Application Box 51 Reduction Oxidation Redox Potentials AG0 and ATP

(1) Redox potentials

The standard redox potential (E0') for a redox couple such as NAD+-NADH is determined by the ability of the couple to transfer electrons to (or take them from) the standard redox couple H+-H2, or standard hydrogen electrode. A strong reducing agent such as NADH coupled to a weak oxidizing agent (NAD+) will transfer electrons (electrons have a negative voltage) to the standard couple. A strong oxidizing agent such as O2 will have a positive voltage with respect to the standard hydrogen electrode. The half-cell potential of NADH with respect to molecular oxygen is indicated below by aE0'.

NAD+ + 2H+ + 2e- i NADH + H+ E0' (volts, V) = -0.320 1/2O2 + 2H+ + 2e- i H2O E0' (volts, V) = +0.816 aE0' = +0.82 V - (-0.32 V) = 1.136 V Note: E0' for FAD-FADH2 is +0.031.

where n = number of electrons transferred and F = Faraday's constant of 23.063 kcal/V mol.

aG°' = -(2)(1.136 V)(23.063 kcal/V mol) = -52.5 kcal/mol

(3) Efficiency of energy transfer to AT

aG°' = +7.3 kcal/mol for synthesis of ATP from ADP and Pi. However, a cell doesn't function under standard conditions, so a more realistic value for ATP synthesis is 10 kcal/mol. If one assumes that three ATP are produced from the energy derived from a pair of electrons in NADH, then the calculated efficiency for energy transfer to ATP would be about 57%, which is a high efficiency for a cellular process.

include flavoproteins, cytochromes (cyt), iron-sulfur proteins (Fe-S), copper atoms, and coenzyme Q (CoQ).

Flavoproteins are polypeptides that bind the prosthetic groups (i.e., non-amino acid compounds attached to proteins) flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD) (Fig. 5.9). Cytochromes are proteins that contain the heme prosthetic group with an iron atom that switches between Fe2+ and Fe3+ oxidation states when it accepts or donates one electron. Fe-S proteins contain iron atoms linked to the sulfur in cysteines. Although the Fe-S centers may contain multiple iron atoms, they are only capable of donating or accepting one electron. Copper atoms in Complex IV switch between Cu2+ and Cu+ oxidation states and like iron, accept or donate a single electron. CoQ is not a protein. It is a lipid-soluble quinone that can be reduced in two steps to carry two electrons. CoQ does not associate with one of the four respiratory complexes; rather, it serves as a mobile electron carrier in the membrane, shuttling electrons from Complex I or II to Complex III. Reduced CoQ also acquires protons from the matrix and upon oxidation, releases the protons into the intermembrane space (Fig. 5.9). By transferring protons across the membrane, CoQ increases the proton electrochemical gradient used to generate ATP.

As noted above, oxidation of glucose via glycolysis, pyruvate oxidation, and the TCA cycle result in electrons being stored in 10 molecules of NADH and 2 molecules of FADH2. All the reduced coenzymes are in the mitochondrion, except for the two NADH produced during glycolysis, a cytosolic pathway. The electrons associated with the cytosolic NADH most often are transferred to FAD in the inner mitochondrial membrane via the glycerol phosphate shuttle. Hence, electrons in the two molecules of cytosolic NADH usually lose energy as they are transferred into the mitochondrion. In some tissues, electrons associated with the cytosolic NADH can be passed to mitochondrial NAD+ via the malate-aspartate shuttle.

Two electrons from an NADH molecule enter Complex I (NADH dehydrogenase complex), which is composed of approximately 45 different subunits and contains 6 to 9 Fe-S centers. The electrons pass to an FMN prothetic group, Fe-S centers and then to CoQ (Fig. 5.9). Energy is released with each succeeding oxidation-reduction. The released energy is somehow coupled to translocation of four protons from the mitochondrial matrix to the intermembrane space. Translocation creates a proton (electrochemical) gradient between the matrix and the intermembrane space that preserves some of the energy released in the oxidation-reduction reactions.

Complex II (succinate dehydrogenase) transfers electrons from succi-nate in step 6 of the TCA cycle to CoQ. The complex consists of four polypeptides, two of which comprise succinate dehydrogenase, and three Fe-S centers. Electrons from succinate pass to FAD, Fe-S centers, and then to CoQ (Fig. 5.9). Electron transfer through Complex II is not coupled to proton translocation; thus, in comparison to the electrons in NADH, the electrons in FADH2 contribute less to the electrochemical gradient (i.e., fewer ATP).

A pair of electrons from reduced CoQ (CoQH2) is transferred to Complex III (cytochrome bc1), which is composed of 11 polypeptides, 2 cyt b, cyt cx, and an Fe-S center. Transfer of the electrons across

Complex III to cyt c is associated with translocation of four protons into the intermembrane space (Fig. 5.9). Two of the protons are derived from reduced CoQ when it enters the complex, and two additional protons are translocated as part of the Q Cycle.

The Q Cycle stems directly from the initial transfer of electrons from CoQ to Complex III (Fig. 5.9). One electron carried by reduced CoQ is passed to the Rieske Fe-S center, which can only accept one electron. The second electron is transferred to cyt bL. The electron transferred to the FeS center passes to cyt cx and then to cyt c (Fig. 5.9). However, the electron passed to cyt bL is transferred to cyt bH (only one cyt b is shown in Fig. 5.9) and then back to a CoQ. After CoQ receives a second electron from cyt bH, it acquires two protons from the matrix and prepares to transfer the electrons to Complex III. Upon transfer of the electrons to the Fe-S center and cyt bL, the second pair of protons is released to the intermembrane space.

Cyt c carries electrons to Complex IV (cytochrome oxidase), which is composed of 13 polypeptides, including cyt a, cyt a3, CuA (dimeric copper center), and CuB. The electron from cyt c is initially transferred to CuA. Reduced CuA then passes its single electron to the heme group (Fe) in cyt a. From cyt a, the electron is transferred to the binuclear redox center consisting of the heme group (Fe) of cyt a3 and the copper atom of CuB (Fig. 5.9). Two electrons are required to reduce the cyt a3-CuB bi-nuclear center. Molecular oxygen then binds to the center and accepts the electrons. The binuclear center essentially holds the oxygen in place until it is fully reduced to two H2O by acquiring two additional electrons and four protons. Release of partially reduced oxygen species that are extremely reactive leads to cellular damage (see Clinical Box 5.1). For each pair of electrons that passes through Complex IV, two protons translocate (are "pumped") from the matrix to the intermembrane space. In addition, two protons are removed from the matrix when they bind to oxygen to form water. Both actions contribute to the electrochemical gradient between the matrix and the intermembrane space.

5.3.4 Chemiosmosis and ATP synthesis

Peter D. Mitchell (1920-1992), the British biochemist and Nobel laureate, first proposed the chemiosmotic coupling model, which suggested that most energy required for ATP synthesis is derived from an electrochemical gradient across the inner mitochondrial membrane. As described in the previous sections, the electrochemical gradient primarily is created by coupling energy released during electron transport to "pumping" of protons into the intermembrane space. Binding of matrix protons to oxygen to form water also contributes to the gradient. The chemical component of the gradient is due to the higher concentration of hydrogen ions (protons) in the intermembrane space relative to

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