MCAT Biochemistry Review

Chapter 10: Carbohydrate Metabolism II: Aerobic Respiration

10.3 The Electron Transport Chain

The electron transport chain is the final common pathway that utilizes the harvested electrons from different fuels in the body. It is important to make the distinction that it is not the flow of electrons but the proton gradient that ultimately produces ATP. Aerobic metabolism is the most efficient way of generating energy in living systems, and the mitochondrion is the reason why. In eukaryotes, the aerobic components of respiration are executed in mitochondria, while anaerobic processes such as glycolysis and fermentation occur in the cytosol. Looking at Figure 10.12, notice how the components of the mitochondria are critical in the harvesting of energy. The citric acid cycle takes place in the mitochondrial matrix. The assemblies needed to complete oxidative phosphorylation are housed adjacent to the matrix in the inner membrane of the mitochondria. The inner mitochondrial membrane is assembled into folds called cristae, which maximize surface area. It is the inner mitochondrial membrane that will be essential for generating ATP using the proton-motive force, an electrochemical proton gradient generated by the complexes of the electron transport chain.

Figure 10.12. Mitochondrial Structure

The final step in aerobic respiration is actually two steps: electron transport along the inner mitochondrial membrane and the generation of ATP via ADP phosphorylation. While these two processes are actually separate entities, they are very much coupled, so explaining these steps together makes a great deal of sense. The electron-rich molecules NADH and FADH2 are formed as byproducts at earlier steps in respiration. They transfer their electrons to carrier proteins located along the inner mitochondrial membrane. Finally, these electrons are given to oxygen in the form of hydride ions (H) and water is formed. While this is happening, energy released from transporting electrons facilitates proton transport at three specific locations in the chain. Protons are moved from the mitochondrial matrix into the intermembrane space of the mitochondria, thereby creating a greater concentration gradient of hydrogen ions that can be used to drive ATP production.

ELECTRON FLOW AND COMPLEXES

The formation of ATP is endergonic and electron transport is an exergonic pathway. By coupling these reactions, the energy yielded by one reaction can fuel the other. In order for energy to be harnessed via electron transport reactions, the proteins along the inner membrane must transfer the electrons donated by NADH and FADH2 in a specific order and direction. The physical property that determines the direction of electron flow is reduction potential. Recall from Chapter 12 of MCAT General Chemistry Review that if you pair two molecules with different reduction potentials, the molecule with the higher potential will be reduced, while the other molecule will become oxidized. The electron transport chain is therefore nothing more than a series of oxidations and reductions that occur via the same mechanism. NADH is a good electron donor, and the high reduction potential of oxygen makes it a great final acceptor in the electron transport chain. The organizational structure of the membrane-bound complexes that make up the transport chain is diagrammed in Figure 10.13, and further detailed below.

Figure 10.13. Respiratory Complexes on the Inner Mitochondrial Membrane

·        Complex I (NADH-CoQ oxidoreductase): the transfer of electrons from NADH to coenzyme Q (CoQ) is catalyzed in this first complex. This complex has over 20 subunits, but the two highlighted here include a protein that has an iron–sulfur cluster and a flavoprotein that oxidizes NADH. The flavoprotein has a coenzyme called flavin mononucleotide (FMN) covalently bound to it. FMN is quite similar in structure to FAD, flavin adenine dinucleotide. The first step in the reaction involves NADH transferring its electrons over to FMN, thereby becoming oxidized to NAD+ as FMN is reduced to FMNH2. Next, the flavoprotein becomes reoxidized while the iron–sulfur subunit is reduced. Finally, the reduced iron–sulfur subunit donates the electrons it received from FMNH2 to coenzyme Q (also called ubiquinone). Coenzyme Q becomes CoQH2. This first complex is one of three sites where proton pumping occurs, as four protons are moved to the intermembrane space.

·        NADH + H+ + FMN → NAD+ + FMNH2

·        FMNH2 + 2 Fe–Soxidized → FMN + 2 Fe–Sreduced + 2 H+

·        2 Fe–Sreduced + CoQ + 2 H+ → 2Fe–Soxidized + CoQH2

The net effect is passing high-energy electrons from NADH to CoQ to form CoQH2:

·        NADH + H+ + CoQ → NAD+ + CoQH2

·        Complex II (Succinate-CoQ oxidoreductase): just like complex I, complex II transfers electrons to coenzyme Q. While complex I received electrons from NADH, complex II actually receives electrons from succinate. Remember that succinate is a citric acid cycle intermediate, and that it is oxidized to fumarate upon interacting with FAD. FAD is covalently bonded to complex II, and once succinate is oxidized, it's converted to FADH2. After this, FADH2 gets reoxidized to FAD as it reduces an iron–sulfur protein. The final step reoxidizes the iron–sulfur protein as coenzyme Q is reduced. Because succinate dehydrogenase was responsible for oxidizing succinate to fumarate in the citric acid cycle, it makes sense that succinate dehydrogenase is also a part of complex II. It should be noted that no hydrogen pumping occurs here to contribute to the proton gradient.

·        succinate + FAD → fumarate + FADH2

·        FADH2 + Fe–Soxidized → FAD + Fe–Sreduced

·        Fe–Sreduced + CoQ + 2 H+ → Fe–Soxidized + CoQH2

REAL WORLD

Unlike iron in cytochromes, iron in the heme group of hemoglobin always remains as Fe2+ during the transport of oxygen through the bloodstream under normal conditions.

The net effect is passing high-energy electrons from succinate to CoQ to form CoQH2:

·        succinate + CoQ + 2 H+ → fumarate + CoQH2

·        Complex III (CoQH2-cytochrome c oxidoreductase): also called cytochrome reductase, this complex facilitates the transfer of electrons from coenzyme Q to cytochrome c in a few steps. The overall reaction is written below. The following steps involve the oxidation and reduction of cytochromes: proteins with heme groups in which iron is reduced to Fe2+ and reoxidized to Fe3+.

·        CoQH2 + 2 cytochrome c [with Fe3+] →
CoQ + 2 cytochrome c [with Fe2+] + 2 H+

In the transfer of electrons from iron, only one electron is transferred per reaction, but because coenzyme Q has two electrons to transfer, two cytochrome c molecules will be needed. Complex III's main contribution to the proton-motive force is via the Q cycle. In the Q cycle, two electrons are shuttled from a molecule of ubiquinol (CoQH2) near the intermembrane space to a molecule of ubiquinone (CoQ) near the mitochondrial matrix. Another two electrons are attached to heme moieties, reducing two molecules to cytochrome c. A carrier containing iron and sulfur assists this process. In shuttling these electrons, four protons are also displaced to the intermembrane space; therefore, the Q cycle continues to increase the gradient of the proton-motive force across the inner mitochondrial membrane.

KEY CONCEPT

Both coenzyme Q and cytochrome c aren't technically part of the complexes we're describing. However, because both are able to move freely in the inner mitochondrial membrane, this degree of mobility allows these carriers to transfer electrons by physically interacting with the next component of the transport chain.

·        Complex IV (cytochrome c oxidase): this complex facilitates the culminating step of the electron transport chain: transfer of electrons from cytochrome c to oxygen, the final electron acceptor. This complex includes subunits of cytochrome a, cytochrome a3, and Cu2+ ions. Together, cytochromes a and a3 make up cytochrome oxidase. Through a series of redox reactions, cytochrome oxidase gets oxidized as oxygen becomes reduced and forms water. This is the final location on the transport chain where proton pumping occurs, as two protons are moved across the membrane. The role proton pumping plays in ATP synthesis is an essential one that we will describe in detail next. The overall reaction is:

·        2 cytochrome c [with Fe2+] + 2 H+ + O2 →
2 cytochrome c [with Fe3+] + H2O

KEY CONCEPT

Cyanide, mentioned in the introduction to this chapter, is an inhibitor to the cytochrome c oxidase subunits a and a3. The cyanide anion is able to attach to the iron group and prevents the transfer of electrons. Tissues that rely heavily on aerobic respiration such as the heart and the central nervous system can be greatly impacted.

THE PROTON-MOTIVE FORCE

Let's take a step back and look at the proton gradient that formed as electrons were passed along the ETC. As [H+] increases in the intermembrane space, two things happen simultaneously: pH drops in the intermembrane space, and the voltage difference between the intermembrane space and matrix increases due to proton pumping. Together, these two changes contribute to what is referred to as an electrochemical gradient: a gradient that has both chemical and electrostatic properties. Because it is based on protons, we often refer to the electrochemical gradient across the inner mitochondrial membrane as the proton-motive force. Any electrochemical gradient stores energy, and it will be the responsibility of ATP synthase to harness this energy to form ATP from ADP and an inorganic phosphate.

NADH SHUTTLES

As we look at the net ATP yield per glucose, note that a range exists between 30–32. This is because efficiency of aerobic respiration varies between cells. This variable efficiency is caused by the fact that cytosolic NADH formed through glycolysis cannot directly cross into the mitochondrial matrix. Because it cannot contribute its electrons to the transport chain directly, it must find alternate means of transportation referred to as shuttle mechanisms. A shuttle mechanism transfers the high-energy electrons of NADH to a carrier that can cross the inner mitochondrial membrane. Depending on which of the two shuttle mechanisms NADH participates in, either 1.5 or 2.5 ATP will end up being produced. Let's take a look at the two mechanisms:

·        Glycerol 3-phosphate shuttle: the cytosol contains one isoform of glycerol-3-phosphate dehydrogenase, which oxidizes cytosolic NADH to NAD+ while forming glycerol 3-phosphate from dihydroxyacetone phosphate (DHAP). On the outer face of the inner mitochondrial membrane, there exists another isoform of glycerol-3-phosphate dehydrogenase that is FAD-dependent. This mitochondrial FAD is the oxidizing agent, and ends up being reduced to FADH2. Once reduced, FADH2 proceeds to transfer its electrons to the ETC via Complex II, thus generating 1.5 ATP for every molecule of cytosolic NADH that participates in this pathway, which is shown in Figure 10.14.

Figure 10.14. Glycerol-3-Phosphate Shuttle

·        Malate–aspartate shuttle: cytosolic oxaloacetate, which cannot pass through the inner mitochondrial membrane, is reduced to malate, which can. This is accomplished by cytosolic malate dehydrogenase. Accompanying this reduction is the oxidation of cytosolic NADH to NAD+. Once malate crosses into the matrix, mitochondrial malate dehydrogenase reverses the reaction to form mitochondrial NADH. Now that NADH is in the matrix, it can pass along its electrons to the ETC via Complex I and generate 2.5 ATP per molecule of NADH. Recycling the malate requires oxidation to oxaloacetate, which can be transaminated to form aspartate. Aspartate crosses into the cytosol, and can be reconverted to oxaloacetate to restart the cycle, as shown in Figure 10.15.

Figure 10.15. Malate–Aspartate Shuttle

BRIDGE

Glycerol 3-phosphate is an important link between lipid metabolism, discussed in Chapter 11 of MCAT Biochemistry Review, and glycolysis, discussed in Chapter 9. Its ability to be converted to DHAP, an intermediate of glycolysis, means that the glycerol of triacylglycerols can be shunted into glycolysis for energy.

MCAT Concept Check 10.3:

Before you move on, assess your understanding of the material with these questions.

1.    Which complex(es) are associated with each of the following? (circle all that apply)

·        Pumping a proton into the intermembrane space

I

II

III

IV

·        Acquiring electrons from NADH

I

II

III

IV

·        Acquiring electrons from FADH2

I

II

III

IV

·        Having the highest reduction potential

I

II

III

IV

2.    What role does the electron transport chain play in the generation of ATP?

3.    Based on its needs, which of the two shuttle mechanisms is cardiac muscle most likely to utilize? Why?