Transport mitochondrial

As in the case of mitochondrial proteins, some of the proteins of chloroplasts are synthesized directly in the organelle, whereas others are synthesized in the cytosol and must be transported. The mechanisms for transport are very similar to those observed for transported mitochondrial proteins. 

ATP synthase, H+ transporting, mitochondrial FI complex, beta polypeptide [Homo sapiens]... 

See also Electron Transport, Mitochondrial Structure and Function... 

Insects poisoned with rotenone exhibit a steady decline ia oxygen consumption and the iasecticide has been shown to have a specific action ia interfering with the electron transport iavolved ia the oxidation of reduced nicotinamide adenine dinucleotide (NADH) to nicotinamide adenine dinucleotide (NAD) by cytochrome b. Poisoning, therefore, inhibits the mitochondrial oxidation of Krebs-cycle iatermediates which is catalysed by NAD. 

Hydramethylnon [67485-29-4] is tetrabydro-5,5-dimetbyl-2-(1 H)-pyrimidinone [bis-l,5-(4-trifluoromethylphenyl)-3-penta-l,4-dienylidene] hydrazone (152) (mp 189°C). It is a slow-acting stomach poison used in baits and traps to control ants and cockroaches. Its mode of action is inhibition of mitochondrial electron transport. 

The decline in immune function may pardy depend on a deficiency of coenzyme Q, a group of closely related quinone compounds (ubiquinones) that participate in the mitochondrial electron transport chain (49). Concentrations of coenzyme Q (specifically coenzyme Q q) appear to decline with age in several organs, most notably the thymus. 

FIGURE 21.3 % J and % values for the components of the mitochondrial electron transport chain. Values indicated are consensus values for animal mitochondria. Black bars represent %r red bars,. ... 

Protein Complexes of the Mitochondrial Electron-Transport Cham ... 

This is a crucial point because (as we will see) proton transport is coupled with ATP synthesis. Oxidation of one FADHg in the electron transport chain results in synthesis of approximately two molecules of ATP, compared with the approximately three ATPs produced by the oxidation of one NADH. Other enzymes can also supply electrons to UQ, including mitochondrial 5w-glyc-erophosphate dehydrogenase, an inner membrane-bound shuttle enzyme, and the fatty acyl-CoA dehydrogenases, three soluble matrix enzymes involved in fatty acid oxidation (Figure 21.7 also see Chapter 24). The path of electrons from succinate to UQ is shown in Figure 21.8. 

As with Complex 1, passage of electrons through the Q cycle of Complex 111 is accompanied by proton transport across the inner mitochondrial membrane. The postulated pathway for electrons in this system is shown in Figure 21.12. A large pool of UQ and UQHg exists in the inner mitochondrial membrane. The Q cycle is initiated when a molecule of UQHg from this pool diffuses to a site (called Q, ) on Complex 111 near the cytosolic face of the membrane. 

Cytochrome c, like UQ is a mobile electron carrier. It associates loosely with the inner mitochondrial membrane (in the intermembrane space on the cytosolic side of the inner membrane) to acquire electrons from the Fe-S-cyt C aggregate of Complex 111, and then it migrates along the membrane surface in the reduced state, carrying electrons to cytochrome c oxidase, the fourth complex of the electron transport chain. 

Thus, Og and cytochrome c oxidase are the final destination for the electrons derived from the oxidation of food materials. In concert with this process, cytochrome c oxidase also drives transport of protons across the inner mitochondrial membrane. These important functions are carried out by a transmembrane protein complex consisting of more than 10 subunits (Table 21.2). 

Complex IV Also Transports Protons Across the Inner Mitochondrial Membrane... 

The reduction of oxygen in Complex IV is accompanied by transport of protons across the inner mitochondrial membrane. Transfer of four electrons through this complex drives the transport of approximately four protons. The mechanism of proton transport is unknown but is thought to involve the steps from state P to state O (Figure 21.20). Four protons are taken up on the matrix side for every two protons transported to the cytoplasm (see Figure 21.17). 

It should be emphasized here that the four major complexes of the electron transport chain operate quite independently in the inner mitochondrial membrane. Each is a multiprotein aggregate maintained by numerous strong associations between peptides of the complex, but there is no evidence that the complexes associate with one another in the membrane. Measurements of the lateral diffusion rates of the four complexes, of coenzyme Q, and of cytochrome c in the inner mitochondrial membrane show that the rates differ considerably, indicating that these complexes do not move together in the membrane. Kinetic studies with reconstituted systems show that electron transport does not operate by means of connected sets of the four complexes. 

In 1961, Peter Mitchell, a British biochemist, proposed that the energy stored in a proton gradient across the inner mitochondrial membrane by electron transport drives the synthesis of ATP in cells. The proposal became known as... 

FIGURE 21.21 A model for the electron transport pathway in the mitochondrial inner membrane. UQ/UQH9 and cytochrome e are mobile electron carriers and function by transferring electrons between the complexes. The proton transport driven by Complexes I, III, and IV is indicated. 

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