PP_i as a Biochemical Energy Source

Abstract

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 PP_i, but in the presence of divalent metal ions (especially Mg²⁺) the ‡G’⁰ value of PP_i hydrolysis is lower than that of ATP hydrolysis, because the product of the former reaction, P_i, 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 PP_i 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(P_i²/PP_i), where P_i (2.42 mM) and PP_i (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 PP_i in rat liver is located in mitochondria, where its concentration is higher (about 0.1 mM; see chapter 3.4).