Oxidation, P:0 ratio and

If the substrate is succinate, also of the citric acid cycle, or an acylCoA formed during the )3-oxidation of fatty acids, then, because their dehydrogenases are flavoproteins, they miss out on the first phosphorylation. site and have a P 0 ratio of 2. 

Wofe These P/0 ratios of 2.5 and 1.5 for mitochondrial oxidation of NADH and FADHj are "consensus values." Since they may not reflect actual values and since these ratios may change depending on metabolic conditions, these estimates of ATP yield from glucose oxidation are approximate.)... 

The implications of the different stoichiometries for the overall P/0 ratios are obvious. If the proton gradient is the only and necessary intermediate in oxidative phosphorylation, the theoretical P/0 ratio can be calculated from the H " /O and H /ATP ratios. The original experiments agreed with the long-accepted P/O ratios of 3, 2 and 1 for NAD-linked substrates, succinate and cytochrome c, respectively. The more recent findings may not always agree with these accepted values [123]. 

In a number of cases, the eflBciency of oxidative phosphorylation in the mitochondria of developing systems, as judged by the magnitude of the P 0 ratio, has been found to be less than that of mitochondria from fully differentiated tissues, and it has been suggested that such mitochondria may not yet have developed all of the fundamental machinery necessary for coupling respiration and phosphorylation [blowfly muscle (Lewis and Slater, 1954), rat muscle (Kiessling, 1962), honeybee flight muscle (Balboni, 1967), rat liver (Mintz et al., 1967), and rat brain (Milstein et al, 1968)]. 

As was noted above, respiration and oxidative enzymic activities increase during adult development of the cockroach. Removal of the corpora cardiaca leads to reduced respiration and levels of enzymic activity, but the P 0 ratios of mitochondria from cockroach tissues in the two physiological states remain unchanged (Keeley, 1972). This result is similar to that obtained when rats are thyroidectomized. Respiration and enzymic activity of muscle and liver mitochondria are depressed after thyroidectomy and restored to normal after administration of thyroxine, but P 0 ratios are the same (Tata et al, 1963 Gustafson et al, 1965 Bronk, 1963,1966 Volfin et al, 1969 Meijr, 1972). 

Assuming that oxidation drives the phosphorylation process, then Xi < 0 and X2 > 0, and J1U2 is the conventional P/0 ratio, while X1/X2 is the ratio of phosphate potential to the applied redox potential. At static head (sh), analogous to an open circuited cell, the net rate of ATP vanishes, and the rate of oxygen consumption and the force (the phosphate potential) are expressed in terms of a as follows (Stucki, 1984) ... 

The number of molecules of phosphate esterified (or ATP molecules synthesized from ADP and Pi) per atom of oxygen consumed is called the P 0 ratio. The P/O ratio for NADH oxidation is therefore 3. However for the oxidation of succinate via succinate dehydrogenase, since the part of the respiratory chain between NADH and ubiquinone is by-passed (page 217) only two coupling sites are traversed and the P/O ratio for succinate is 2. Similarly, if isolated mitochondria are presented with ascorbic acid which reacts directly with cytochrome c, the first two coupling sites are by-passed and the P/O ratio for the oxidation of ascorbic acid is 1. These considerations will be applied later to calculate the number of ATP molecules that can be synthesized during the oxidation of a molecule of glucose or of fatty acids. 

Lehninger has extended these studies by the use of the iS-hydroxybuty-rate dehydrogenase system, which is DPN-linked, and in isolated mitochondria oxidizes /3-hydroxybutrate in a one-step reaction to acetoacetate, a product which is quite inert in the system studied. The dehydrogenase appears to act merely as a source of enzymatically generated reduced DPN. P/0 ratios of 3.0 have been reported by Lehninger. The value of 3.0 would appear to be likely for every DPN or TPN-linked oxidation in... 

Many attempts have been made to demonstrate phosphorylation coupled to oxidation of reduced cytochrome c. Judah has presented evidence which suggests that phosphorylation occurs when cytochrome c is reduced in situ by a non-enzymatic reaction with ascorbic acid, and then subsequently oxidized enzymatically. Slater, on the other hand, has reported experiments which he interprets as meaning that phosphorylation does not occur when cytochrome c is oxidized. Slater s experiments seem to be open to some criticism on technical grounds. In any event, it is difficult to explain the P/0 ratio of 2.0 which has been observed in the oxidation of succinate unless one assumes that phosphorylation occurs coupled to oxidations above the level of cytochrome c. The potential span between succinate and cytochrome c is only about 0.3 v., which is not sufficient to generate the 24,000 cal. needed to bring about the formation of 2 moles of ADP to ATP. 

Very little is known about the oxidative reactions which lead to the production of acetyl-coenzyme A from fatty acids in animal tissues, but, since it is likely that these oxidations utilize the same electron transfer system as Krebs cycle oxidations, and since fatty acid oxidation takes place in the same enzyme preparations which are used for the study of oxidative phosphorylation, it would seem likely on a priori grounds that the over-all P/0 ratio for fatty acid oxidation must be similar to that observed for the complete oxidation of pyruvate. 

The P 0 ratio for the oxidation of malate and citrate is 3, whereas that for succinate is 2. At present there is detailed knowledge of only one oxidative step which can be linked with phosphate esterification, viz., oxidation of a-ketoglutarate to succinyl CoA. The succinate activation enzyme catalyzes reaction 5 of the following sequence initiated by the Kg.d. ... 

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