Phosphorylation potential

This equation shows that the contribution of the two components of the PMF differ with different conditions. In state 4, the electrical potential gradient across the inner membrane can be as high as 300,000 Vcm" and the A pH difference one unit. ATP synthesis only occurs when the PMF is sufficiently large. The phosphorylation potential (AG atp) is lower for ATP synthesis in the matnx (AG atp in = 3AP) for ATP exported to the cytosol (AG ajp out = 4AP) because an extra proton is consumed in importing ADP into the matnx (see text). 

Slater, E.C., Rosing, J., Mol, A. (1973). The phosphorylation potential generated by respiring mitochondria. Biochim. Biophys. Acta 292, 534-553. 

Kamo, N., Muratsugu, M., Hongoh, R. and Kobatake, Y., (1979) Membrane potential of mitochondria measured with an electrode sensitive to tetraphenyl phosphonium and relationship between proton electrochemical potential and phosphorylation potential in steady state. Journal of Membrane Biology, 49 (2), 105-121. 

K. Kawada, M. Namba, T. Nishida T. Imai, et al.. Relationship between cerebral oxygenation and phosphorylation potential during secondary energy failure in hypoxic-ischemic newborn piglets. Pediatr. Res., 2009,65, 317-322. 

AG °, ranging from -50 to -65 kJ/mol. AGP is often called the phosphorylation potential. In the following discussions we use the standard free-energy change for ATP hydrolysis, because this allows comparison, on the same basis, with the energetics of other cellular reactions. Remember, however, that in living cells AG is the relevant quantity—for ATP hydrolysis and all other reactions—and may be quite different from AG °. 

The problem of oxidative phosphorylation has been approached through model studies that utilize the phosphorylating potential of metaphosphate. In the mitrochondrial process inorganic phosphate and adenosine diphosphate are converted to adenosine triphosphate. Wieland, in a series of papers (e.g. ref. 31), has shown that a variety of thiolactones can activate inorganic phosphate in the presence of bromine for transfer to adenosine diphosphate (ADP). The intermediate may be an acyl phosphate or a sulfonium salt similar to that postulated by Higuchi and Gensch32 and by Lambeth and Lardy33, viz. 

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... 

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. 

Be able to calculate the energy charge and phosphorylation potential for a cell. 

Another way of expressing the availability of energy, that is, ATP, is to calculate the so-called phosphorylation potential ... 

There are numerous enzyme systems in the human organism, which respond to changes in the energy status of cells. A high-energy charge (or phosphorylation potential) tends to shut down those systems that produce ATP directly or indirectly. On the other hand, a low-energy status of cells turns on such pathways and turns off those that utilize ATP. 

Figure 5.7 Comparison of GTPase timing for physiological (non-equilibrium) and non-physiological (equilibrium) cases. Results at cellular phosphorylation potential (AG gtp = -bOkJ-mor1) are plotted as solid lines results at equilibrium (AG gtp = 0) are plotted as dashed lines. The top panel plots piwit), the probability that the G protein is in the GTP-bound state given that it is in the state Ggtp at t = 0. The bottom panel plots fj t) the probability distribution of dwell time in the GTP-bound state. See text for details and parameter values.

Inasmuch as ATP and polyP have similar phosphorylating potentials, the ability of [32P] (polyP) to phosphorylate the CaATPase was examined by autoradiography of an SDS-PAGE gel (Figure 21 A). 

Since aspartyl phosphates have higher phosphorylating potentials than histidine phosphates, it was postulated that the [32P]polyP-phosphorylated Ca2+-ATPase could transfer the phosphate to a polyP chain as well as back to ADP. The capacity of the [32P]polyP phosphorylated Ca2+-ATPase to carry out these reactions was confirmed (Figure 22A,B), thus demonstrating that the CaATPase has all the enzymatic activities associated with polyphosphate kinases.129... 

The phosphorylation potential, in contrast with the energy charge, depends on the concentration of Pj and is directly related to the free energy-storage available from ATP. 

Any considered mechanism must, first of all, be consistent with the thermodynamic constraints of the system. Such limits are set by the span of oxidoreduction potentials in the respiratory chain and by the protonmotive force that opposes the proton movement. The relative magnitudes of these two forces set an absolute upper limit for H" /e stoicheiometry of proton translocation. The stoicheiometry in turn, puts limits on the underlying mechanisms. Analogous limits for the H /ATP stoicheiometry of ATP synthesis are obtained from the relative magnitudes of phosphorylation potential and pmf. An elementary thermodynamic analysis of the system can therefore be helpful in defining the degree of freedom in discussions of chemical mechanisms (see Ref. 8). 

Mitochondrial respiration has certain characteristic states, defined originally by the classical work of Chance and Williams [23]. States 3 and 4 are of particular relevance here. In State 4 the rate of respiration is minimal ( resting state ) due to a maximal back-pressure from the pmf (or from the phosphorylation potential via the pmf). This is a state termed static head in the theory of thermodynamics of irreversible processes (see, e.g., Refs. 24, 25). 

State 3 ( active , phosphorylating state) is characterised by high respiratory activity due to a lowered phosphorylation potential (addition of ADP). However, respiration is usually not at its maximum, and the measured pmf is only little... 

The near-equilibrium h fpothesis has been championed by the studies of Wilson and co-workers. Erecinska et al. [224] have shown that respiration coupled to ATP synthesis is proportional to the inorganic phosphate (Pj) concentration as well as the ADP/ATP ratio. According to the near-equilibrium theory of control, respiration should respond to changes in the extramitochondrial phosphorylation potential as a whole and not to alterations in the concentrations of individual reactants, such as ATP/ADP ratios. Furthermore, at a constant NAD /NADH ratio, respiration should respond to the phosphorylation potential and, if this latter term remains constant, respiration also should remain constant. 

Interpretation of the validity of the near-equilibrium concept is dependent on the accuracy of intramitochondrial free NAD/NADH measurements and the difference between extra- and intramitochondrial phosphorylation potentials. In a series of studies, Wilson and associates [40,41,225,226] have presented evidence in support of their hypothesis. Utilizing rat liver mitochondria, Forman and Wilson [225] compared the mass action ratios to calculated equilibrium constants under conditions promoting either forward (net ATP synthesis) or reversed (net ATP hydrolysis) electron transport. Since the mass action ratios calculated under various conditions were similar to the calculated K, these findings were said to support a near-equi-... 

The optimal pH values for the forward (Cr + ATP ADP + CrP) and reverse (CrP + ADP <— ATP + Cr) reactions are 9.0 and 6.7, respectively. At neutral pH, CrP has a much higher phosphorylating potential than does ATP this higher potential favors the reverse reaction, with ATP being formed from CrP. The reverse reaction proceeds two to six times faster than the forward reaction, depending on the reaction conditions. 

The High Phosphoryl Potential of ATP Results from Structural Differences Between ATP and Its Hydrolysis Products... 

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