Most of the energy liberated during the oxidation of carbohydrate, fatty acids, and amino acids is made available within mitochondria as reducing equivalents (—H or electrons) (Figure 13–2). The enzymes of the citric acid cycle and β-oxidation (see Chapters 22 and 16), the respiratory chain complexes, and the machinery for oxidative phosphorylation are all found in mitochondria. The respiratory chain collects and transports reducing equivalents, directing them to their final reaction with oxygen to form water, and oxidative phosphorylation is the process by which the liberated free energy is trapped as high-energy phosphate.
Role of the respiratory chain of mitochondria in the conversion of food energy to ATP. Oxidation of the major foodstuffs leads to the generation of reducing equivalents (2H) that are collected by the respiratory chain for oxidation and coupled generation of ATP.
Components of the Respiratory Chain Are Contained in Four Large Protein Complexes Embedded in the Inner Mitochondrial Membrane
Electrons flow through the respiratory chain through a redox span of 1.1 V from NAD+/NADH to O2/2H2O (see Table 12–1), passing through three large protein complexes: NADH-Q oxidoreductase (Complex I), where electrons are transferred from NADH to coenzyme Q (Q) (also called ubiquinone) (Figure 13–6); Q-cytochrome c oxidoreductase (Complex III), which passes the electrons on to cytochrome c; and cytochrome c oxidase (Complex IV), which completes the chain, passing the electrons to O2 and causing it to be reduced to H2O (Figure 13–3). Some substrates with more positive redox potentials than NAD+/NADH (eg, succinate) pass electrons to Q via a fourth complex, succinate-Q reductase (Complex II), rather than Complex I. The four complexes are embedded in the inner mitochondrial membrane, but Q and cytochrome c are mobile. Q diffuses rapidly within the membrane, while cytochrome c is a soluble protein. The flow of electrons through Complexes I, III, and IV results in the pumping of protons from the matrix across the inner mitochondrial membrane into the intermembrane space (Figure 13–7).
Overview of electron flow through the respiratory chain. (cyt, cytochrome; Q, coenzyme Q or ubiquinone.)
Flavoproteins & Iron-Sulfur Proteins (Fe-S) Are Components of the Respiratory Chain Complexes
Flavoproteins (see Chapter 12) are important components of Complexes I and II. The oxidized flavin nucleotide (FMN or FAD) can be reduced in reactions involving the transfer of two electrons (to form FMNH2 or FADH2), but they can also accept one electron to form the semiquinone (see Figure 12–2). Iron-sulfur proteins (nonheme iron proteins, Fe-S) are found in Complexes I, II, and III. These may contain one, two, or four Fe atoms linked to inorganic sulfur atoms and/or via cysteine-SH groups to the protein (Figure 13–4). The Fe-S take part in single electron transfer reactions in which one Fe atom undergoes oxidoreduction between Fe2+ and Fe3+.
Iron–sulfur proteins (Fe–S). (A) The simplest Fe-S with one Fe bound by four cysteines. (B) 2Fe-2S center. (C) 4Fe-4S center. (Cys, cysteine; Pr, apoprotein; Ⓢ, Inorganic sulfur.)
Q Accepts Electrons via Complexes I & II
NADH-Q oxidoreductase or Complex I is a large L-shaped multisubunit protein that catalyzes electron transfer from NADH to Q, coupled with the transfer of four H+ across the membrane:
Electrons are transferred from NADH to FMN initially, then to a series of Fe-S centers, and finally to Q (Figure 13–5). In Complex II (succinate-Q reductase), FADH2 is formed during the conversion of succinate to fumarate in the citric acid cycle (see Figure 16–3) and electrons are then passed via several Fe-S centers to Q (Figure 13–5). Glycerol-3-phosphate (generated in the breakdown of triacylglycerols or from glycolysis, Figure 17–2) and acyl-CoA also pass electrons to Q via different pathways involving flavoproteins (Figure 13–5).
Flow of electrons through the respiratory chain complexes, showing the entry points for reducing equivalents from important substrates. Q and cyt c are mobile components of the system as indicated by the dotted arrows. The flow through Complex III (the Q cycle) is shown in more detail in Figure 13–6. (cyt, cytochrome; ETF, electron transferring flavoprotein; Fe-S, iron-sulfur protein; Q, coenzyme Q or ubiquinone.)
The Q Cycle Couples Electron Transfer to Proton Transport in Complex III
Electrons are passed from QH2 to cytochrome c via Complex III (Q-cytochrome c oxidoreductase):
The process is believed to involve cytochromes c1, bL, and bH and a Rieske Fe-S (an unusual Fe-S in which one of the Fe atoms is linked to two histidine residues rather than two cysteine residues) (Figure 13–5) and is known as the Q cycle (Figure 13–6). Q may exist in three forms: the oxidized quinone, the reduced quinol, or the semiquinone (Figure 13–6). The semiquinone is formed transiently during the cycle, one turn of which results in the oxidation of 2QH2 to Q, releasing 4H+ into the intermembrane space, and the reduction of one Q to QH2, causing 2H+ to be taken up from the matrix (Figure 13–6). Note that while Q carries two electrons, the cytochromes carry only one, thus the oxidation of one QH2 is coupled to the reduction of two molecules of cytochrome c via the Q cycle.
The Q cycle. During the oxidation of QH2 to Q, one electron is donated to cyt c via a Rieske Fe-S and cyt c1 and the second to a Q to form the semiquinone via cyt βL and cyt βH, with 2H+ being released into the intermembrane space. A similar process then occurs with a second QH2, but in this case the second electron is donated to the semiquinone, reducing it to QH2, and 2H+ are taken up from the matrix. (cyt, cytochrome; Fe-S, iron-sulfur protein; Q, coenzyme Q or ubiquinone.)
Molecular Oxygen Is Reduced to Water via Complex IV
Reduced cytochrome c is oxidized by Complex IV (cytochrome c oxidase), with the concomitant reduction of O2 to two molecules of water:
This transfer of four electrons from cytochrome c to O2 involves two heme groups, a and a3, and Cu (Figure 13–5). Electrons are passed initially to a Cu center (CuA), which contains 2Cu atoms linked to two protein cysteine-SH groups (resembling an Fe-S), then in sequence to heme a, heme a3, a second Cu center, CuB, which is linked to heme a3, and finally to O2. Of the eight H+ removed from the matrix, four are used to form two water molecules and four are pumped into the intermembrane space. Thus, for every pair of electrons passing down the chain from NADH or FADH2, 2H+ are pumped across the membrane by Complex IV. The O2 remains tightly bound to Complex IV until it is fully reduced, and this minimizes the release of potentially damaging intermediates such as superoxide anions or peroxide which are formed when O2 accepts one or two electrons, respectively (see Chapter 12).