Inorganic pyrophosphatase kinetics

Kinetic studies of inorganic pyrophosphatases from mouse (23) and rat (24) liver cytoplasm have previously implicated free MgZt as an... 

Fio. 2. Proposed kinetic model for yeast inorganic pyrophosphatase. Here M represents Mg + but may also apply to any divalent cation with which the enzyme is active. In the rate equation A represents all mono-magnesium PPi complexes, B represents the di-magnesium complex, and I represents free PPi. Hydrogen ion equilibria are not considered. Kinetic runs were done at pH 7.4, 30° (9). Best values for kinetic constants were obtained from a computer program for nonlinear regression (9S). 

Fig. 3. Rate of PPi hydrolysis as a function of total added MgCls. Experimental conditions 0.45M tris (Cl"), pH 7.4, ionic strength adjusted to 1.0 with KC1, 30°, 2.58 X 10 M PPi. Experimental data are shown by the points. The solid line is that predicted by the kinetic model described in Fig. 2 the broken line is that predicted by a model which best fits data obtained with E. coli inorganic pyrophosphatase (22), which postulates no effect of MgiPPi or free Mg8 on the reaction rate.

Kinetic and Thermodynamic Studies of Yeast Inorganic Pyrophosphatase... 

AlO. Arion, E. J., and Nordlie, R. C., Liver microsomal glucose-6-phosphatase, inorganic pyrophosphatase, and pyrophosphate-glucose-phosphotransferase. II. Kinetic studies. J. Biol. Chem. 239, 2752-2757 (1964). 

H. Hirano, Y. Baba, N. Yoza, and S. Ohashi, Measurements of Kinetic Parameters of Inorganic Pyrophosphatase by Flow-Injection Procedures. Anal. Chim. Acta, 179 (1986) 209. 

Activation energies and kinetic constants (Km, Vmax) were calculated for inorganic pyrophosphatases in several soils. Some residual activity remained after steam sterilization and formaldehyde treatment of soils. Enzyme activities were significantly correlated with organic C of soil profiles, and with organic C and clay contents of acidic top soils ". ... 

The splitting of inorganic pyrophosphafe (PPj) into two inorganic phosphate ions is catalyzed by pyrophosphatases (p. 636) that apparently occur universally. Their function appears to be simply to remove the product PPj from reactions that produce it, shifting the equilibrium toward formation of a desired compound. An example is the formation of aminoacyl-tRNA molecules needed for protein synthesis. As shown in Eq. 17-36, the process requires the use of two ATP molecules to activate one amino acid. While the "spending" of two ATPs for the addition of one monomer imif to a polymer does not appear necessary from a thermodynamic viewpoint, it is frequently observed, and there is no doubt that hydrolysis of PP ensures thaf the reaction will go virtually to completion. Transfer RNAs fend to become saturated with amino acids according to Eq. 17-36 even if the concentration of free amino acid in the cytoplasm is low. On the other hand, kinetic considerations may be involved. Perhaps the biosynthetic sequence would move too slowly if if were nof for the extra boost given by the removal of PPj. Part of the explanation for the complexity may depend on control mechanisms which are only incompletely understood. 

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