Only 10 of the Energy Stored in an Organism Can Be Passed

eleven.four Stage Iii of Catabolism

Learning Objectives

1. Depict the reactions of the citric acid cycle.
2. Describe the function of the citric acid cycle and identify the products produced.
3. Depict the role of the electron ship chain in energy metabolism.
4. Describe the office of oxidative phosphorylation in free energy metabolism.

The acetyl group enters a cyclic sequence of reactions known collectively as the citric acid cycle (or Krebs cycle or tricarboxylic acid [TCA] bicycle). The cyclical design of this circuitous series of reactions, which bring about the oxidation of the acetyl grouping of acetyl-CoA to carbon dioxide and h2o, was get-go proposed by Hans Krebs in 1937. (He was awarded the 1953 Nobel Prize in Physiology or Medicine.) Acetyl-CoA's entrance into the citric acid cycle is the showtime of phase 3 of catabolism. The citric acid cycle produces adenosine triphosphate (ATP), reduced nicotinamide adenine dinucleotide (NADH), reduced flavin adenine dinucleotide (FADH_ii), and metabolic intermediates for the synthesis of needed compounds.

Steps of the Citric Acid Cycle

At first glance, the citric acrid bicycle appears rather complex (Figure xi.12 "Reactions of the Citric Acid Bicycle"). All the reactions, nonetheless, are familiar types in organic chemistry: hydration, oxidation, decarboxylation, and hydrolysis. Each reaction of the citric acid cycle is numbered, and in Effigy 11.12 "Reactions of the Citric Acid Bicycle", the two acetyl carbon atoms are highlighted in cerise. Each intermediate in the wheel is a carboxylic acrid, existing as an anion at physiological pH. All the reactions occur within the mitochondria, which are minor organelles inside the cells of plants and animals. We volition look more than closely at the construction of mitochondria in Department xi.v "Stage Ii of Sugar Catabolism".

Figure eleven.12 Reactions of the Citric Acid Bicycle

In the first reaction, acetyl-CoA enters the citric acrid bike, and the acetyl group is transferred onto oxaloacetate, yielding citrate. Note that this step releases coenzyme A. The reaction is catalyzed by citrate synthase.

In the next footstep, aconitase catalyzes the isomerization of citrate to isocitrate. In this reaction, a third booze, which cannot be oxidized, is converted to a secondary alcohol, which can exist oxidized in the adjacent footstep.

Isocitrate then undergoes a reaction known equally oxidative decarboxylation because the alcohol is oxidized and the molecule is shortened by one carbon atom with the release of carbon dioxide (decarboxylation). The reaction is catalyzed by isocitrate dehydrogenase, and the production of the reaction is α-ketoglutarate. An important reaction linked to this is the reduction of the coenzyme nicotinamide adenine dinucleotide (NAD⁺) to NADH. The NADH is ultimately reoxidized, and the energy released is used in the synthesis of ATP, as we shall see.

The fourth step is another oxidative decarboxylation. This time α-ketoglutarate is converted to succinyl-CoA, and another molecule of NAD⁺ is reduced to NADH. The α-ketoglutarate dehydrogenase complex catalyzes this reaction. This is the only irreversible reaction in the citric acid bike. As such, it prevents the wheel from operating in the reverse direction, in which acetyl-CoA would be synthesized from carbon dioxide.

Comment: And then far, in the first four steps, two carbon atoms have entered the wheel as an acetyl group, and 2 carbon atoms have been released as molecules of carbon dioxide. The remaining reactions of the citric acrid bicycle employ the four carbon atoms of the succinyl group to resynthesize a molecule of oxaloacetate, which is the compound needed to combine with an incoming acetyl grouping and brainstorm another round of the wheel.

In the 5th reaction, the energy released by the hydrolysis of the high-energy thioester bond of succinyl-CoA is used to form guanosine triphosphate (GTP) from guanosine diphosphate (GDP) and inorganic phosphate in a reaction catalyzed by succinyl-CoA synthetase. This step is the only reaction in the citric acrid bike that directly forms a high-energy phosphate compound. GTP can readily transfer its terminal phosphate group to adenosine diphosphate (ADP) to generate ATP in the presence of nucleoside diphosphokinase.

Succinate dehydrogenase then catalyzes the removal of ii hydrogen atoms from succinate, forming fumarate. This oxidation-reduction reaction uses flavin adenine dinucleotide (FAD), rather than NAD⁺, every bit the oxidizing agent. Succinate dehydrogenase is the only enzyme of the citric acid bike located within the inner mitochondrial membrane. We will see soon the importance of this.

In the following step, a molecule of water is added to the double bond of fumarate to form 50-malate in a reaction catalyzed by fumarase.

One revolution of the bike is completed with the oxidation of L-malate to oxaloacetate, brought about by malate dehydrogenase. This is the tertiary oxidation-reduction reaction that uses NAD⁺ equally the oxidizing amanuensis. Oxaloacetate can accept an acetyl group from acetyl-CoA, allowing the cycle to brainstorm again.

Cellular Respiration

Respiration can be defined as the process past which cells oxidize organic molecules in the presence of gaseous oxygen to produce carbon dioxide, water, and energy in the course of ATP. We accept seen that ii carbon atoms enter the citric acid cycle from acetyl-CoA (step one), and two different carbon atoms get out the cycle equally carbon dioxide (steps iii and 4). Yet nowhere in our discussion of the citric acid wheel have nosotros indicated how oxygen is used. Recollect, yet, that in the iv oxidation-reduction steps occurring in the citric acid wheel, the coenzyme NAD⁺ or FAD is reduced to NADH or FADH_ii, respectively. Oxygen is needed to reoxidize these coenzymes. Recollect, as well, that very petty ATP is obtained directly from the citric acid cycle. Instead, oxygen participation and significant ATP production occur subsequent to the citric acid cycle, in 2 pathways that are closely linked: electron transport and oxidative phosphorylation.

All the enzymes and coenzymes for the citric acid cycle, the reoxidation of NADH and FADH_2, and the production of ATP are located in the mitochondria, which are pocket-size, oval organelles with double membranes, often referred to as the "power plants" of the cell (Figure xi.thirteen "Respiration"). A cell may contain 100–five,000 mitochondria, depending on its function, and the mitochondria tin can reproduce themselves if the energy requirements of the prison cell increment.

Figure eleven.13 Respiration

Cellular respiration occurs in the mitochondria.

Effigy eleven.thirteen "Respiration" shows the mitochondrion's two membranes: outer and inner. The inner membrane is extensively folded into a series of internal ridges called cristae. Thus at that place are two compartments in mitochondria: the intermembrane infinite, which lies between the membranes, and the matrix, which lies inside the inner membrane. The outer membrane is permeable, whereas the inner membrane is impermeable to most molecules and ions, although water, oxygen, and carbon dioxide can freely penetrate both membranes. The matrix contains all the enzymes of the citric acid wheel with the exception of succinate dehydrogenase, which is embedded in the inner membrane. The enzymes that are needed for the reoxidation of NADH and FADH₂ and ATP production are likewise located in the inner membrane. They are arranged in specific positions so that they function in a manner analogous to a saucepan brigade. This highly organized sequence of oxidation-reduction enzymes is known as the electron send chain (or respiratory chain).

Electron Transport

Figure 11.fourteen "The Mitochondrial Electron Transport Chain and ATP Synthase" illustrates the organisation of the electron transport chain. The components of the chain are organized into four complexes designated I, II, III, and Iv. Each complex contains several enzymes, other proteins, and metal ions. The metal ions can be reduced and so oxidized repeatedly as electrons are passed from 1 component to the next. A compound is reduced when it gains electrons or hydrogen atoms and is oxidized when it loses electrons or hydrogen atoms.

Figure xi.fourteen The Mitochondrial Electron Transport Concatenation and ATP Synthase

The red line shows the path of electrons.

Electrons can enter the electron transport chain through either complex I or II. We will await first at electrons entering at complex I. These electrons come from NADH, which is formed in 3 reactions of the citric acid bike. Let'south apply pace eight as an example, the reaction in which L-malate is oxidized to oxaloacetate and NAD⁺ is reduced to NADH. This reaction can be divided into 2 half reactions:

Oxidation half-reaction:

Reduction half-reaction:

In the oxidation one-half-reaction, two hydrogen (H⁺) ions and ii electrons are removed from the substrate. In the reduction half-reaction, the NAD⁺ molecule accepts both of those electrons and one of the H⁺ ions. The other H⁺ ion is transported from the matrix, across the inner mitochondrial membrane, and into the intermembrane space. The NADH diffuses through the matrix and is jump by complex I of the electron transport chain. In the complex, the coenzyme flavin mononucleotide (FMN) accepts both electrons from NADH. By passing the electrons along, NADH is oxidized back to NAD⁺ and FMN is reduced to FMNH₂ (reduced class of flavin mononucleotide). Again, the reaction can exist illustrated by dividing it into its corresponding half-reactions.

Oxidation half-reaction:

Reduction half-reaction:

Complex I contains several proteins that accept iron-sulfur (Fe·Due south) centers. The electrons that reduced FMN to FMNH_two are at present transferred to these proteins. The iron ions in the Fe·S centers are in the Fe(III) class at first, but past accepting an electron, each ion is reduced to the Fe(2) class. Because each Fe·Southward center can transfer only i electron, two centers are needed to accept the two electrons that will regenerate FMN.

Oxidation one-half-reaction:

FMNH₂ → FMN + 2H⁺ + 2e⁻

Reduction one-half-reaction:

2Fe(III) · S + 2e⁻ → 2Fe(Two) · Due south

Electrons from FADH_ii, formed in step 6 of the citric acrid cycle, enter the electron transport chain through complex 2. Succinate dehydrogenase, the enzyme in the citric acid bike that catalyzes the formation of FADH_two from FAD is part of complex Ii. The electrons from FADH₂ are then transferred to an Fe·S protein.

Oxidation half-reaction:

FADH₂ → FAD + 2H⁺ + 2e⁻

Reduction half-reaction:

2Fe(III) · South + 2e⁻ → 2Fe(Ii) · Due south

Electrons from complexes I and II are then transferred from the Fe·S protein to coenzyme Q (CoQ), a mobile electron carrier that acts as the electron shuttle betwixt complexes I or II and complex 3.

Note

Coenzyme Q is besides called ubiquinone because information technology is ubiquitous in living systems.

Oxidation half-reaction:

2Fe(II) · S → 2Fe(Three) · South + 2e⁻

Reduction half-reaction:

Complexes Iii and 4 include several iron-containing proteins known as cytochromes. The fe in these enzymes is located in substructures known equally iron porphyrins (Effigy 11.xv "An Fe Porphyrin"). Like the Fe·S centers, the feature feature of the cytochromes is the ability of their iron atoms to be as either Fe(II) or Atomic number 26(III). Thus, each cytochrome in its oxidized course—Iron(Iii)—tin accept i electron and be reduced to the Fe(2) form. This change in oxidation state is reversible, so the reduced course can donate its electron to the adjacent cytochrome, and so on. Complex III contains cytochromes b and c, equally well as Iron·South proteins, with cytochrome c interim as the electron shuttle between complex Iii and Four. Complex IV contains cytochromes a and a₃ in an enzyme known as cytochrome oxidase. This enzyme has the power to transfer electrons to molecular oxygen, the last electron acceptor in the chain of electron transport reactions. In this last stride, h2o (H₂O) is formed.

Oxidation half-reaction:

4Cyt a_three–Atomic number 26(II) → 4Cyt a₃–Fe(3) + 4e⁻

Reduction half-reaction:

O₂ + 4H⁺ + 4e⁻ → 2H₂O

Figure xi.15 An Iron Porphyrin

Iron porphyrins are present in cytochromes besides as in myoglobin and hemoglobin.

Oxidative Phosphorylation

Each intermediate compound in the electron transport chain is reduced by the add-on of ane or two electrons in one reaction and then later restored to its original course past delivering the electron(s) to the side by side compound along the concatenation. The successive electron transfers result in energy product. But how is this energy used for the synthesis of ATP? The process that links ATP synthesis to the performance of the electron transport concatenation is referred to as oxidative phosphorylation.

Electron transport is tightly coupled to oxidative phosphorylation. The coenzymes NADH and FADH_ii are oxidized by the respiratory chain only if ADP is simultaneously phosphorylated to ATP. The currently accepted model explaining how these 2 processes are linked is known as the chemiosmotic hypothesis, which was proposed by Peter Mitchell, resulting in Mitchell beingness awarded the 1978 Nobel Prize in Chemistry.

Looking once again at Figure 11.14 "The Mitochondrial Electron Send Concatenation and ATP Synthase", nosotros see that as electrons are being transferred through the electron transport chain, hydrogen (H⁺) ions are being transported across the inner mitochondrial membrane from the matrix to the intermembrane space. The concentration of H⁺ is already higher in the intermembrane infinite than in the matrix, so energy is required to transport the boosted H⁺ at that place. This energy comes from the electron transfer reactions in the electron transport chain. But how does the extreme difference in H⁺ concentration then lead to ATP synthesis? The buildup of H⁺ ions in the intermembrane space results in an H⁺ ion gradient that is a large energy source, like water behind a dam (considering, given the opportunity, the protons will flow out of the intermembrane infinite and into the less full-bodied matrix). Current inquiry indicates that the flow of H⁺ down this concentration gradient through a fifth enzyme complex, known every bit ATP synthase, leads to a change in the structure of the synthase, causing the synthesis and release of ATP.

In cells that are using energy, the turnover of ATP is very loftier, so these cells incorporate high levels of ADP. They must therefore eat large quantities of oxygen continuously, so equally to take the energy necessary to phosphorylate ADP to form ATP. Consider, for case, that resting skeletal muscles use about 30% of a resting adult'south oxygen consumption, but when the same muscles are working strenuously, they account for almost ninety% of the full oxygen consumption of the organism.

Experiment has shown that 2.v–3 ATP molecules are formed for every molecule of NADH oxidized in the electron ship chain, and 1.5–two ATP molecules are formed for every molecule of FADH₂ oxidized. Table 11.two "Maximum Yield of ATP from the Complete Oxidation of 1 Mol of Acetyl-CoA" summarizes the theoretical maximum yield of ATP produced by the complete oxidation of 1 mol of acetyl-CoA through the sequential action of the citric acrid bike, the electron ship chain, and oxidative phosphorylation.

Table xi.2 Maximum Yield of ATP from the Complete Oxidation of 1 Mol of Acetyl-CoA

Reaction	Comments	Yield of ATP (moles)
Isocitrate → α-ketoglutarate + COtwo	produces 1 mol NADH
α-ketoglutarate → succinyl-CoA + COtwo	produces 1 mol NADH
Succinyl-CoA → succinate	produces 1 mol GTP	+1
Succinate → fumarate	produces 1 mol FADH2
Malate → oxaloacetate	produces ane mol NADH
1 FADH2 from the citric acid cycle	yields 2 mol ATP	+two
3 NADH from the citric acid cycle	yields iii mol ATP/NADH	+9
Net yield of ATP:		+12

Concept Review Exercises

1. What is the main function of the citric acrid cycle?

2. Two carbon atoms are fed into the citric acid cycle as acetyl-CoA. In what course are two carbon atoms removed from the bike?

3. What are mitochondria and what is their office in the cell?

Answers

1. the consummate oxidation of carbon atoms to carbon dioxide and the germination of a high-energy phosphate compound, energy rich reduced coenzymes (NADH and FADH_ii), and metabolic intermediates for the synthesis of other compounds

2. as carbon dioxide

3. Mitochondria are small-scale organelles with a double membrane that contain the enzymes and other molecules needed for the production of near of the ATP needed by the trunk.

Key Takeaways

• The acetyl grouping of acetyl-CoA enters the citric acid cycle. For each acetyl-CoA that enters the citric acid cycle, 2 molecules of carbon dioxide, three molecules of NADH, 1 molecule of ATP, and 1 molecule of FADH_ii are produced.
• The reduced coenzymes (NADH and FADH_two) produced past the citric acid cycle are reoxidized past the reactions of the electron transport chain. This series of reactions likewise produces a pH slope beyond the inner mitochondrial membrane.
• The pH gradient produced by the electron send chain drives the synthesis of ATP from ADP. For each NADH reoxidized, 2.v–3 molecules of ATP are produced; for each FADH₂ reoxidized, 1.5–two molecules of ATP are produced.

Exercises

1. Replace each question mark with the correct compound.

a. $\text{?}\stackrel{\text{aconitase}}{\to }\text{ isocitrate}$

b. $\text{? + ?}\stackrel{\text{citrate synthase}}{\to }\text{ citrate + coenzyme A}$

c. $\text{fumarate}\stackrel{\text{fumarase}}{\to }\text{ ?}$

d.

two. Supersede each question mark with the right chemical compound.

a. ${\text{malate + NAD}}^{\text{+}}\stackrel{\text{?}}{\to }\text{ oxaloacetate + NADH}$

b. $\text{? + ?}\stackrel{\text{nucleoside diphosphokinase}}{\to }\text{ GDP + ATP}$

c. $\text{succinyl-CoA}\stackrel{\text{succinyl-CoA synthetase}}{\to }\text{ ? + ?}$

d.

3. From the reactions in Exercises 1 and 2, select the equation(s) past number and letter in which each type of reaction occurs.

a. isomerization

b. hydration

c. synthesis

4. From the reactions in Exercises i and 2, select the equation(south) by number and letter in which each type of reaction occurs.

a. oxidation

b. decarboxylation

c. phosphorylation

5. What similar role practice coenzyme Q and cytochrome c serve in the electron ship chain?

6. What is the electron acceptor at the end of the electron transport chain? To what product is this chemical compound reduced?

7. What is the function of the cytochromes in the electron ship concatenation?

8.

a. What is meant past this statement? "Electron transport is tightly coupled to oxidative phosphorylation."

b. How are electron ship and oxidative phosphorylation coupled or linked?

Answers

1.

a. citrate

b. oxaloacetate + acetyl-CoA

c. malate

d. α-ketoglutarate hydrogenase complex

3.

a. reaction in 1a

b. reaction in 1c

c. reaction in 1b

five. Both molecules serve as electron shuttles between the complexes of the electron ship chain.

7. Cytochromes are proteins in the electron transport concatenation and serve as one-electron carriers.

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Source: https://guides.hostos.cuny.edu/che120/chapter11

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