Chapter 19 bioenergetics
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Chapter 19 Bioenergetics. How the Body Converts Food to Energy. Metabolism. Metabolism: the sum of all chemical reactions involved in maintaining the dynamic state of a cell or organism pathway: a series of consecutive biochemical reactions

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Chapter 19 Bioenergetics

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Chapter 19 Bioenergetics

How the Body Converts Food to Energy


Metabolism

  • Metabolism: the sum of all chemical reactions involved in maintaining the dynamic state of a cell or organism

    • pathway: a series of consecutive biochemical reactions

    • catabolism: the biochemical pathways that are involved in generating energy by breaking down large nutrient molecules into smaller molecules with the concurrent production of energy

    • anabolism: the pathways by which biomolecules are synthesized (use ATP energy to build larger molecules from smaller building blocks)


Metabolism

  • metabolism is the sum of catabolism and anabolism


Stages of Catabolism

Catabolic reactions are organized into three stages:

  • In Stage 1, digestion breaks down large molecules into smaller ones that enter the bloodstream

  • In Stage 2, molecules enter the cells and are broken down into two- and three-carbon compounds

  • In Stage 3, compounds are oxidized in the citric acid cycle to provide energy (ATP) for anabolic processes


Stages of Catabolism


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: responsible for generation of most of the energy for cells


Components of Eukaryotic Cells


A Rat Liver Cell


A Mitochondrion

  • Schematic of a mitochondrion cut to reveal its inner organization


Common Catabolic Pthwy

  • The two parts to the common catabolic pathway

    • citric acid cycle, also called the tricarboxylic acid or Krebs cycle

    • oxidative phosphorylation, also called the electron transport chain, or the respiratory chain

  • The four principal compounds participating in the common catabolic pathway are:

    • AMP, ADP, and ATP

    • NAD+/NADH

    • FAD/FADH2

    • coenzyme A; abbreviated CoA or CoA-SH


Adenosine Triphosphate

  • ATP 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

  • hydrolysis of the terminal phosphate of ATP gives ADP, phosphate ion, and energy

  • hydrolysis of a phosphoric anhydride liberates more energy than hydrolysis of a phosphoric ester

  • we say that ATP and ADP contain high-energy phosphoric anhydride bonds

  • ATP is a universal carrier of phosphate groups

  • it is also a common currency for the storage and transfer of energy


Hydrolysis of ATP

  • The hydrolysis of ATP to ADP releases 7.3 kcal (31 kJ/mole)

    ATP  ADP + Pi + 7.3 kcal (31 kJ/mole)

  • The hydrolysis of ADP to AMP releases 7.3 kcal (31 kJ/mole)

    ADP  AMP + Pi + 7.3 kcal (31 kJ/mole)


Coenzymes NAD+/NADH2

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


Coenzymes 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+

  • NAD+ and NADH are also hydrogen ion transporting molecules


Coenzymes NAD+/NADH

  • When a compound is oxidized by an enzyme, 2H+ and 2e- are removed by a coenzyme, which is reduced

  • NAD+ (nicotinamide adenine dinucleotide) participates in reactions that produce a carbon-oxygen double bond (C=O)

  • For example, NAD+ participates in the oxidation of ethanol:


Coenzymes FAD/FADH2

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


Coenzymes 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


Coenzymes FAD/FADH2

  • FAD participates in reactions that produce a carbon-carbon double bond (C=C)

    Oxidation

    —CH2—CH2—  —CH=CH— + 2H+ + 2e-

    Reduction

    FAD + 2H+ + 2e- FADH2


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


Coenzyme A


Citric Acid Cycle

  • overview: the two carbon acetyl group of acetyl CoA is fed into the cycle and oxidized to 2 CO2

  • there are four oxidation steps in the cycle


Citric Acid Cycle

  • Step 1: 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 is an allosteric enzyme; it is inhibited by NADH, ATP, and succinyl-CoA


Citric Acid Cycle

  • Step 2: dehydration and rehydration, catalyzed by aconitase, gives isocitrate

    • citrate is achiral; it has no stereocenter

    • aconitate is also achiral

    • 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 Cycle

  • Step 2 (cont’d): Citrate isomerizes to isocitrate

  • The tertiary –OH group in citrate is converted to a secondary –OH that can be oxidized


Citric Acid Cycle

  • Step 3: oxidation of isocitrate 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+


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 and in succinate

    • this multienzyme complex is inhibited by ATP, NADH, and succinyl CoA; it is activated by ADP and NAD+


Citric Acid Cycle

  • Step 5: formation of succinate

    • the two CH2-COO- groups of succinate are now equivalent

    • this is the first energy-yielding step of the cycle; a molecule of GTP is produced


Citric Acid Cycle

  • Step 6: oxidation of succinate to fumarate

  • Step 7: hydration of fumarate to L-malate

    • L-malate is chiral and can exist as a pair of enantiomers; it is produced in the citric acid cycle as a single stereoisomer


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


Citric Acid Cycle

In one turn of the citric acid cycle:

  • Two decarboxylations remove two carbons as 2CO2

  • Four oxidations provide hydrogen for 3NADH and one FADH2

  • A direct phosphorylation forms GTP which is used to form ATP

    Overall reaction of citric acid cycle:


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+


CA Cycle in Catabolism

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


Electron Carriers

  • The electron transport chain consists of electron carriers that accept H+ ions and electrons from the reduced coenzymes NADH and FADH2

  • The H+ ions and electrons are passed down a chain of carriers until in the last step they combine with oxygen to form H2O

  • Oxidative phosphorylation is the process by which the energy from transport is used to synthesize ATP


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


Electron Transport System

  • The electron carriers in the electron transport system are attached to the inner membrane of the mitochondrion

  • They are organized into four protein complexes:

    Complex I NADH dehydrogenase

    Complex IISuccinate dehydrogenase

    Complex IIICoQ-Cytochrome c reductase

    Complex IVCytochrome c Oxidase


Electron Transport Chain


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 this reaction is used to move 2H+ from the matrix into the intermembrane space


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


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 CoQH2 are pumped from the matrix into the intermembrane space


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


  • Coupling of Ox and Phos

    • To explain how electron and H+ transport produce the chemical energy of ATP, Peter Mitchell proposed the chemiosmotic theory

      • the energy-releasing oxidations give rise to proton pumping and a pH gradient across the inner mitochondrial membrane

      • there is a higher concentration of H+ in the intermembrane space than inside the mitochondrion

      • this proton gradient provides the driving force to propel protons back into the mitochondrion through the enzyme complex called proton translocating ATPase


    Coupling of Ox and Phos

    • 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


  • Coupling of Ox and Phos

    • The overall reactions of oxidative phosphorylation are:


    The 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


    Other Energy Forms

    • 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

      • Special transport proteins in cell membranes constantly pump K+ into and Na+ out of the cells

      • 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 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


    Bioenergetics

    End

    Chapter 19


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