At least one third of enzymes requires metal ions to function, the majority of these enzymes require two metal ions for functionality however the are examples of multinuclear metal clusters (Schenk, Elliott, Leung, Carrington, Mitić, & Guddat, 2008). Metalloenzymes catalyse the hydrolysis of phosphodiester bonds, a crucial biochemical reaction in a number of different metabolic and regulatory pathways (Boyer, 1971). The substrates for this reaction are usually mono- and dinucleotides, polynucleotides and phosphorylated proteins (Koonin, 1994). Here we have characterised the hypothetical protein LOC56985, which contains the conserved motif GN/GNH[ED] common in phosphoresterases. Secondary structure analysis has indicated that structural homologues of this protein include purple acid phosphatases and glycerophosphodiesterases that are all defined as Calcineurin-like phosphoesterases (Pfam domain PF00149). This domain classification is large and diverse group of proteins that contain 47 structures and 113 protein architectures (Finn, et al., 2006; Bateman, et al., 2004).
Purple acid phosphatases (PAPs) (EC 22.214.171.124) are metalloenzymes that hydrolyse phosphate esters and anhydrides. Usually these proteins are referred to as PAPs when in plants, and titrate-resistant acid phosphatases (TRAPs) when in animals. In biology, most of the high-energy phosphate compounds are formed from the condensation of the phosphate ion with a phosphorylated sugar (like ribose on a nucleotide like ADP/ CDP). The resulting pyrophosphate bond is a classic anhydride bond and an unpublished paper by Canales et. al. (2008) links the rat ortholog as a hydrolase of such bonds. Mammalian and plant PAPs only show <20% sequence similarity but all the critical residues are conserved (Schenk, Korsinczky, Hume, Hamilton, & Jersey, 2000).
Purple acid phosphatases have been characterised as enzymes that catalyse phosphate ester hydrolysis under acidic conditions, are ~35kDa in size and contain two Fe ions at their catalytic centres (Wilcox, 1996). Similar acid phosphatases found in kidney bean, sweet potato and other plants contain Zn and Fe, two Fe or Mn at their cores.
Although the physiological role of purple acid phosphatases in the cell in not known, some of the proposed biological functions include iron transport, generation of reactive oxygen species and bone resorption (Oddie, et al., 2000); plant functions may include phosphate metabolism and generation of reactive oxygen species (Leung, Teixeira, Guddat, Mitić, & Schenk, 2007). Their localizations in the lysosomes of osteoclasts and macrophages support an intracellular monophosphate function in animals (Doi, Antanaitis, & Aisen, 1988).
Through the use of bioinformatics techniques we have attempted to infer the functional characteristics of the hypothetical protein LOC56985 by taking into account its relative structure and evolutionary characteristics.