8885ac19690-7503/1/0411:32 AM Page703mac76mac76:385 19.1 Electron-Transfer Reactions in Mitochondria Intermembrane space(P side C q NADH H+ NAD+ Succinate Fumarate 2O2+ 2H H2o Matrix(N side) FIGURE 19-15 Summary of the flow of electrons and protons then transfers electrons from reduced cytochrome c to O2 Electron through the four complexes of the respiratory chain. Electrons reach flow through Complexes L, l, and IV is accompanied by proton flow Q through Complexes I and Il QH2 serves as a mobile carrier of elec. from the matrix to the intermembrane space. Recall that electrons from trons and protons. It passes electrons to Complex Ill, which passes B oxidation of fatty acids can also enter the respiratory chain through them to another mobile connecting link, cytochrome c Q(see Fig. 19-8) Much of this energy is used to pump protons out of units more alkaline than that of the intermembrane the matrix. For each pair of electrons transferred to O2, space, so the calculated free-energy change for pump four protons are pumped out by Complex I, four by Com- ing protons outward is about 20 k/mol (of H), most plex Ill, and two by Complex IV (Fig. 19-15). The vec- of which is contributed by the electrical portion of the torial equation for the process is therefore electrochemical potential. Because the transfer of two NADH 1lHN +5O2-NAD+ 10HP+H20(19-7 electrons from NADH to O, is accompanied by the out- ward pumping of 10 H(Egn 19-7, roughly 200 kJ of The electrochemical energy inherent in this difference the 220 kj released by oxidation of a mole of NADH is in proton concentration and separation of charge rep conserved in the proton gradient resents a temporary conservation of much of the energy When protons flow spontaneously down their elec of electron transfer. The energy stored in such a gradi- trochemical gradient, energy is made available to do it, termed the proton-motive force, has two com- work. In mitochondria, chloroplasts, and aerobic bacte- ponents: (1) the chemical potential energy due to the ria, the electrochemical energy in the proton gradient difference in concentration of a chemical species(h) drives the synthesis of ATP from ADP and Pi. We return in the two regions separated by the membrane, and (2) to the energetics and stoichiometry of ATP synthesis the electrical potential energy that results from the driven by the electrochemical potential of the proton separation of charge when a proton moves across the gradient in Section 19.2 membrane without a counterion(Fig. 19-16) As we showed in Chapter ll, the free-energy change for the creation of an electrochemical gradient by an ion pump is +Z Ay (19-8) [HTI =C [HTJ=C1 where C2 and Ci are the concentrations of an ion in two HT OH regions, and C2>Cl; Z is the absolute value of its elec H+ OH trical charge(1 for a proton), and Ay is the transmem- H+ OH brane difference in electrical potential, measured in volts HOHˉ For protons at25°C H+ 2.3(log [H]P- log [H IN) OH =23(pHN-pHp)=23△pH AG=RThn(C2C1)+Z子M and Equation 19-8 reduces to =23 RT ApH+3△ψ FIGURE 19-16 Proton-motive force. The inner mitochondrial mem. △G=23RT△pH+Mψ brane separates two compartments of different [H"I, resulting in dif- =(5.70kJ/mol)△pH+(96.5kN·mol) ferences in chemical concentration(ApH) and charge distribution (AM) across the membrane. The net effect is the proton-motive force In actively respiring mitochondria, the measured A is (AC), which can be calculated as shown here. This is explained more 0. 15 to 0.20 V and the ph of the matrix is about 0. 75 fullyMuch of this energy is used to pump protons out of the matrix. For each pair of electrons transferred to O2, four protons are pumped out by Complex I, four by Com￾plex III, and two by Complex IV (Fig. 19–15). The vec￾torial equation for the process is therefore NADH
11H
N
1 2 O2 On NAD

10H
P
H2O (19–7) The electrochemical energy inherent in this difference in proton concentration and separation of charge rep￾resents a temporary conservation of much of the energy of electron transfer. The energy stored in such a gradi￾ent, termed the proton-motive force, has two com￾ponents: (1) the chemical potential energy due to the difference in concentration of a chemical species (H
) in the two regions separated by the membrane, and (2) the electrical potential energy that results from the separation of charge when a proton moves across the membrane without a counterion (Fig. 19–16). As we showed in Chapter 11, the free-energy change for the creation of an electrochemical gradient by an ion pump is G  RT ln

Z  (19–8) where C2 and C1 are the concentrations of an ion in two regions, and C2 C1; Z is the absolute value of its elec￾trical charge (1 for a proton), and  is the transmem￾brane difference in electrical potential, measured in volts. For protons at 25 C, ln
 2.3(log [H
]P  log [H
]N)  2.3(pHN  pHP)  2.3 pH and Equation 19–8 reduces to G  2.3RT pH
 (19–9)  (5.70 kJ/mol)pH
(96.5 kJ/V  mol)∆ In actively respiring mitochondria, the measured ∆ is 0.15 to 0.20 V and the pH of the matrix is about 0.75 C2 C1 C2 C1 units more alkaline than that of the intermembrane space, so the calculated free-energy change for pump￾ing protons outward is about 20 kJ/mol (of H
), most of which is contributed by the electrical portion of the electrochemical potential. Because the transfer of two electrons from NADH to O2 is accompanied by the out￾ward pumping of 10 H
(Eqn 19–7), roughly 200 kJ of the 220 kJ released by oxidation of a mole of NADH is conserved in the proton gradient. When protons flow spontaneously down their elec￾trochemical gradient, energy is made available to do work. In mitochondria, chloroplasts, and aerobic bacte￾ria, the electrochemical energy in the proton gradient drives the synthesis of ATP from ADP and Pi. We return to the energetics and stoichiometry of ATP synthesis driven by the electrochemical potential of the proton gradient in Section 19.2. 19.1 Electron-Transfer Reactions in Mitochondria 703 Intermembrane space (P side) Matrix (N side) 4H+ 1 –2 O2 + 2H+ H2O Succinate Fumarate 4H+ II NADH + H+ NAD+ Cyt c IV 2H+ III I Q FIGURE 19–15 Summary of the flow of electrons and protons through the four complexes of the respiratory chain. Electrons reach Q through Complexes I and II. QH2 serves as a mobile carrier of elec￾trons and protons. It passes electrons to Complex III, which passes them to another mobile connecting link, cytochrome c. Complex IV then transfers electrons from reduced cytochrome c to O2. Electron flow through Complexes I, III, and IV is accompanied by proton flow from the matrix to the intermembrane space. Recall that electrons from
oxidation of fatty acids can also enter the respiratory chain through Q (see Fig. 19–8). N side [H
]N  C1 OH OH OH OH OH OH OH H
H
H
H
H
H
H
P side [H
]P  C2 G  RT ln (C2/C1)
Zℑ   2.3RT pH
ℑ ∆ H
Proton pump   FIGURE 19–16 Proton-motive force. The inner mitochondrial mem￾brane separates two compartments of different [H
], resulting in dif￾ferences in chemical concentration (pH) and charge distribution () across the membrane. The net effect is the proton-motive force (G), which can be calculated as shown here. This is explained more fully in the text. 8885d_c19_690-750 3/1/04 11:32 AM Page 703 mac76 mac76:385_reb: