Oxidative phosphorylating system

Not all the cellular DNA is in the nucleus some is found in the mitochondria. In addition, mitochondria contain RNA as well as several enzymes used for protein synthesis. Interestingly, mitochond-rial RNA and DNA bear a closer resemblance to the nucleic acid of bacterial cells than they do to animal cells. For example, the rather small DNA molecule of the mitochondrion is circular and does not form nucleosomes. Its information is contained in approximately 16,500 nucleotides that func-tion in the synthesis of two ribosomal and 22 transfer RNAs (tRNAs). In addition, mitochondrial DNA codes for the synthesis of 13 proteins, all components of the respiratory chain and the oxidative phosphorylation system. Still, mitochondrial DNA does not contain sufficient information for the synthesis of all mitochondrial proteins most are coded by nuclear genes. Most mitochondrial proteins are synthesized in the cytosol from nuclear-derived messenger RNAs (mRNAs) and then transported into the mito-chondria, where they contribute to both the structural and the functional elements of this organelle. Because mitochondria are inherited cytoplasmically, an individual does not necessarily receive mitochondrial nucleic acid equally from each parent. In fact, mito-chondria are inherited maternally. 

According to one favored theory, mitochondria evolved from aerobic bacteria that were endocytosed by an anaerobic ancestor of eukaryotic cells. The endocytic event is thought to have occurred when oxygen emerged in the atmosphere (about two billion years ago) and threatened most life systems. Under the selective pressure of oxygen, a stable relationship developed in which the host cell had acquired the ability to exploit the bacterial oxidative phosphorylation system for their own use (Green and... 

This sequence of reactions, namely oxidation of CH2-CH2 to CH=CH, then hydration to CH2-CHOH, followed by oxidation to CH2-CO, is a sequence we shall meet again in the -oxidation of fatty acids (see Section 15.4.1). The first oxidation utilizes FAD as coenzyme, the second NAD+. In both cases, participation of the oxidative phosphorylation system allows regeneration of the oxidized coenzyme and the subsequent generation of energy in the form of ATP. 

The oxidative phosphorylation system contains over 80 polypeptides. Only 13 of them are encoded by mtDNA, which is contained within mitochondria, and all the other proteins that reside in the mitochondrion are nuclear gene products. Mitochondria depend on nuclear genes for the synthesis and assembly of the enzymes for mtDNA replication, transcription, translation, and repair (Tl). The proteins involved in heme synthesis, substrate oxidation by TCA cycle, degradation of fatty acids by /i-oxidalion, part of the urea cycle, and regulation of apoptosis that occurs in mitochondria are all made by the genes in nuclear DNA. 

Hatefi Y. The mitochondrial electron transport and oxidative phosphorylation system. Annu Rev Biochem 54 1015-1069, 1985. 

Carrying out this optimization, we find that 18.667 ATP may be synthesized in this system for each glucose molecule consumed. The flux distribution at this optimal solution is illustrated in Figure 9.4. Here, 14.667 ATP/glucose are synthesized by the ATPase reaction of the oxidative phosphorylation system, 2 by glycolysis and 2 by the TCA cycle. 

The preceding use of the reciprocity relationships is evidently of a phe nomenological nature the validity of the relationship per se does not eluci date the specific mechanisms of functioning the oxidative phosphorylation systems. 

NADH-ubiquinone reductase was isolated by Hatefi et al. in 1961 (27-B9). A procedure was developed for the resolution of the mitochondrial electron transport system into four enzyme complexes. Recently, a fifth fraction, which is capable of energy conservation and ATP-Pi exchange, was also isolated (30, 31). The overall scheme for the isolation of the five component enzyme complexes of the mitochondrial electron transport-oxidative phosphorylation system is given in Fig. 1. It is seen... 

The involvement of oxidative stress in AD has opened a new door for potential therapeutic targets. In this regard, several antioxidants are currently in clinical trials such as Idebenone, a-Lipoic acid, acetyl-L-carnitine (ALC), vitamin E, vitamin C, flavonoids, P-carotene, gingko biloba, and metal-chelating agents. Idebenone is a metabolic antioxidant and is normally synthesized as part of the mitochondrial oxidative phosphorylation system. Improvements in clinical status after treatment with idebenone have been shown in a dose-dependent manner compared to placebo and tacrine (Thai et al., 2003). 

Although there is a 5-fold difference between the sizes of the mitochondrial genomes of yeast (84 kb) and mammals (16 kb), the number of proteins synthesized within mitochondria is similar. Proteins produced by mammalian mitochondria are those involved in electron-transport and oxidative-phosphorylation systems. These include cytochrome b, three subunits of cytochrome oxidase, one subunit of ATPase, and six subunits of NADH dehydrogenase. Apart from these differences, protein synthesis in mitochondria follows the same steps and mechanisms as those in the cytoplasm. 

FCCP permeabilizes the inner mitochondrial membrane to protons, destroying the proton gradient and, in doing so, uncouples the electron transport system from the oxidative phosphorylation system. In this situation, electrons continue to pass through the electron transport system and reduce oxygen to water, but ATP is not synthesized... 

Hatefi, Y. The Mitochondrial Electron Transport and Oxidative Phosphorylation System. Ann. Rev. Biochem. 54, 1015-1069 (1985). [A review that emphasizes the coupling between oxidation and phosphorylation.]... 

Another approach to overcoming the limitations inherent in the lEF dimension of 2-DE is to use alternative types of 2-D separations. 2-D blue native (BN) electrophoresis (Schagger and von Jagow, 1991) can be used to separate membrane and other functional protein complexes as intact, enzymatically active complexes in the first dimension. This is followed with a second-dimension separation by Tricine-SDS-PAGE to separate the complexes into their component subunits. This method, combined with protein identification by MALDI PMF, has been applied to several studies of the mitochondrial proteome (Brookes et al., 2002 Kraft et al., 2001). In a study of human heart mitochondria using BN/SDS-PAGE, the individual subunits of all five complexes of the oxidative phosphorylation system were represented and a novel variant of cytochrome c oxidase subunit Vic was reported (Devreese et al., 2002). 

In addition, it has become increasingly evident that there is significant mitochondrial dysfunction and impairment of the oxidative phosphorylation system [29, 41, 66-69]. This impairment is felt to be secondary to inhibition of the Krebs cycle enzymes citrate synthase, aconitase, and isocitrate dehydrogenase by methylcitrate, inhibition of pyruvate carboxylase by methylmalonic acid, and inhibition of pyruvate dehydrogenase complex. 

Rhein, V., Song, X., Wiesner, A., et al. Amyloid-beta and tau synergistically impair the oxidative phosphorylation system in triple transgenic Alzheimer s disease mice. Pioc. Natl. Acad. Sci. USA 106, 20057-20062 (2009). doi 10.1073/pnas.0905529106... 

Whereas mitochondrial oxidation utilizes NAD" and flavoproteins coupled to the oxidative phosphorylation system, the glyoxysomal system differs in two m jor respects (Fig. 5). First, the flavoprotein reduced in the acyl-CoA dehydrogenase step is oxidized directly by molecular oxygen with the formation of HjOj, which is then degraded by catalase second, NADHj produced in the oxidation of 2-hydroxyacyl-CoA is not reoxidized in the glyoxy-somes thus an external NAD -NADH redox system is required. 

The substrate-level oxidative phosphorylation system comprised of enolase plus pyruvate kinase differs in several respects from those given in Table 3-1. 

Riboflavin is an important constituent of the flavoproteins.The prosthetic group of these compound proteins contains riboflavin in the form of the phosphate (flavin mononucleotide, FMN) or in a more complex form as flavin adenine dinucleotide (FAD). There are several flavoproteins that function in the animal body they are all concerned with chemical reactions involving the transport of hydrogen. Further details of the importance of flavoproteins in carbohydrate and amino acid metabolism are discussed in Chapter 9. Flavin adenine dinucleotide plays a role in the oxidative phosphorylation system (see Fig. 9.2 on p. 196) and forms the prosthetic group of the enzyme succinic dehydrogenase, which converts succinic acid to fumaric acid in the citric acid cycle. It is also the coenzyme for acyl-CoA dehydrogenase. 

Nicotinamide functions in the animal body as the active group of two important coenzymes nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP).These coenzymes are involved in the mechanism of hydrogen transfer in living cells (see Chapter 9) NAD is involved in the oxidative phosphorylation system, the tricyclic acid (TCA) cycle and the metabolism of many molecules, including pyruvate, acetate, (3-hydroxy-butyrate, glycerol, fatty acids and glutamate NADPH is the hydrogen acceptor in the pentose phosphate pathway. 

---