Electron Transport
Chain and Oxidative
Phosphorylation
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Oxidative Phosphorylation: Electron Transport Chain and ATP Synthesis, Lecture notes of Medical Biochemistry
electron transport chain and oxidative phosphorylation
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2018/2019
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Electron Transport
Chain and Oxidative
Phosphorylation
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(^) Energy-rich molecules, such as glucose, are metabolized by a series of oxidation reactions ultimately yielding CO2 and water. (^) The metabolic intermediates of these reactions donate electrons to specific coenzymes—nicotinamide adenine dinucleotide (NAD+) and flavin adenine dinucleotide (FAD)—to form the energy-rich reduced coenzymes, NADH and FADH. (^) These reduced coenzymes can, in turn, each donate a pair of electrons to a specialized set of electron carriers, collectively called the electron transport chain, described in this section. (^) As electrons are passed down the electron transport chain, they lose much of their free energy. (^) A: Part of this energy can be captured and stored by the production of ATP from ADP and inorganic phosphate (Pi). (^) This process is called oxidative phosphorylation. (^) B: The remainder of the free energy not trapped as ATP is used to drive ancillary reactions such as Ca2+ transport into mitochondria, and to generate heat.
The metabolic breakdown of energy yielding
molecules.
Membranes of the mitochondrion:
(^) The components of the electron transport chain are located in the inner membrane. (^) Although the outer membrane contains special pores, making it freely permeable to most ions and small molecules, the inner mitochondrial membrane is a specialized structure that is impermeable to most small ions, including H+, Na+, and K+, and small molecules such as ATP, ADP, pyruvate, and other metabolites important to mitochondrial function (^) Specialized carriers or transport systems are required to move ions or molecules across this membrane. (^) The inner mitochondrial membrane is unusually rich in protein , half of which is directly involved in electron transport and oxidative phosphorylation. (^) The inner mitochondrial membrane is highly convoluted. (^) The convolutions, called cristae, serve to greatly increase the surface area of the membrane
Matrix of the mitochondrion:
(^) This gel-like solution in the interior of mitochondria is 50% protein. (^) These molecules include the enzymes responsible for the oxidation of pyruvate, amino acids, fatty acids (by β-oxidation), and those of the tricarboxylic acid (TCA) cycle.
- The synthesis of glucose,
- urea, and
- heme occur partially in the matrix of mitochondria. (^) In addition, the matrix contains NAD+ and FAD (the oxidized forms of the two coenzymes that are required as hydrogen acceptors) and ADP and Pi, which are used to produce ATP. (^) Note: The matrix also contains mitochondrial RNA and DNA (mtRNA and mtDNA) and mitochondrial ribosomes.
Electron transport chain
Complex V
Complex V is a protein complex that contains a domain
(Fo) that spans the inner mitochondrial membrane, and a
domain (F1) that appears as a sphere that protrudes into
the mitochondrial matrix
Complex V catalyzes ATP synthesis and so is referred to
as ATP synthase.
Reactions of the electron transport chain (^) Formation of NADH: NAD+ is reduced to NADH by dehydrogenases that remove two hydrogen atoms from their substrate. (^) Both electrons but only one proton (that is, a hydride ion, : H–) are transferred to the NAD+, forming NADH plus a free proton, H+. (^) NADH dehydrogenase: The free proton plus the hydride ion carried by NADH are next transferred to NADH dehydrogenase, a protein complex (Complex I) embedded in the inner mitochondrial membrane. (^) Complex I has a tightly bound molecule of flavin mono nucleotide (FMN, a coenzyme structurally related to FAD) that accepts the two hydrogen atoms (2e– + 2H+), becoming FMNH2. (^) NADH dehydrogenase also contains iron atoms paired with sulfur atoms to make iron–sulfur centers. (^) These are necessary for the transfer of the hydrogen atoms to the next member of the chain, coenzyme Q (ubiquinone).
Iron-sulfur center of Complex I.
Cytochromes
(^) The remaining members of the electron transport chain are cytochromes. (^) Each contains a heme group (a porphyrin ring plus iron). (^) Unlike the heme groups of hemoglobin, the cytochrome iron is reversibly converted from its ferric (Fe3+) to its ferrous (Fe2+) form as a normal part of its function as a reversible carrier of electrons. (^) Electrons are passed along the chain from CoQ to cytochromes bc1 (Complex III), c, and a + a3 (Complex IV) (^) Note: Cytochrome c is associated with the outer face of the inner membrane and, like CoQ, is a mobile carrier of electrons.
Cytochrome a + a3:
(^) This cytochrome complex is the only electron carrier in which the heme iron has an available coordination site that can react directly with O2, and so also is called cytochrome oxidase. (^) At this site, the transported electrons, O2, and free protons (^) are brought together, and O2 is reduced to water. (^) Cytochrome oxidase contains copper atoms that are required for this complex reaction to occur.
ROS
(^) Incomplete reduction of oxygen to water produces reactive oxygen species (ROS),
- such as superoxide (O2– • ),
- hydrogen peroxide (H2O2) and
- hydroxyl radicals (OH•). (^) ROS damage DNA and proteins, and cause lipid peroxidation. (^) Enzymes such as superoxide dismutase (SOD), catalase, and glutathione peroxidase are cellular defenses against ROS. (^) Release of free energy during electron transport (^) Free energy is released as electrons are transferred along the electron transport chain from an electron donor (reducing agent or reductant) to an electron acceptor (oxidizing agent or oxidant).
Redox pairs:
(^) Oxidation (loss of electrons) of one compound is always accompanied by reduction (gain of electrons) of a second substance. (^) For example, Figure 6.11 shows the oxidation of NADH to NAD+ accompanied by the reduction of FMN to FMNH2. (^) Such oxidation-reduction reactions can be written as the sum of two separate half-reactions, one an oxidation reaction and the other a reduction reaction. (^) NAD+ and NADH form a redox pair, as do FMN and FMNH2. (^) Redox pairs differ in their tendency to lose electrons. (^) This tendency is a characteristic of a particular redox pair, and can be quantitatively specified by a constant, Eo ( the standard reduction potential ), with units in volts.