Inorganic pyrophosphatase mechanism

There is evidence (but not conclusive evidence) that the mechanisms involves direct phosphoryl transfer to water rather than the formation of a phosphoenzyme intermediate.293 This utilizes the fact that ATP also serves as a substrate to inorganic pyrophosphatase. Hydrolysis of the ATP analogue adenosine 5 -0-(3-thiotriphosphate) chirally labelled with I70 and I80 at the y-phosphate proceeds with inversion of configuration to give the chiral [170,180]-thiophosphate (Figure 13).293... 

M. Baltscheffsky and H. Baltscheffsky (1992). Inorganic pyrophosphate and inorganic pyrophosphatase. In L. Ernster (Ed.), Molecular Mechanisms in bioenergetics, Elsevier, Amsterdam, pp. 331-348. 

Baltscheffsky, M. and Baltscheffsky, H. (1992). Inorganic pyrophosphate and inorganic pyrophosphatases. In Molecular Mechanisms in Bioenergetics, ed. L.Ernster. Amsterdam Elsevier, pp. 331 8. 

These model systems imply an important role for the multiple metal ions in the yeast inorganic pyrophosphatase reaction. A plausible mechanism consistent with its known properties and requirements is shown in Scheme 17. Clearly, the metal ions would reduce the charge on the phosphate residues and activate the P centers to nucleophilic attack. They also probably provide intramolecular hydroxide nucleophiles. 

The mechanism of this revermble reaction is, at the present time, unknown. Arginosuccinate formation can be greatly stimulated by hydrolysis of PP by inorganic pyrophosphatase 36i). 

In addition to orthophosphate cleavage (Figure 10.2a), ATP may be hydrolysed by pyrophosphate cleavage (Figure 10.2b) so called because it produces pyrophosphate and AMP. This mechanism is important since a freely reversible coupled reaction may be converted into an essentially irreversible one by the subsequent removal of pyrophosphate by hydrolytic inorganic pyrophosphatase (Sections 11.5, 14.6, 15.2). 

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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