Inorganic pyrophosphatase

Inorganic pyrophosphatase (or inorganic diphosphatase, PPase) is an enzyme (EC 3.6.1.1) that catalyzes the conversion of one ion of pyrophosphate to two phosphate ions. This is a highly exergonic reaction, and therefore can be coupled to unfavorable biochemical transformations in order to drive these transformations to completion. The functionality of this enzyme plays a critical role in lipid metabolism (including lipid synthesis and degradation), calcium absorption and bone formation, and DNA synthesis, as well as other biochemical transformations.Thermostable soluable pyrophosphatase had been isolated from the extremophile Thermococcus litoralis. The 3-dimensional structure was determined using x-ray crystallography, and was found to consist of two alpha-helices, as well as an antiparallel closed beta-sheet. The form of inorganic pyrophosphatase isolated from Thermococcus litoralis was found to contain a total of 174 amino acid residues and have a hexameric oligomeric organization (Image 1).Though the precise mechanism of catalysis via inorganic pyrophosphatase in most organisms remains uncertain, site-directed mutagenesis studies in Escherichia coli have allowed for analysis of the enzyme active site and identification of key amino acids. In particular, this analysis has revealed 17 residues of that may be of functional importance in catalysis.The hydrolysis of inorganic pyrophosphate (PPi) to two phosphate ions is utilized in many biochemical pathways to render reactions effectively irreversible. This process is highly exergonic (accounting for approximately a −19kJ change in free energy), and therefore greatly increases the energetic favorability of reaction system when coupled with a typically less-favorable reaction.Examination of prokaryotic and eukaryotic forms of soluble inorganic pyrophosphatase (sPPase, Pfam PF00719) has shown that they differ significantly in both amino acid sequence, number of residues, and oligomeric organization. Despite differing structural components, recent work has suggested a large degree of evolutionary conservation of active site structure as well as reaction mechanism, based on kinetic data. Analysis of approximately one million genetic sequences taken from organisms in the Sargasso Sea identified a 57 residue sequence within the regions coding for proton-pumping inorganic pyrophosphatase (H+-PPase) that appears to be highly conserved; this region primarily consisted of the four early amino acid residues Gly, Ala, Val and Asp, suggesting an evolutionarily ancient origin for the protein.