BIOCHEMISTRY AND HUMAN NUTRITION

Module 3. Metabolism

Lesson 22
ELECTRON TRANSPORT CHAIN AND ATP SYNTHESIS

22.1 Introduction

• In eukaryotes, electron transport and oxidative phosphorylation occur in the inner membrane of mitochondria.
• These processes re-oxidize the NADH and FADH₂ that arise from the citric acid cycle (located in the mitochondrial matrix ), glycolysis (located in the cytoplasm ) and fatty acid oxidation (located in the mitochondrial matrix ) and trap the energy released as ATP.
• Oxidative phosphorylation is by far the major source of ATP in the cell. In prokaryotes, the components of electron transport and oxidative phosphorylation are located in the plasma membrane.

22.2 Redox Potential

• The oxidation of a molecule involves the loss of electrons. The reduction of a molecule involves the gain of electrons. Since in a chemical reaction, if one molecule is oxidized, another must be reduced (i.e. it is an oxidation-reduction reaction). Thus, by definition, oxidation-reduction reactions involve the transfer of electrons.
• In the oxidation-reduction reaction when the NADH is oxidized to NAD⁺, it loses electrons. When the molecular oxygen is reduced to water, it gains electrons.

• The oxidation-reduction potential, E, (or redox potential) is a measure of the affinity of a substance for electrons and is measured relative to hydrogen. A positive redox potential means that the substance has a higher affinity for electrons than does hydrogen and so would accept electrons from hydrogen. A substance with a negative redox potential has a lower affinity for electrons than does hydrogen and would donate electrons to H+, forming hydrogen. In the example above, NADH is a strong reducing agent with a negative redox potential and has a tendency to donate electrons. Oxygen is a strong oxidizing agent with a positive redox potential and has a tendency to accept electrons.
• For biological systems, the standard redox potential for a substance (E0’) is measured under standard conditions, at pH 7, and is expressed in volts. In an oxidation-reduction reaction, where electron transfer is occurring, the total voltage change of the reaction (change in electric potential, ΔE) is the sum of the voltage changes of the individual oxidation-reduction steps. The standard free energy change of a reaction at pH 7, ΔG0’, can be readily calculated from the change in redox potential ΔE0’ of the substrates and products:

ΔG^0’ = -nF ΔE₀^’

Where n is the number of electrons transferred, ΔE₀’ is in volts (V), ΔG^0’ is in kilocalories per mole (kcal mol^-1) and F is a constant called the Faraday (23.06 kcal V^-1 mol^-1). Note that a reaction with a positive ΔE0’ has a negative ΔG^0’ (i.e., is exergonic).
Thus for the reaction:

22.3 Electron Transport from NADH

• NADH oxidation and ATP synthesis do not occur in a single step. Electrons are not transferred from NADH to oxygen directly. Rather the electrons are transferred from NADH to oxygen along a chain of electron carriers collectively called the electron transport chain (also called the respiratory chain).

Fig. 22.1 Organization of ETC complexes

22.4 Electron Transport Chain

Electron transport chain consists of three large protein complexes embedded in the inner mitochondrial membrane,

• NADH dehydrogenase complex (Complex I)
• Succinate Q reductase (Complex II)
• The cytochrome bc1 complex (Complex III)
• cytochrome oxidase ( Complex IV)

Electrons flow from NADH to oxygen through these three complexes as shown in Fig 22.1

Each complex contains several electron carriers that work sequentially to carry electrons down the chain. Two free electron carriers are also needed to link these large complexes;

• Ubiquinone also called as coenzyme Q (CoQ)
• cytochrome c

22.5 ATP Synthesis (Oxidative Phosphorylation)

• Oxidative phosphorylation is also the name given to the synthesis of ATP (phosphorylation) that occurs when NADH and FADH₂ are oxidized (hence oxidative) by electron transport through the respiratory chain. Unlike substrate level phosphorylation, it does not involve phosphorylated chemical intermediates.
• This proposes that energy liberated by electron transport is used to create a proton gradient across the mitochondrial inner membrane and that is used to drive ATP synthesis (chemiosmotic hypothesis). Thus the proton gradient couples electron transport and ATP synthesis, not a chemical intermediate as in substrate level phosphorylation.
• The actual synthesis of ATP is carried out by an enzyme called ATP synthase located in the inner mitochondrial membrane. (Note that the enzyme was originally called an ATPase because, without the input of energy from electron transport, the reaction can reverse and actually hydrolyzes ATP.)

22.6 Summary

• In brief, Electron transport down the respiratory chain from NADH oxidation causes H⁺ ions to be pumped out of the mitochondrial matrix across the inner mitochondrial membrane into the intermembrane space by the three H⁺ pumps; NADH dehydrogenase, the cytochrome bc1 complex and cytochrome oxidase.
• The free energy change in transporting an electrically charged ion across a membrane leads to the formation of electrochemical proton gradient. The pumping out of the H⁺ ions generates a higher concentration of H+ ions in the intermembrane space and an electrical potential. Thus, the side of the inner mitochondrial membrane facing the intermembrane space being positive.
• The protons flow back into the mitochondrial matrix according to electrochemical gradient through the ATP synthase and this drives ATP synthesis. The ATP synthase is driven by proton-motive force, which is the sum of the pH gradient (i.e. the chemical gradient of H⁺ ions) and the membrane potential (i.e. the electrical charge potential across the inner mitochondrial membrane).
• FADH₂ is reoxidized via ubiquinone , its oxidation causes H⁺ ions to be pumped out only by the cytochrome bc1 complex and cytochrome oxidase and so the amount of ATP made from FADH₂ is less than from NADH. Measurements made have shown that 2.5 ATP molecules are synthesized per NADH oxidized whereas 1.5 ATPs are synthesized per FADH₂ oxidized.

Fig. 22.2 Summary of electron flow