Phosphorylation state ratio adenylate system

Various measures of the phosphorylating potential of the adenylate system within cells have been proposed. One measure is the product [ATP] / [ADP][P ], which will be called the phosphorylation state ratio... 

From these data Krebs and Veech concluded that the oxidation state of the NAD system is determined largely by the phosphorylation state ratio of the adenylate system.169 If the ATP level is high the equilibrium in Eq. 17-42a will be reached at a higher [NAD+]/[NADH] ratio and lactate may be oxidized to pyruvate to adjust the [lactate]/[pyruvate] ratio. 

An interesting experiment is to allow oxidative phosphorylation to proceed until the mitochondria reach state 4 and to measure the phosphorylation state ratio Rp, which equals the value of [ATP] / [ADP][PJ that is attained. This mass action ratio, which has also been called the "phosphorylation ratio" or "phosphorylation potential" (see Chapter 6 and Eq. 6-29), often reaches values greater than 104-105 M 1 in the cytosol.164 An extrapolated value for a zero rate of ATP hydrolysis of log Rf) = 6.9 was estimated. This corresponds (Eq. 6-29) to an increase in group transfer potential (AG of hydrolysis of ATP) of 39 kj/mol. It follows that the overall value of AG for oxidation of NADH in the coupled electron transport chain is less negative than is AG. If synthesis of three molecules of ATP is coupled to electron transport, the system should reach an equilibrium when Rp = 106 4 at 25°C, the difference in AG and AG being 3RT In Rp = 3 x 5.708 x 6.4 = 110 kj mol-1. This value of Rp is, within experimental error, the same as the maximum value observed.165 There apparently is an almost true equilibrium among NADH, 02 and the adenylate system if the P/O ratio is 3. 

However, if the electron transport between 3-hydroxybutyrate and cytochrome b562 is tightly coupled to the synthesis of one molecule of ATP, the observed potential of the carrier will be determined not only by the imposed potential E of the equilibrating system but also by the phosphorylation state ratio of the adenylate system (Eq. 18-7). Here AG atp is the group transfer potential (-AG of hydrolysis) of ATP at pH 7 (Table 6-6), and n is the number of electrons passing through the chain required to synthesize one ATP. In the upper part of the equation n is the number of electrons required to reduce the carrier, namely one in the case of cytochrome b562. 

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