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DENF 1521 Biochemistry

Dental Biochemistry

Lesson 5.1 Oxidative phosphorylation overview

5.1 An overview of the reactions of the
oxidative phosphorylation pathway

5.1A Lesson objectives

The objectives of this lesson are to understand the

1. Function of oxidative phosphorylation in the cell
2. Importance of the mitochondrial structure to this process
3. Sources of NADH and FADH₂ for electron transport
4. Basis of oxidation/reduction reactions

5.1B Introduction

Oxidative phosphorylation is the process by which ATP is formed as electrons are transferred from NADH or FADH₂ to molecular oxygen (O₂) by a series of electron carriers. The energy released from the oxidation of glucose, fatty acids, and amino acids is stored as the reduced coenzymes NADH or FADH₂. There is a step by step transfer of electrons from NADH or FADH₂ to specific complexes which are part of the electron transport chain. These respiratory assemblies are located in the inner mitochondrial membrane. The ultimate acceptor of these electrons is O₂. Another important feature to remember is that the process of electron transport and ATP synthesis are distinct, but coupled processes.

The structure of the mitochondria is a very important feature of the oxidative phosphorylation process. The diagram below reviews the important properties of the mitochondria with respect to oxidative phosphorylation. It is very important to remember that the inner mitochondrial membrane is impermeable to ions and small molecules.

This structure is very important because the process of electron transport establishes a proton gradient across the inner mitochondria membrane which is then used to synthesize ATP. If the inner mitochondrial membrane was permeable to protons, no gradient could be established. In a later lesson, you will see how compounds which break down the permeability barrier prevent the synthesis of ATP.

5.1C Sources of reduced coenzymes

The reduced coenzymes NADH and FADH₂, are derived mainly from the oxidation of glucose, fatty acids, and amino acids to form Acetyl CoA as indicated by the following diagram. Note how pathways leading to the formation of acetyl CoA also produce NADH and FADH₂.

Dental Biochemistry

Lesson 5.1 Oxidative phosphorylation overview

CAC introduction
Practice Exercise 1:	The inner mitochondrial membrane is impermeable to ions and small molecules. No Response True False
Practice Exercise 2:	The ultimate acceptor of the electrons from the reduced coenzymes which enter oxidative phosphorylation is No Response ATP Acetyl CoA Cytochrome c O2
Practice Exercise 3:	The establishment of a proton gradient across the inner mitochondrial membrane is due to the No Response Synthesis of ATP Passage of electrons between carriers Conversion of NADH to NADPH Hydrolysis of ATP
Practice Exercise 4:	Most of the NADH used in oxidative phosphorylation is formed by pathways leading to the formation of pyruvate. No Response True False

Dental Biochemistry

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Lesson 5.1 Oxidative phosphorylation overview

5.1D Redox potential and free energy

This section is a review of the basic principles needed to understand biological oxidative phosphorylation. If you need additional information consult a basic biochemistry or chemistry text or see the following sites containing a FAQ on REDOX reactions or an index to information of electrochemistry.

Oxidation-reduction (Redox) reactions involve the transfer of electrons from one substance to another. Redox reactions must occur together. One compound must lose electrons and the other gain electrons. The substance which loses electrons is called the reducing agent while the substance which gains electrons is called the oxidizing agent. The reduction potential is the measure of the ability of one compound to reduce another. For example, in the following reaction O₂ is the reducing agent (and is oxidized) and NADH is the oxidizing agent (and is reduced).

1/2 O2 + NADH + H+

H2O + NAD+

Dental Biochemistry

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Lesson 5.1 Oxidative phosphorylation overview

Redox potential/Free energy
Practice Exercise 5:	A compound which loses electrons as part of a redox reaction is classified as an oxidizing agent. No Response True False
Practice Exercise 6:	If a reaction has an overall E0' which is positive, then the reaction is a favorable reaction. No Response True False

Dental Biochemistry

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Lesson 5.1 Oxidative phosphorylation overview

5.1E Summary

After completing this lesson you should understand the following general aspects concerning oxidative phosphorylation.

1. The transport of electrons from NADH and FADH₂ provide "energy" for the synthesis of ATP
2. The integrity of the mitochondrial structure is essential for oxidative phosphorylation
3. NADH and FADH₂ used for electron transport result from the oxidation of fuels, mainly glucose
4. Oxidation/reduction reactions are coupled
5. The ultimate acceptor of electrons in oxidative phosphorylation is molecular oxygen (O₂)