P : O ratio

Catalyst Para, wt % Ortho, wt % Meta, wt % p/o ratio Conversion, % Reference... 

Engelhardt s experiments in 1930 led to the notion that ATP is synthesized as the result of electron transport, and, by 1940, Severo Ochoa had carried out a measurement of the P/O ratio, the number of molecules of ATP generated per atom of oxygen consumed in the electron transport chain. Because two electrons are transferred down the chain per oxygen atom reduced, the P/O ratio also reflects the ratio of ATPs synthesized per pair of electrons consumed. After many tedious and careful measurements, scientists decided that the P/O ratio was 3 for NADH oxidation and 2 for succinate (that is, [FADHg]) oxidation. Electron flow and ATP synthesis are very tightly coupled in the sense that, in normal mitochondria, neither occurs without the other. 

What Is the P/O Ratio for Mitochondrial Electron Transport and Oxidative Phosphorylation ... 

Note The.se P/O ratios of 2.5 and 1.5 for mitochondrial oxidation of NADH and [FADHg] are consen.sns values. Because diey may not reflect actual values and because these ratios may change depending on metabolic conditions, the.se estimates of ATP yield from glucose oxidation are approximate. 

In an early paper by our group (ref. 19) halobenzenes are brominated at 298 K in the liquid phase over various Y-zeolites (Fig. 7). Improved p/o-ratios with respect to conventional (FeBr3) catalysis are observed. 

De la Vega and Sasson (ref. 20) added propene oxide as a HBr-scavenger in the bromination of toluene over NaY in CCI4 and obtained an initial p/o-ratio... 

The P/O ratio is the number of ATPs made for each O atom consumed by mitochondrial respiration. The P stands for high-energy phosphate equivalents, and the O actually stands for the number of I 02 s that are consumed by the electron transport chain. The full reduction of 02 to 2 H20 takes 4 electrons. Therefore, 2 electrons reduce of an 02. The oxidation of NADH to NAD and the oxidation of FADH2 to FAD are both 2-electron oxidations. O can be read as the transfer of 2 electrons. It s not quite as obscure as it sounds.2... 

Acetyl-CoA metabolized through the TCA cycle yields 3 NADH, 1 FADH2, and 1 GTP—a total of 12 ATP equivalents (3 from each NADH, 2 from each FADH2, and 1 GTP—12 in all). Four O s are used, 1 for each NADH and FADH2. The P in this case is 10 (12 — 2 for the activation of acetate to acetyl-CoA). The P/O = 10/4 = 2.5. P/O ratios for anything else are calculated in the same way. 

The patent literature discloses alkylation performances of several additional structure types. A Mobil patent (182) describes the use of VTM-A, a pillared titanosilicate of the MCM-27 family. The catalyst produced about 80 wt% of octanes under relatively mild conditions (OSV = 0.05 h 1, P/O ratio = 20). A number of patents describe the use of MCM-36. MCM-49, which is closely related to MCM-22, has also been tested as an alkylation catalyst. In general,... 

In principle, the same rules hold true when zeolitic alkylation catalysts are used. A detailed study of the influence of PO and OSV on the performance of zeolite H-BEA in a backmix reactor was reported by de Jong et al. (80). The authors developed a simple model of the kinetics, which predicted catalyst lifetimes as a function of P/O and OSV. Catalyst lifetime (which is equivalent to the catalyst productivity, the reciprocal of acid consumption) increased with increasing P/O ratio and decreasing OSV. Furthermore, the authors persuasively demonstrated the superiority of a backmix reactor over a plug flow reactor. Qualitatively similar results were obtained by Taylor and Sherwood (222) employing a USY zeolite catalyst in a backmix reactor. The authors stressed the detrimental effect of unreacted alkene on the catalyst lifetime and product quality. Feller et al. (89) tested LaX zeolites in a backmix reactor and found the catalyst productivity to be nearly independent of the OSV within the examined OSV range. At higher values of OSV, the catalyst life was shorter, but in this shorter time the same total amount of product was produced. The P/O ratio had only a moderate influence on the catalyst performance. 

Both Kalckar and the Russians measured the ratio of atoms of phosphorus yielding ATP to the atoms of oxygen utilized (P/O ratio) and found values significantly greater than 1, the figure expected if a phosphate group had been introduced into a cycle intermediate, as in the glycolytic pathway. 

Interpretation of these early experiments with crude tissue preparations was greatly complicated by the presence of very active phosphatases (ATPases) which rapidly hydrolyzed any ATP which might have been formed. Ochoa suggested that the amount of Pj apparently esterified should be corrected for the measurable rate of ATP hydrolysis by the preparation. This gave P/O ratios approaching 3. Belitzer and Tsibakowa and Ochoa realized that phosphorylation must occur not only when the substrate is dehydrogenated. .. but also during... 

With these improved techniques P-hydroxybutyrate, which penetrates mitochondria easily and is oxidized to acetoacetate using NAD+ as H acceptor, gave a P/O ratio of 3, the value equivalent to that from the reoxidation of NADH found by Lehninger. Succinate, which bypassed the NAD+/NADH step, gave a ratio of 2. When cytochrome c-Fe2+ was... 

A different and simpler approach to the measurement of P/O ratios came from the introduction of an oxygen electrode suitable for biochemical studies. Chance and Williams (1955) established conditions under which mitochondrial respiration, in the presence of excess substrate, was totally dependent on the amount of ADP available, i.e., the mitochondria were exhibiting respiratory control. From the change in potential when a known amount of ADP was admitted into the electrode vessel, the oxygen uptake and thus the P/O ratio could be determined, completely confirming the earlier results. 

The anodic chlorination in some cases allows one to achieve better regioselec-tivities than chemical alternatives (p/o ratio of chlorotoluene in chlorination of toluene anodic 2.2, chemical alternative 0.5-1.0) [215]. Anodic oxidation of iodine in trimethyl orthoformate afforded a positive iodine species, which led to a more selective aromatic iodination than known methods ]216]. Aryliodination is achieved in good yield, when an aryhodide is oxidized in HOAc, 25% AC2O, 5% H2SO4 in the presence of an arene ]217, 218]. Alkyl nitroaromatic compounds, nitroaromatic ketones, and nitroanihnes are prepared in good yields and regioselectivity by addition of the corresponding nucleophile to a nitroarene and subsequent anodic oxidation of the a-complex (Table 13, number 11) ]219, 220]. 

To figure out a P/O ratio you have to figure out two things—the P and the O. The P is easy if you ve learned to count ATPs made by various metabolic pathways. The P is the net number of ATPs made by the metabolism of the substance you re dealing with. The O is a little harder. Here you must figure out how many times 2 electrons have been passed down the electron transport chain. For each NADH or each FADH2 made during the metabolism of your substance, 1 O is consumed as 2 electrons are passed down the chain. 

For transfer of two hydrogens from NADH to oxygen, the P/O ratio is 2.5. 

One view to explain different P/O ratios for different classes of organisms is to consider variability in both the molecular mechanism as well as the stoichiometry of proton transport and ATP synthesis with the source of the enzyme [67]. However, considering our molecular mechanism and the energetics of the oxidative phosphorylation process, we believe that a universality in the mechanistic, kinetic and thermodynamic characteristics of the system is operative. 

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