Mitochondrial chain

Although less active than the microsomal system, mitochondrial chain elongation has been extensively investigated, particularly in liver and brain. The two-carbon elongation donor in mitochondria is acetyl-CoA. Generally, a monounsaturated fatty acyl-CoA substrate is more active than saturated CoA and both support higher activity than PUFA, particularly in brain. 

Mitochondrial elongation occurs by successive addition and reduction of acetyl units in a reversal of fatty acid oxidation. Although fatty acid P-oxidation and mitochondrial chain elongation have the same organelle location, reversal of a tra/ii-2-enoyl-CoA reductase P-oxidation is not feasible the FAD-dependent acyl-CoA dehydrogenase of P-oxidation is substituted by a more thermodynamically favorable enzyme reaction, catalyzed by NADPH-dependent enoyl-CoA reductase, to produce overall negative free-energy for the sequence. Enoyl-CoA reductase firom liver mitochondria is distinct from... 

In view of the large number of metal-containing electron carriers in the mitochondrial chain, there are many possible locations for proton pumps. However, the presence of the three isopotential groups of Table... 

The mitochondrial electron transport chain reduces oxygen to water, where oxygen is reduced by accepting four electrons, leading to the formation of two water molecules. However, upon mitochondrial damage, a variable fraction of electrons can leak from the mitochondrial chain, leading to the univalent reduction of molecular oxygen, which... 

This reaction may be considered as an abiotic mimicking of the classical biological oxidation reaction involving the oxidized and the reduced form of a substrate. However, at variance with the biological pattern, observed for instance in the redox cross reaction between NAD / NADH and O2 / H2O in the respiratory mitochondrial chain, no additional redox intermediate... 

Many iron-sulfur centers are known to function in the mitochondrial chain. At least five different iron-sulfur centers have been characterized in beef-heart mitochondrial complex I (Albracht et al, 1977). Complex II contains two to three different iron-sulfur centers (Ohnishi et al, 1974a Beinert et al, 1975), whereas complex III contains one iron-sulfur center (Rieske et al, 1964 Orme-Johnson et al, 1974). Complex II exhibits an ESR signal in the oxidized state (Ohnishi et al, 1974b). The ESR spectra and redox properties are similar to those of the signal of the HiPIP from Chromatiwn vinosum. This signal therefore probably occurs for the [4Fe-4S]1-(i- 2-) cluster. The number of iron atoms per center is not known for the other iron-sulfur centers in the respiratory chain. 

Catenanes are formed when two or more closed-circular DNAs are linked together to form a chain. Catenanes were first isolated in human mitochondrial DNA and have since been identified in a number of biological systems. These stmctures often occur as intermediates during the repHcation of circular DNA molecules. 

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,. ... 

All these intermediates except for cytochrome c are membrane-associated (either in the mitochondrial inner membrane of eukaryotes or in the plasma membrane of prokaryotes). All three types of proteins involved in this chain— flavoproteins, cytochromes, and iron-sulfur proteins—possess electron-transferring prosthetic groups. 

Adapted from Hatefi, Y, 1985. The mitochondrial electron tran.sport chain and oxidative pho.sphorylation. sy.stem. Annual Review of Biochemistry 54 1015-1069 and DePierre, J., and Ern.ster, L., 1977. Enzyme topology of intracellular membrane.s. Annual Review of Biochemistry 46 201-262. 

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. 

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. 

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. 

When Mitchell first described his chemiosmotic hypothesis in 1961, little evidence existed to support it, and it was met with considerable skepticism by the scientific community. Eventually, however, considerable evidence accumulated to support this model. It is now clear that the electron transport chain generates a proton gradient, and careful measurements have shown that ATP is synthesized when a pH gradient is applied to mitochondria that cannot carry out electron transport. Even more relevant is a simple but crucial experiment reported in 1974 by Efraim Racker and Walther Stoeckenius, which provided specific confirmation of the Mitchell hypothesis. In this experiment, the bovine mitochondrial ATP synthasereconstituted in simple lipid vesicles with bac-teriorhodopsin, a light-driven proton pump from Halobaeterium halobium. As shown in Eigure 21.28, upon illumination, bacteriorhodopsin pumped protons... 

ATP results from the movement of approximately three protons from the cytosol into the matrix through Fg. Altogether this means that approximately four protons are transported into the matrix per ATP synthesized. Thus, approximately one-fourth of the energy derived from the respiratory chain (electron transport and oxidative phosphorylation) is expended as the electrochemical energy devoted to mitochondrial ATP-ADP transport. 

The second electron shuttle system, called the malate-aspartate shuttle, is shown in Figure 21.34. Oxaloacetate is reduced in the cytosol, acquiring the electrons of NADH (which is oxidized to NAD ). Malate is transported across the inner membrane, where it is reoxidized by malate dehydrogenase, converting NAD to NADH in the matrix. This mitochondrial NADH readily enters the electron transport chain. The oxaloacetate produced in this reaction cannot cross the inner membrane and must be transaminated to form aspartate, which can be transported across the membrane to the cytosolic side. Transamination in the cytosol recycles aspartate back to oxaloacetate. In contrast to the glycerol phosphate shuttle, the malate-aspartate cycle is reversible, and it operates as shown in Figure 21.34 only if the NADH/NAD ratio in the cytosol is higher than the ratio in the matrix. Because this shuttle produces NADH in the matrix, the full 2.5 ATPs per NADH are recovered. 

All of the other enzymes of the /3-oxidation pathway are located in the mitochondrial matrix. Short-chain fatty acids, as already mentioned, are transported into the matrix as free acids and form the acyl-CoA derivatives there. However, long-chain fatty acyl-CoA derivatives cannot be transported into the matrix directly. These long-chain derivatives must first be converted to acylearnitine derivatives, as shown in Figure 24.9. Carnitine acyltransferase I, located on the outer side of the inner mitochondrial membrane, catalyzes the formation of... 

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