Electron transport chain respiratory control

Salicylic acid, the major metabolite of aspirin, uncouples the electron transport chain in the mitochondria. This results in (a) increased use of oxygen and production of carbon dioxide, (b) lack of ATP, and (c) excess energy no longer utilized in ATP production. The result is increased respiration and raised temperature. The alterations in respiration lead to alkalosis followed by acidosis. The lack of ATP and loss of respiratory control will cause increased metabolic activity and hypoglycemia after an initial mobilization of glucose from glycogen. 

Electron transport is normally tightly coupled to ATP synthesis electrons do not flow through the electron transport chain to oxygen unless ADP is simultaneously phosphorylated to ATP. If ADP is available, electron transport proceeds and ATP is made as the ADP concentration falls, electron transport slows down. This process, called respiratory control, ensures that electron flow occurs only when ATP synthesis is required. 

The increases in total number of mitochondria, in the amount of cytochrome c, and in the activity of cytochrome enzymes might be adaptive changes resulting from increased metabolism. These changes are unlikely to determine the rate of oxygen uptake because oxygen use is not controlled by the concentration of the enzymes of the electron transport chain, but rather by the amount of work performed and the amount of ATP used. The concentration of respiratory enzyme even in normal tissues is greater than is needed for maximal respiration rates. 

The mitochondrial electron transport chain (ETC) or respiratory chain comprises a series of membrane-bound redox-aetive intermediates including flavoproteins, quinones, cytochromes, and iron-sulfur elusters [9] (Fig. 1). The latter facilitate the thermodynamically controlled transfer of electrons from conjugate redox pairs of low redox potential (E°) [e.g., —320 mV for reduced nicotinamide-adenine dinueleotide/oxidized nieotinamide-adenine dinucleotide (NADH/ NAD )] to the final electron donor, O2 (with a high redox potential of -1-820... 

Now that it is established that cestodes possess all the components of a electron transport system, is the latter functional Weinbach von Brand (952) failed to demonstrate either respiratory control or oxidative phosphorylation in T. taeniaeformis, although they regarded this as a technical rather than a physiological problem. However, there is good evidence that isolated mitochondria from M. expansa (124-127) and H. diminuta (663, 978) are capable of oxidative phosphorylation and respiratory control. The demonstration that a preparation of H. diminuta mitochondria will oxidise a range of substrates, exhibiting respiratory control, is shown in Table 5.14. Similarly, mitochondria from Diphyllo-bothrium latum can oxidise NADH (728) and succinate (729). It is likely that the classical mammalian-type part of the cytochrome chain in cestodes is capable of oxidative phosphorylation, but there is no evidence for ATP synthesis occurring on the alternative branch from the quinone or vitamin K/cytochrome b complex to cytochrome o. 

Maintenance of respiratory control depends on the structural integrity of the mitochondrion. Disruption of the organelle causes electron transport to become uncoupled from ATP synthesis. Under these conditions, oxygen uptake proceeds at high rates under all conditions. ATP synthesis is inhibited, even though electrons are being passed along the respiratory chain and used to reduce 02 to water. 

Table 1 shows the effect of several inhibitors. The major result of these inhibitor studies is that an inhibition of proton efflux is possible without impairing respiratory electron transport. This can also be demonstrated advantageously by the lowered H /e ratios given (in percent of control) in the last column of Table 1. These results are in accordance with the hypothesis that an ATP-consuming cytoplasmic-membrane ATPase mediates the proton efflux and contradicts the model of a respiratory chain located on the plasma membrane.

Triacylglycerols can be mobilized by the hydrolytic action of lipases that are under hormonal control. Fatty acids are activated to acyl CoAs, transported across the inner mitochondrial membrane by carnitine, and degraded in the mitochondrial matrix by a recurring sequence of four reactions oxidation by FAD, hydration, oxidation by NAD+, and thiolysis by CoA. The FADH2 and NADH formed in the oxidation steps transfer their electrons to O2 by means of the respiratory chain, whereas the acetyl CoA formed in the thiolysis step normally enters the citric acid cycle by condensing with oxaloacetate. Mammals are unable to convert fatty acids into glucose, because they lack a pathway for the net production of oxaloacetate, pyruvate, or other gluconeogenic intermediates from acetyl CoA. 

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