isocitrate is chiral it has 2 stereocenters and
  4 stereoisomers are possible.
• Only one of the 4 possible stereoisomers is
  formed in the cycle.

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Citric Acid Cycle

• Step 3 Oxidation of isocitrate to
  oxalosuccinate followed by decarboxylation gives
  a-ketoglutarate.
• Isocitrate dehydrogenase is an allosteric enzyme
  it is inhibited by ATP and NADH, and activated by
  ADP and NAD.

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Citric Acid Cycle

• Step 4 Oxidative decarboxylation of
  ?-ketoglutarate to succinyl-CoA.
• The two carbons of the acetyl group of acetyl CoA
  are still present in succinyl CoA.
• This multienzyme complex is inhibited by ATP,
  NADH, and succinyl CoA. It is activated by ADP
  and NAD.

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Citric Acid Cycle

• Step 5 Formation of succinate.
• The two CH2-COO- groups of succinate are now
  equivalent.
• This is the first and only energy-yielding step
  of the cycle. A molecule of GTP is produced.

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Citric Acid Cycle

• Step 6 Oxidation of succinate to fumarate.
• Step 7 Hydration of fumarate to L-malate.
• Malate is chiral and can exist as a pair of
  enantiomers It is produced in the cycle as a
  single stereoisomer.

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Citric Acid Cycle

• Step 8 Oxidation of malate.
• Oxaloacetate now can react with acetyl CoA to
  start another round of the cycle by repeating
  Step 1.
• The overall reaction of the cycle is

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Citric Acid Cycle

• Control of the cycle
• Controlled by three feedback mechanisms.
• Citrate synthase inhibited by ATP, NADH, and
  succinyl CoA also product inhibition by citrate.
• Isocitrate dehydrogenase activated by ADP and
  NAD, inhibited by ATP and NADH.
• ?-Ketoglutarate dehydrogenase complex inhibited
  by ATP, NADH, and succinyl CoA activated by ADP
  and NAD.

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TCA Cycle in Catabolism

• The catabolism of proteins, carbohydrates, and
  fatty acids all feed into the citric acid cycle
  at one or more points

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Oxidative Phosphorylation

• Carried out by four closely related multisubunit
  membrane-bound complexes and two electron
  carriers, coenzyme Q and cytochrome c.
• In a series of oxidation-reduction reactions,
  electrons from FADH2 and NADH are transferred
  from one complex to the next until they reach O2.
• O2 is reduced to H2O.
• As a result of electron transport, protons are
  pumped across the inner membrane to the
  intermembrane space.

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Oxidative Phosphorylation

• Figure 19.10 Schematic diagram of the electron
  and H transport chain and subsequent
  phosphorylation.

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Complex I

• The sequence starts with Complex I
• This large complex contains some 40 subunits,
  among them are a flavoprotein, several
  iron-sulfur (FeS) clusters, and coenzyme Q (CoQ,
  ubiquinone).
• Complex I oxidizes NADH to NAD.
• The oxidizing agent is CoQ, which is reduced to
  CoQH2.
• Some of the energy released in the oxidation of
  NAD is used to move 2H from the matrix into the
  intermembrane space.

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Complex II

• Complex II oxidizes FADH2 to FAD.
• The oxidizing agent is CoQ, which is reduced to
  CoQH2.
• The energy released in this reaction is not
  sufficient to pump protons across the membrane.

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Complex III

• Complex III delivers electrons from CoQH2 to
  cytochrome c (Cyt c).
• This integral membrane complex contains 11
  subunits, including cytochrome b, cytochrome c1,
  and FeS clusters.
• Complex III has two channels through which the
  two H from each CoQH2 oxidized are pumped from
  the matrix into the intermembrane space.

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Complex IV

• Complex IV is also known as cytochrome oxidase.
• It contains 13 subunits, one of which is
  cytochrome a3
• Electrons flow from Cyt c (oxidized) in Complex
  III to Cyt a3 in Complex IV.
• From Cyt a3 electrons are transferred to O2.
• During this redox reaction, H are pumped from
  the matrix into the intermembrane space.
• Summing the reactions of Complexes I - IV, six H
  are pumped out per NADH and four H per FADH2.

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Chemiosmotic Pump

• To explain how electron and H transport produce
  the chemical energy of ATP, Peter Mitchell
  proposed the chemiosmotic theory that electron
  transport is accompanied by an accumulation of
  protons in the intermembrane space of the
  mitochondrion, which in turn creates osmotic
  pressure the protons driven back to the
  mitochondrion under this pressure generate ATP.
• The energy-releasing oxidations give rise to
  proton pumping and a pH gradient is created
  across the inner mitochondrial membrane.
• There is a higher concentration of H in the
  intermembrane space than inside the mitochondria.
• This proton gradient provides the driving force
  to propel protons back into the mitochondrion
  through the enzyme complex called proton
  translocating ATPase.

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Chemiosmotic Pump

• Protons flow back into the matrix through
  channels in the F0 unit of ATP synthase.
• The flow of protons is accompanied by formation
  of ATP in the F1 unit of ATP synthase.
• The functions of oxygen are
• To oxidize NADH to NAD and FADH2 to FAD so that
  these molecules can return to participate in the
  citric acid cycle.
• Provide energy for the conversion of ADP to ATP.

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Chemiosmotic Pump

• The overall reactions of oxidative
  phosphorylation are
• Oxidation of each NADH gives 3ATP.
• Oxidation of each FADH2 gives 2 ATP.

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Energy Yield

• A portion of the energy released during electron
  transport is now built into ATP.
• For each two-carbon acetyl unit entering the
  citric acid cycle, we get three NADH and one
  FADH2.
• For each NADH oxidized to NAD, we get three ATP.
• For each FADH2 oxidized to FAD, we get two ATP.
• Thus, the yield of ATP per two-carbon acetyl
  group oxidized to CO2 is

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Other Forms of Energy

• The chemical energy of ATP is converted by the
  body to several other forms of energy
• Electrical energy
• The body maintains a K concentration gradient
  across cell membranes higher inside and lower
  outside.
• It also maintains a Na concentration gradient
  across cell membranes lower inside, higher
  outside.
• This pumping requires energy, which is supplied
  by the hydrolysis of ATP to ADP.
• Thus, the chemical energy of ATP is transformed
  into electrical energy, which operates in
  neurotransmission.

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Other Forms of Energy

• Mechanical energy
• ATP drives the alternating association and
  dissociation of actin and myosin and,
  consequently, the contraction and relaxation of
  muscle tissue.
• Heat energy
• Hydrolysis of ATP to ADP yields 7.3 kcal/mol.
• Some of this energy is released as heat to
  maintain body temperature.

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Example

• How many ATP molecules are generated for each H
  translocated through the ATPase complex?