Let’s examine more closely the energetically favored movement of electrons from NADH and FADH2 to the final electron acceptor, O2. For simplicity, we will focus our discussion on NADH. In respiring mitochondria, each NADH molecule releases two electrons to the electron-transport chain; these electrons ultimately reduce one oxygen atom (half of an O2 molecule), forming one molecule of water:
As electrons move from NADH to O2, their electric potential declines by 1.14 V, which corresponds to 26.2 kcal/mol of electrons transferred, or about 53 kcal/mol for a pair of electrons. As noted earlier, much of this energy is conserved in the proton-motive force generated across the inner mitochondrial membrane.
Four large multiprotein complexes (complexes I–IV) compose the electron-transport chain in the inner mitochondrial membrane that is responsible for the generation of the proton-motive force (see Figure 1, stage III). Each complex contains several prosthetic groups that participate in the process of moving electrons from donor molecules to acceptor molecules in coupled oxidation-reduction reactions. These small nonpeptide organic molecules or metal ions are tightly and specifically associated with the multiprotein complexes (Table 1).
Fig1. Summary of aerobic oxidation of glucose and fatty acids. Stage I: In the cytosol, glucose is converted to pyruvate (glycolysis) and fatty acid to fatty acyl CoA. Pyruvate and fatty acyl CoA then move into the mitochondrion. Mitochondrial porins make the outer membrane permeable to these metabolites, but specific transport proteins (colored ovals) in the inner membrane are required to import pyruvate (yellow) and fatty acids (blue) into the matrix. Fatty acyl groups are transferred from fatty acyl CoA to an intermediate carrier, transported across the inner membrane, and then reattached to CoA on the matrix side. Stage II: In the mitochondrial matrix, pyruvate and fatty acyl CoA are converted to acetyl CoA and then oxidized, releasing CO2. Pyruvate is converted to acetyl CoA with the formation of NADH and CO2; two carbons from fatty acyl CoA are converted to acetyl CoA with the formation of FADH2 and NADH. Oxidation of acetyl CoA in the citric acid cycle generates NADH and FADH2, GTP, and CO2. Stage III: Electron transport reduces O2 to H2O and generates a proton motive force. Electrons (blue) from reduced coenzymes are transferred via electron-transport complexes (blue boxes) to O2 concomitant with transport of H+ ions (red) from the matrix to the intermembrane space, generating the proton-motive force. Electrons from NADH flow directly from complex I to complex III, bypassing complex II. Electrons from FADH2 flow directly from complex II to complex III, bypassing com plex I. Stage IV: ATP synthase, also called the F0F1 complex (orange), harnesses the proton-motive force to synthesize ATP in the matrix. Antiporter proteins (purple and green ovals) transport ADP and Pi into the matrix and export hydroxyl groups and ATP. NADH generated in the cytosol is not transported directly to the matrix because the inner membrane is impermeable to NAD+ and NADH; instead, a shuttle system (red) transports electrons from cytosolic NADH to NAD+ in the matrix. O2 diffuses into the matrix, and CO2 diffuses out.
Table1. Electron-Carrying Prosthetic Groups in the Electron-Transport Chain
Heme and the Cytochromes Several types of heme, an iron containing prosthetic group similar to that found in hemoglobin and myoglobin (Figure 2a), are tightly bound (covalently or noncovalently) to a set of mitochondrial proteins called cytochromes. Each cytochrome is designated by a letter, such as a, b, c, or c1. Electron flow through the cytochromes occurs by oxidation and reduction of the Fe atom in the center of the heme molecule:
Because the heme ring in cytochromes consists of alternating double- and single-bonded atoms, a large number of resonance hybrid forms exist. These forms allow the extra electron delivered to the cytochrome to be spread throughout the heme carbon and nitrogen atoms as well as the Fe ion.
Fig2. Heme and iron-sulfur pros thetic groups in the electron-transport chain. (a) Heme portion of cytochromes bL and bH, which are components of CoQH2–cytochrome c reductase (complex III). The same porphyrin ring (yellow) is present in all hemes. The chemical substituents attached to the porphyrin ring differ in the other cytochromes in the electron-transport chain. All hemes accept and release one electron at a time. (b) Dimeric iron-sulfur cluster (Fe-S). Each Fe atom is bonded to four S atoms: two are inorganic sulfur, and two are in cysteine side chains of the associated protein. All Fe-S clusters accept and release one electron at a time.
The various cytochromes each have slightly different heme groups and surrounding atoms (called axial ligands), which generate different environments for the Fe ion. Therefore, each cytochrome has a different reduction potential, or tendency to accept an electron—an important property that dictates the unidirectional, energetically “downhill” electron flow along the chain. Just as water spontaneously flows downhill from a higher to a lower potential energy state—but not uphill—electrons flow in only one direction from one heme (or other prosthetic group) to another due to their differing reduction potentials. (For more on the concept of reduction potential, E.) All the cytochromes except cytochrome c are components of integral membrane multiprotein complexes in the inner mitochondrial membrane.
Iron-Sulfur Clusters Iron-sulfur clusters are nonheme, iron containing prosthetic groups consisting of Fe atoms bonded both to inorganic sulfur (S) atoms and to S atoms on cysteine residues in a protein (Figure 2b). Some Fe atoms in the cluster bear a +2 charge; others have a +3 charge. However, the net charge of each Fe atom is actually between +2 and +3, because electrons in their outermost orbitals, together with the extra electron delivered via the transport chain, are dispersed among the Fe atoms and move rapidly from one atom to another. Iron-sulfur clusters accept and release electrons one at a time.
Coenzyme Q Coenzyme Q (CoQ), also called ubiquinone, is the only small-molecule electron carrier in the electron transport chain that is not an essentially irreversibly protein bound prosthetic group (Figure 3). It is a carrier of both protons and electrons. The oxidized quinone form of CoQ can accept a single electron to form a semiquinone, a charged free radical denoted by CoQ●−. Addition of a second electron and two protons (thus a total of two hydrogen atoms) to CoQ●− forms dihydroubiquinone (CoQH2), the fully reduced form. Both CoQ and CoQH2 are soluble in phospholipids and diffuse freely in the hydrophobic center of the inner mitochondrial membrane. These properties under lie ubiquinone’s role in the electron-transport chain: carrying electrons and protons between the membrane- embedded protein complexes of the chain.
Fig3. Oxidized and reduced forms of coenzyme Q (CoQ), which can carry two protons and two electrons. Because of its long hydrocarbon “tail” of isoprene units, CoQ, also called ubiquinone, is soluble in the hydrophobic core of phospholipid bilayers and is very mobile. Reduction of CoQ to the fully reduced form, QH2 (dihydro quinone), occurs in two steps with a half-reduced free-radical intermediate, called semiquinone.
Next we consider in detail the multiprotein complexes that use these prosthetic groups and the paths taken by electrons and protons as they pass through these complexes.
