II. BACTERIAL GROWTH AND MICROBIAL METABOLISM

C. Energy Conversion in Microorganisms

3. Mechanisms for Generating ATP

c. Oxidative Phosphorylation

Fundamental statements for this learning object:

1. Oxidative phosphorylation is the production of ATP using energy derived from the transfer of electrons in an electron transport system and occurs by chemiosmosis.
2. An atom of hydrogen contains only one proton (H⁺) and one electron.
3. Electrons have stored energy, or potential energy, ready to do work. When an atom or molecule loses that electron (becomes oxidized) that energy is released and able to do cellular work.
4. Oxidation-reduction reactions are coupled chemical reactions in which one atom or molecule loses one or more electrons (oxidation) while another atom or molecule gains those electrons (reduction).
5. An oxidation reaction during which both a proton and an electron are lost is called dehydrogenation.
6. A reduction reaction during which both a proton and an electron are gained is called hydrogenation.
7. Cells use specific molecules such as NAD⁺, FAD, and NADP⁺ to carry the electrons that are removed during the oxidation of an energy source. These molecules are called electron carriers and they alternately become oxidized and reduced during electron and proton transfer.
8. Coupled oxidation-reduction reactions and electron carriers are often part of what is called an electron transport chain.
9. The chemiosmotic theory explains the functioning of electron transport chains. According to this theory, the transfer of electrons down an electron transport system through a series of oxidation-reduction reactions releases energy. This energy allows certain carriers in the chain to transport hydrogen ions (H⁺ or protons) across a membrane.
10. In prokaryotic cells, the protons are transported from the cytoplasm of the bacterium across the cytoplasmic membrane to the periplasmic space located between the cytoplasmic membrane and the cell wall; in eukaryotic cells, protons are transported from the matrix of the mitochondria across the inner mitochondrial membrane to the intermembrane space located between the inner and outer mitochondrial membranes; in plant cells and the cells of algae, protons are transported from the stroma of the chloroplast across the thylakoid membrane into the interior space of the thylakoid.
11. As the hydrogen ions accumulate on one side of a membrane, the concentration of hydrogen ions creates an electrochemical gradient or potential difference (voltage) across the membrane called proton motive force (PMF).
12. This proton motive force provides the energy necessary for enzymes called ATP synthases to catalyze the synthesis of ATP from ADP and phosphate.

 

Learning Objectives for this Section

Oxidative phosphorylation (def) is the production of ATP using energy derived from the transfer of electrons in an electron transport system (def) and occurs by chemiosmosis.

To understand oxidative phosphorylation, it is important to first review the hydrogen atom and the process of oxidation and reduction. An atom of hydrogen contains only one proton (def) (H⁺) and one electron (def) (e^-). Therefore, the term proton and the term hydrogen ion (H⁺) are interchangeable. Also remember that electrons have stored energy, or potential energy, ready to do work and when an atom or molecule loses that electron (becomes oxidized) that energy is released and able to do cellular work.

Oxidation-reduction reactions are coupled chemical reactions in which one atom or molecule loses one or more electrons (oxidation (def)) while another atom or molecule gains those electrons (reduction (def)). The compound that loses electrons becomes oxidized; the compound that gains those electrons becomes reduced. In covalent compounds, however, it is usually easier to lose a whole hydrogen (H) atom - a proton and an electron - rather than just an electron. An oxidation reaction during which both a proton and an electron are lost is called dehydrogenation (def) . A reduction reaction during which both a proton and an electron are gained is called hydrogenation (def).

Cells use specific molecules to carry the electrons that are removed during the oxidation of an energy source. These molecules are called electron carriers and they alternately become oxidized and reduced during electron and proton transfer. These include three freely diffusible coenzymes known as NAD⁺, FAD, and NADP⁺. The reduced forms of these coenzymes (NADH, FADH₂, and NADPH) have reducing power because their bonds contain a form of usable energy.

• NAD⁺ (def) , or nicotinamide adenine dinucleotide, is a coenzyme (def) that often works in conjunction with an enzyme called a dehydrogenase. The enzyme removes two hydrogen atoms (2H⁺ and 2e^-) from its substrate. Both electrons but only one proton are accepted by the NAD⁺ to produce its reduced form, NADH, plus H⁺. NADH is used to generate proton motive force (discussed below) that can drive the synthesis of ATP.

• FAD (def), or flavin adenine dinucleotide, is a coenzyme (def) that also works in conjunction with an enzyme called a dehydrogenase. The enzyme removes two hydrogen atoms (2H⁺ and 2e^-) from its substrate. Both electrons and both protons are accepted by the FAD to produce its reduced form, FADH₂. FADH₂ is used to generate proton motive force (discussed below) that can drive the synthesis of ATP.

• NADP⁺, or nicotinamide adenine dinucleotide phosphate, is a coenzyme that uses dehydrogenase to remove two hydrogen atoms (2H⁺ and 2e^-) from its substrate. Both electrons but only one proton are accepted by the NADP⁺ to produce its reduced form, NADPH, plus H⁺. NADPH is not used for ATP synthesis but its electrons provide the energy for certain biosynthesis reactions such as ones involved in photosynthesis.

During the process of aerobic respiration, discussed in the next section, coupled oxidation-reduction reactions and electron carriers are often part of what is called an electron transport chain (def), a series of electron carriers that eventually transfers electrons from NADH and FADH₂ to oxygen. The diffusible electron carriers NADH and FADH₂ carry hydrogen atoms (protons and electrons) from substrates in exergonic catabolic pathways such as glycolysis and the citric acid cycle to other electron carriers that are embedded in membranes. These membrane-associated electron carriers include flavoproteins, iron-sulfur proteins, quinones, and cytochromes. The last electron carrier in the electron transport chain transfers the electrons to the terminal electron acceptor, oxygen.

The chemiosmotic theory (def) explains the functioning of electron transport chains. According to this theory, the transfer of electrons down an electron transport system through a series of oxidation-reduction reactions (def) releases energy (see Fig. 1). This energy allows certain carriers in the chain to transport hydrogen ions (H⁺ or protons) across a membrane.

Depending on the type of cell, the electron transport chain may be found in the cytoplasmic membrane, the inner membrane of mitochondria, and the inner membrane of chloroplasts.

• In prokaryotic cells, the protons are transported from the cytoplasm of the bacterium across the cytoplasmic membrane to the periplasmic space located between the cytoplasmic membrane and the cell wall.
• In eukaryotic cells, protons are transported from the matrix of the mitochondria across the inner mitochondrial membrane to the intermembrane space located between the inner and outer mitochondrial membranes.
• In plant cells and the cells of algae, protons are transported from the stroma of the chloroplast across the thylakoid membrane into the interior space of the thylakoid.

As the hydrogen ions accumulate on one side of a membrane, the concentration of hydrogen ions creates an electrochemical gradient or potential difference (voltage) across the membrane. (The fluid on the side of the membrane where the protons accumulate acquires a positive charge; the fluid on the opposite side of the membrane is left with a negative charge.) The energized state of the membrane as a result of this charge separation is called proton motive force (def) or PMF.

This proton motive force provides the energy necessary for enzymes called ATP synthases (see Fig. 5), also located in the membranes mentioned above, to catalyze the synthesis of ATP from ADP and phosphate. This generation of ATP occurs as the protons cross the membrane through the ATP synthase complexes and re-enter either the bacterial cytoplasm (see Fig. 5), the matrix of the mitochondria, or the stroma of the chloroplasts. As the protons move down the concentration gradient through the ATP synthase, the energy released causes the rotor and rod of the ATP synthase to rotate. The mechanical energy from this rotation is converted into chemical energy as phosphate is added to ADP to form ATP.

Proton motive force is also used to transport substances across membranes during active transport and to rotate bacterial flagella.

At the end of the electron transport chain involved in aerobic respiration, the last electron carrier in the membrane transfers 2 electrons to half an oxygen molecule (an oxygen atom) that simultaneously combines with 2 protons from the surrounding medium to produce water as an end product (see Fig. 3). The electron transport chains involved in photosynthesis ultimately transfer 2 electrons to NADP⁺ that simultaneously combines with 2 protons from the surrounding medium to produce NADPH.

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