Chapter 19 Chapter 19 Bioenergetics; How the BodyBioenergetics; How the Body

Converts Food to EnergyConverts Food to Energy

MetabolismMetabolismMetabolism:Metabolism: The sum of all chemical reactions involved in maintaining the dynamic state of a cell or organism.• Pathway:Pathway: A series of biochemical reactions.• Catabolism:Catabolism: The process of breaking down large

nutrient molecules into smaller molecules with the concurrent production of energy.

• Anabolism:Anabolism: The process of synthesizing larger molecules from smaller ones.

MetabolismMetabolismMetabolism is the sum of catabolism and anabolism.

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

Cells and MitochondriaCells and MitochondriaAnimal cells have many components, each with specific functions; some components along with one or more of their functions are:• Nucleus:Nucleus: Where replication of DNA takes place.• Lysosomes:Lysosomes: Remove damaged cellular components

and some unwanted foreign materials.• Golgi bodies:Golgi bodies: Package and process proteins for

secretion and delivery to other cellular components.• Mitochondria: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.

The Common Metabolic PathwayThe Common Metabolic Pathway• The two parts to the common catabolic pathway:• TheThe citric acid cyclecitric acid cycle, also called the tricarboxylic acid

(TCA) or Krebs cycle.• Electron transport chain Electron transport chain and phosphorylationphosphorylation,

together called oxidative phosphorylationoxidative 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.

Adenosine Triphosphate (ATP)Adenosine Triphosphate (ATP)ATPATP is the most important compound involved in the transfer of phosphate groups.• ATP contains two phosphoric anhydride bonds and one

phosphoric ester bond.

Adenosine Triphosphate (ATP)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.

NADNAD++/NADH/NADH• Nicotinamide adenine dinucleotide (NADNicotinamide adenine dinucleotide (NAD++)) is a

biological oxidizing agent.

NADNAD++/NADH/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

FAD/FADHFAD/FADH22• Flavin adenine dinucleotide (FAD)Flavin adenine dinucleotide (FAD) is also a biological

oxidizing agent.

FAD/FADHFAD/FADH22• 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.

Coenzyme ACoenzyme A• Coenzyme A (CoACoenzyme A (CoA)) is an acetyl-carrying group.• Like NAD+ and FAD, coenzyme A contains a unit of

ADP.• CoA is often written CoA-SHCoA-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

Coenzyme ACoenzyme A• Figure 19.7 The structure of coenzyme A The business

end is the -SH (sulfhydryl) group at the left end.

Mercaptoethylamine Pantothenic acid Phosphorylated ADP

Citric Acid CycleCitric Acid CycleKrebs Cycle

Citric Acid CycleCitric 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.

Citric Acid CycleCitric Acid CycleStep 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

Citric Acid CycleCitric Acid CycleStep 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.

Citric Acid CycleCitric Acid CycleStep 3: Oxidation of isocitrate to oxalosuccinate followed by decarboxylation gives -ketoglutarate.

• Isocitrate dehydrogenase is an allosteric enzyme; it is inhibited by ATP and NADH, and activated by ADP and NAD+.

Citric Acid CycleCitric Acid CycleStep 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+.

Citric Acid CycleCitric 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.

Citric Acid CycleCitric 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.

Citric Acid CycleCitric 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:

Citric Acid CycleCitric Acid CycleControl of the cycle:

• Controlled by three feedback mechanisms.• Citrate synthase:Citrate synthase: inhibited by ATP, NADH, and

succinyl CoA; also product inhibition by citrate.

• Isocitrate dehydrogenase:Isocitrate dehydrogenase: activated by ADP and NAD+, inhibited by ATP and NADH.

• -Ketoglutarate dehydrogenase complex:-Ketoglutarate dehydrogenase complex: inhibited by ATP, NADH, and succinyl CoA; activated by ADP and NAD+.

TCA Cycle in CatabolismTCA Cycle in CatabolismThe catabolism of proteins, carbohydrates, and fatty acids all feed into the citric acid cycle at one or more points:

Oxidative PhosphorylationOxidative PhosphorylationCarried 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.

Oxidative PhosphorylationOxidative Phosphorylation• Figure 19.10 Schematic diagram of the electron

and H+ transport chain and subsequent phosphorylation.

Complex IComplex IThe 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.

Complex IIComplex 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.

Complex IIIComplex 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.

Complex IVComplex 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.

Chemiosmotic PumpChemiosmotic PumpTo explain how electron and H+ transport produce the chemical energy of ATP, Peter Mitchell proposed the chemiosmotic theory chemiosmotic theory that electron transport is that electron transport is accompanied by an accumulation of protons in the accompanied by an accumulation of protons in the intermembrane space of the mitochondrion, which in turn intermembrane space of the mitochondrion, which in turn creates osmotic pressure; the protons driven back to the creates osmotic pressure; the protons driven back to the mitochondrion under this pressure generate ATP.mitochondrion under this pressure generate ATP. • The energy-releasing oxidations give rise to proton

pumping and a pH gradientgradient 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 proton translocating ATPaseATPase..

Chemiosmotic PumpChemiosmotic 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.

Chemiosmotic PumpChemiosmotic Pump• The overall reactions of oxidative phosphorylation are:

• Oxidation of each NADH gives 3ATP.• Oxidation of each FADH2 gives 2 ATP.

Energy YieldEnergy YieldA 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:

Other Forms of EnergyOther Forms of EnergyThe chemical energy of ATP is converted by the body to several other forms of energy:Electrical energyElectrical 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.

Other Forms of EnergyOther Forms of EnergyMechanical energyMechanical energy

• ATP drives the alternating association and dissociation of actin and myosin and, consequently, the contraction and relaxation of muscle tissue.

Heat energyHeat energy• Hydrolysis of ATP to ADP yields 7.3 kcal/mol.• Some of this energy is released as heat to maintain

body temperature.

ExampleExample How many ATP molecules are generated for each H+

translocated through the ATPase complex?