Mitochondria electron-transport complexes

FIGURE 4.1 Schematics of the mitochondrion. Electron transport chain, proton pumps, and ATP synthase are components of oxidative phosphorylation. Major complexes of electron transport chain are NADH dehydrogenase, bcl complex, and cytochrome c oxidase. The quinone, Q, and cytochrome c are intermediates that shuttle electrons between the complexes. Each of the three complexes is a proton pump. Cytochrome c oxidase is the terminal complex of the electron transport chain. 

Rotenone is a complex flavonoid found in the plant Derris ellyptica. It acts by inhibiting electron transport in the mitochondrion. Derris powder is an insecticidal preparation made from the plant, which is highly toxic to hsh. 

Rotenone A complex flavonoid produced by the plant Denis ellyptica. It has insecticidal activity due to its ability to inhibit electron transport in the mitochondrion. 

Ered Sanger, a double Nobel Prize winner, sequenced the human mitochondrial genome back in 1981. This genome codes for 13 proteins and the mitochondrion possesses the genetic machinery needed to synthesize them. Thus, the mitochondria are a secondary site for protein synthesis in eukaryotic cells. It turns out that the 13 proteins coded for by the mitochondrial genome and synthesized in the mitochondria are critically important parts of the complexes of the electron transport chain, the site of ATP synthesis. The nuclear DNA codes for the remainder of the mitochondrial proteins and these are synthesized on ribosomes, and subsequently transported to the mitochondria. 

Oxidizible substrates from glycolysis, fatty acid or protein catabolism enter the mitochondrion in the form of acetyl-CoA, or as other intermediaries of the Krebs cycle, which resides within the mitochondrial matrix. Reducing equivalents in the form of NADH and FADH pass electrons to complex I (NADH-ubiquinone oxidore-ductase) or complex II (succinate dehydrogenase) of the electron transport chain, respectively. Electrons pass from complex I and II to complex III (ubiquinol-cyto-chrome c oxidoreductase) and then to complex IV (cytochrome c oxidase) which accumulates four electrons and then tetravalently reduces O2 to water. Protons are pumped into the inner membrane space at complexes I, II and IV and then diffuse down their concentration gradient through complex V (FoFi-ATPase), where their potential energy is captured in the form of ATP. In this way, ATP formation is coupled to electron transport and the formation of water, a process termed oxidative phosphorylation (OXPHOS). 

The chemiosmotic model requires that flow of electrons through the electron-transport chain leads to extrusion of protons from the mitochondrion, thus generating the proton electrochemical-potential gradient. Measurements of the number of H+ ions extruded per O atom reduced by complex IV of the electron-transport chain (the H+/0 ratio) are experimentally important because the ratio can be used to test the validity of mechanistic models of proton translocation (Sec. 14.6). 

The proteins of the respiratory chain are NADH dehydrogenase, cytochrome b, cytochrome Cj, cytochrome c, cytochrome aj, and cytochrome a. Cytochromes ai and as form a complex known as cytochrome c oxidase. The proteins are listed in the order in which they are used in the electron transport pathway. The proteins are all membrane-bound proteins, though cytochrome c is only weakly bound to the outer surface of the irmer membrane. Its polypeptide chain is not inserted in the membrane. Electrons are delivered in pairs, via NAD, to-the respiratory chain. It is thought that three protons are driven out of the mitochondrion for each pair of electrons from NAD passing down the respiratory chain and through cytochrome oxidase to oxygen (Cross, 1981 Hatefi, 1985). Further details are given in Chapter 5 and xmder Iron in Chapter 10. 

The electron transport chain (ETC) or electron transport system (ETS) shown in Figure 16-1 is located on the inner membrane of the mitochondrion and is responsible for the harnessing of free energy released as electrons travel from more reduced (more negative reduction potential, E to more oxidized (more positive carriers to drive the phosphorylation of ADP to ATP. Complex 1 accepts a pair of electrons from NADH ( = -0.32 V)... 

In eukaryotic cells aerobic metabolism occurs within the mitochondrion. Acetyl-CoA, the oxidation product of pyruvate, fatty acids, and certain amino acids (not shown), is oxidized by the reactions of the citric acid cycle within the mitochondrial matrix. The principal products of the cycle are the reduced coenzymes NADH and FADH, and C02. The high-energy electrons of NADH and FADH2 are subsequently donated to the electron transport chain (ETC), a series of electron carriers in the inner membrane. The terminal electron acceptor for the ETC is 02. The energy derived from the electron transport mechanism drives ATP synthesis by creating a proton gradient across the inner membrane. The large folded surface of the inner membrane is studded with ETC complexes, numerous types of transport proteins, and ATP synthase, the enzyme complex responsible for ATP synthesis. 

NADH carries electrons to the electron transport system inside the mitochondrion via a shuttle system (Figure 15.11). Electrons that enter via the shuttle in Figure 15.11a bypass complex I of the electron transport system, whereas electrons that enter via the shuttle in Figure 15,11b enter at complex L... 

Mitochondria (singular = mitochondrion) are the so-called "power plants" of eukaryotic cells because they are the major source of energy for these cells under aerobic conditions (when oxygen is present). Mitochondria are the sites where complex processes involved in energy generation (such as electron transport and oxidative phosphorylation) are found. The product of mitochondrial action is chemical energy stored in the form of adenosine triphosphate, more commonly called ATP. 

Overall, each NADH donates two electrons, equivalent to the reduction of V2 of an O2 molecule. A generally (but not universally) accepted estimate of the stoichiometry of ATP synthesis is that four protons are pumped at complex I, four protons at complex III, and two at complex IV. With four protons translocated for each ATP synthesized, an estimated 2.5 ATPs are formed for each NADH oxidized and 1.5 ATPs for each of the other FAD(2H)-containing flavoproteins that donate electrons to CoQ. (This calculation neglects proton requirements for the transport of phosphate and substrates from the cytosol, as well as the basal proton leak.) Thus, only approximately 30% of the energy available from NADH and FAD(2H) oxidation by O2 is used for ATP synthesis. Some of the remaining energy in the electrochemical potential is used for the transport of anions and Ca into the mitochondrion. The remainder of the energy is released as heat. Consequently, the electron transport chain is also our major source of heat. 

The oxidation reactions involved are catalyzed by a series of nicotinamide adenine dinucleotide (NAD+) or flavin adenine dinucleotide (FAD) dependent dehydrogenases in the highly conserved metabolic pathways of glycolysis, fatty acid oxidation and the tricarboxylic acid cycle, the latter two of which are localized to the mitochondrion, as is the bulk of coupled ATP synthesis. Reoxidation of the reduced cofactors (NADH and FADH2) requires molecular oxygen and is carried out by protein complexes integral to the inner mitochondrial membrane, collectively known as the respiratory, electron transport, or cytochrome, chain. Ubiquinone (UQ), and the small soluble protein cytochrome c, act as carriers of electrons between the complexes (Fig. 13.1.1). 

The degree of conservation, in terms of subunit composition and protein sequence, between mammalian respiratory chain complexes and those characterized from fungi and other organisms depends on the subunit and complex being considered (detailed in specific sections below), but in general, those subunits which are known to have a central role in electron transport are well conserved in terms of protein sequence and, where known, tertiary structure. For these subunits, a dear relationship to bacterial respiratory chain components can also be seen, which leads to the condusion that the mitochondrial respiratory chain complexes have evolved and adapted from those of the symbiotic bacterial ancestor of the mitochondrion [23]. Mitochondrial complexes have in most cases acquired many additional subunits whose function remains obscure. 

Four protein complexes, three ofthem function as proton pumps, are embedded in the inner mitochondrial membrane and constitute essential components of the electron transport chain. Every complex consists of a different set of proteins with a variety of redox-active prosthetic groups. AU in all, through oxidation of NADH, a proton gradient between the matrix and the intermembrane space is created, which eventually drives the ATP synthase-complex. The correspondingly released electrons are consumed in the reduction of oxygen to water. Both, the NADH oxidation and the oxygen reduction, as well as the ATP synthesis, take place in the matrix of the mitochondrion (Fig. 8.14). 

Figure 8.7. Reconstructions from electron micrographs of Complex I (NADHmbiquinone oxidore-ductase) of the electron transport chain of the inner membrane of the mitochondrion. (A) A more detailed L-shaped structure obtained at high ionic strength. (B,C) Structures obtained at high ionic...

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