PPi as a Biochemical Energy Source
The most common energy source for biochemical reactions is the hydrolysis of the phosphoanhydride bond between the β- and γ-phosphates of ATP. The similar phosphoanhydride bond is opened also in the hydrolysis of PPi, but in the presence of divalent metal ions (especially Mg2+) the ‡G’0 value of PPi hydrolysis is lower than that of ATP hydrolysis, because the product of the former reaction, Pi, has a much lower affinity for Mg2+ than the product of the latter reaction, ADP (Wood et al. 1966, Alberty 1969, Flodgaard and Fleron 1974). ‡G’o values have, however, little relevance in vivo, because in living cells the concentrations of the reactants are usually far away from 1 M. Some attempts have been made to determine ‡G’ of PPi hydrolysis in more natural conditions. Flodgaard and Fleron (1974) determined the equilibrium constant, K’, of the reaction to be about 1000 M in tetra-n-propyl ammoniumphosphate buffer (pH 7.4) at 25°C and at the physiological ionic strength and Mg2+ concentration. By using the equation ‡G’o = -RTlnK’ they got ‡G’o = −4.0 kcal/mol. The physiological free energy change, ‡G’, was then calculated according to the equation ‡G’ = ‡G’o + RTln(Pi2/PPi), where Pi (2.42 mM) and PPi (0.0062 mM) are the cytoplasmic concentrations of the corresponding compounds in rat liver. With these values ‡G’ = ‡G’o = −4.0 kcal/mol or −16.7 kl/mol, The physiological relevance of this value is, however, dubious, because most of PPi in rat liver is located in mitochondria, where its concentration is higher (about 0.1 mM; see chapter 3.4).
KeywordsHydrolysis Fermentation Photosynthesis NADPH Fructose
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