| P:O RATIOS AND RESPIRATORY CONTROL
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| The P:O ratio is a measure of the number of high-energy phosphates (i.e. amount of ATP) synthesized per atom of oxygen (½O2) consumed, or per mole of water produced. The P:O ratio can be calculated from the amount of ADP used to synthesize ATP and the amount of oxygen taken up by mitochondria. For example, if 2 mmol of ADP is converted to ATP, and 0.5 mmol of oxygen (1.0 atom of oxygen) is taken up, the P:O ratio is 2.0. As discussed earlier, the theoretical yield of ATP per mole of NADH is about 7 moles; however, by actual measurement with isolated mitochondria, the P:O ratio for oxidation of metabolites that yield NADH is about 3 and the ratio for those that yield FADH2 is about 2.
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| 'Respiratory control' is the dependence of oxygen uptake by mitochondria on the availability of ADP
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| Normally, oxidation and phosphorylation are tightly coupled: substrates are oxidized, electrons are transported, and oxygen is consumed only when synthesis of ATP is required (coupled respiration). Thus, resting mitochondria consume oxygen at a slow rate, which can be greatly stimulated by addition of ADP (Fig. 8.13). ADP is taken up by the mitochondria, and it stimulates ATP synthase, which lowers the proton gradient. Respiration increases, because the proton pumps are stimulated to re-establish the proton gradient. When the ADP is depleted, ATP synthesis terminates and respiration returns to the original rate. Oxygen uptake declines to the original rate when the concentration of ADP is depleted and ATP synthesis terminates.
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| Mitochondria can become partially uncoupled if the inner membrane loses its structural integrity. They are said to be 'leaky', because protons can diffuse through the inner membrane without involving ATP synthase. This occurs if isolated mitochondria are treated with mild detergents that disrupt the inner membrane, or if they have been stored for a period of time. Such mitochondria become uncoupled and lose respiratory control, and their P:O ratio declines.
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| Figure 8.13 Effect of ADP on the uptake of oxygen by isolated mitochondria. This may be studied in an isolated (sealed) system with an oxygen electrode and a recording device. The graph shows a typical recording of oxygen consumption (pO2, partial pressure of oxygen) by normal mitochondria on introduction of ADP. |
| The mechanism of respiratory control probably depends on the requirement for ADP and Pi binding to the ATP synthase complex: in the absence of ADP and Pi, protons cannot enter
the mitochondrion through this complex, and oxygen consumption markedly decreases, because the proton pumps cannot transport protons against a high proton back-pressure. This happens because the free energy of the electron transport reactions is sufficient to generate a pH gradient of only 2 units across the membrane.
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