Activation of the respiratory chains

Chemiosmotic theory readily explains the dependence of electron transfer on ATP synthesis in mitochondria. When the flow of protons into the matrix through the proton channel of ATP synthase is blocked (with oligomycin, for example), no path exists for the return of protons to the matrix, and the continued extrusion of protons driven by the activity of the respiratory chain generates a large proton gradient. The proton-motive force builds up until the cost (free energy) of pumping... 

An increase in thyroid hormone levels results in an increase in the basal metabolic rate (BMR). BMR measurements can be used to assess thyroid status, as discussed in detail in Chapter 5. This method for the diagnosis of hypo- or hyperthyroidism is not in coitimon use because it is cumbersome. The increase in BMR has been associated with increases in various reactions that use ATP. The increased use of ATP is matched by an increase in activity of the respiratory chain and in Oj... 

PG is made in mitochondria and microsomes of animal cells and appears to be primarily converted to DPG. DPG is biosynthesized exclusively on the matrix side of the mitochondrial inner membrane and is found only in this organelle. There is evidence that the rate-limiting step in DPG biosynthesis is the conversion of PA into CDP-DG (G.M. Hatch, 1994). Consistent with this idea, the levels of CTP regulate DPG biosynthesis in cardiac myoblasts (G.M. Hatch, 1996). Using techniques developed by Raetz and co-workers [14], a temperature-sensitive mutant of PG-P synthase in CHO cells was isolated (M. Nishijima 1993). The mutant had only 1% of wild-type PG-P synthase activity at 40°C and exhibited a temperature-sensitive defect in PG and DPG biosynthesis. This mutant was used to show that DPG is required for the NADH-ubiquinone reductase (complex I) activity of the respiratory chain. 

Mitochondrial dysfnnction during seizures can also alter neuronal excitability. The inhibition of the mitochondrial respiratory chain enzymes, such as cytochrome c oxidase and snccinate dehydrogenase, evokes seizures. This may be due to an intracellular decrease in ATP levels and alterations in neuronal calcium homeostasis (Kunz, 2002). Alternatively, free radicals may attack mitochondria, inhibit the activity of the respiratory chain and induce a transient permeability. This results in a decline of ATP production and excessive release of free radicals, consequently causing cell death (Arzimanoglou et al, 2002). 

Shugaev, A. G. Some aspects of structural organization and oxidative activity of the respiratory chain of plant mitichondria. Uspekhi sovremennoi biologii, 1991, 111 2), 178-191... 

Accumulation of compounds related to the mitochondrial pathway can be detected in one or more body fluids of most patients [1, 2, 15]. Special attention has to be paid to the lactate concentration. Excess of lactate and alanine will be produced after reduction or transamination of accumulated pyruvate (see Fig. 27.1). If there is a severe block in the pyruvate oxidation pathway, and the produced lactate can not adequately be removed by peripheral tissues, it accumulates in blood, urine and/or cerebrospinal fluid, dependent upon the affected tissue(s). A decreased activity of the respiratory chain will shift the equilibrium of the lactate dehydrogenase reaction to conversion of pyruvate to lactate (see also Sect. 1). Thus, patients with a respiratory chain defect should demonstrate an increased lactate/pyruvate ratio in blood, whereas pyruvate dehydrogenase deficiency should result in a normal lactate/pyruvate ratio. However, this tool for differential diagnosis is not helpful in all cases. Furthermore, some patients do not accumulate lactate in blood or urine. 

The respiratory chain can be separated by various techniques into three multienzyme complexes (Figure 16.3). Complex I is NADH-ubiquinone reductase. Complex III is known as ubiquinone-cytochrome c reductase and contains cytochromes b and Ci. Complex IV is cytochrome oxidase. (Succinate dehydrogenase is referred to as Complex II). Ubiquinone and cytochrome c are small molecules which do not form part of these complexes. Reconstitution of the isolated Complexes I-IV with cytochrome c and ubiquinone leads to recovery of the activity of the respiratory chain. 

Certain compounds inhibit the activity of the respiratory chain by blocking the transfer of electrons at certain points. Rotenone and amytal inhibit electron transfer through Complex I. Antimycin A inhibits at the level of Complex III. Cytochrome oxidase activity is inhibited by carbon monoxide, cyanide and hydrogen sulphide. The prevention of electron transport by cyanide which is very rapidly absorbed is responsible for the high toxicity of this compound. 

The thiazolidinediones have also been reported to act as inhibitors of the respiratory chain at high concentrations, and this appears to account for their ability to activate AMGPK in cultured cells. However, the primary target of the thiazolidinediones appears to be the peroxisome proliferator-activated receptor-y ( PPAR-y), a member of the nuclear receptor superfamily expressed in adipocytes. One of the major effects of stimulation of PPAR-y in adipocytes is the release ofthe... 

Mitochondrial permeability transition involves the opening of a larger channel in the inner mitochondrial membrane leading to free radical generation, release of calcium into the cytosol and caspase activation. These alterations in mitochondrial permeability lead eventually to disruption of the respiratory chain and dqDletion of ATP. This in turn leads to release of soluble intramito-chondrial membrane proteins such as cytochrome C and apoptosis-inducing factor, which results in apoptosis. 

Abnormalities of the respiratoiy chain. These are increasingly identified as the hallmark of mitochondrial diseases or mitochondrial encephalomyopathies [13]. They can be identified on the basis of polarographic studies showing differential impairment in the ability of isolated intact mitochondria to use different substrates. For example, defective respiration with NAD-dependent substrates, such as pyruvate and malate, but normal respiration with FAD-dependent substrates, such as succinate, suggests an isolated defect of complex I (Fig. 42-3). However, defective respiration with both types of substrates in the presence of normal cytochrome c oxidase activity, also termed complex IV, localizes the lesions to complex III (Fig. 42-3). Because frozen muscle is much more commonly available than fresh tissue, electron transport is usually measured through discrete portions of the respiratory chain. Thus, isolated defects of NADH-cytochrome c reductase, or NADH-coenzyme Q (CoQ) reductase suggest a problem within complex I, while a simultaneous defect of NADH and succinate-cytochrome c reductase activities points to a biochemical error in complex III (Fig. 42-3). Isolated defects of complex III can be confirmed by measuring reduced CoQ-cytochrome c reductase activity. 

Mitochondrial cytochrome c is a water-soluble basic protein, residing in the mitochondrial inner membrane where complexes HI and IV of the respiratory chain can reversibly interact with the protein. When the basic protein shuttles between complexes III and IV, cardiolipin facilitates the binding of cytochrome c to cytochrome c oxidase-containing membranes (Salamon and Tollin, 1996). In other words, cytochrome c oxidase in complex IV efficiently receives electrons from cytochrome c only in the presence of cardiolipin. It is well documented that cytochrome c oxidase binds one cardiolipin molecule as an activator and 2 others for anchoring itself to the inner membrane (Awasthi et al, 1971 Suter et al, 2000). 

The toxicity of fluoroacetic acid and of its derivatives has played an historical decisive role at the conceptual level. Indeed, it demonstrates that a fluorinated analogue of a natural substrate could have an activity profile that is far different from that of the nonfluorinated parent compound. The toxicity of fluoroacetic acid is due to its ability to block the citric acid cycle (Krebs cycle), which is an essential process of the respiratory chain. The fluoroacetate is transformed in vivo into 2-fluorocitrate by the citrate synthase. It is generally admitted that aconitase (the enzyme that performs the following step of the Krebs cycle) is inhibited by 2-fluorocitrate the formation of aconitate through elimination of the water molecule is a priori impossible from this substrate analogue (Figure 7.1). 

In juvenile liver fluke and miracidia, a respiratory chain up to cytochrome c oxidase is active and all evidence obtained so far indicates that in F. hepatica at least this electron-transport chain is not different from the classical one present in mammalian mitochondria (Figs 20.1 and 20.2). In the aerobically functioning stages, electrons are transferred from NADH and succinate to ubiquinone via complex I and II of the respiratory chain, respectively. Subsequently, these electrons are transferred from the formed ubiquinol to oxygen via the complexes III and IV of the respiratory chain. 

Enzymatic activity of the substrate in the respiratory process (3.49) is implemented by Krebs cycle hydrogenases (3.52) (the endergonic component of the respiratory process), where chemical energy is accumulated in the form of an active intermediate compound (linked hydrogen atom). There is no doubt that linked 8H and 2C02 forms, in which these intermediates exist in the cell environment, will significantly decrease AG° [21]. The final product of the respiratory process is synthesized by enzymes of the respiratory chain (exer-gonic component). 

Note also that chemical reactions accompanied by water formation are always thermodynamically profitable. Therefore, H+ ion participation in water formation during the ADP phosphorylation process makes this reaction much simpler. On the other hand, the water formed is bound to the / j factor. It is common knowledge that the reaction is intensified by water-binding compounds. Then water is dissociated to ions, emitted to different sides of the membrane, and this process is synchronized with ATP desorption. Desorbed protons are localized at H+-ATP-synthase, directly at the cytochrome system of the respiratory chain. Therefore, they quickly interact with activated oxygen, producing water at the final stage of respiration (3.50). 

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