histidine acid phosphatases (HAPs). HAPs have been isolated from filamentous fungi, bacteria, yeast, and plants. All members of this class of phytase share a common active site sequence motif (Arg-His-Gly-X-Arg-X-Pro) and have a two-step mechanism that hydrolyzes phytic acid (as well as some other phosphoesters). The phytase from the fungus Aspergillus niger is a HAP and is well known for its high specific activity and its commercially marketed role as an animal feed additive to increase the bioavailability of phosphate from phytic acid in the grain-based diets of poultry and swine. HAPs have also been overexpressed in several transgenic plants as a potential alternative method of phytase production for the animal feed industry and very recently, the HAP phytase gene from E. coli has been successfully expressed in a transgenic pig. β-propeller phytases β-propeller phytases make up a recently discovered class of phytase. These first examples of this class of enzyme were originally cloned from Bacillus species, but numerous microorganisms have since been identified as producing β-propeller phytases. The three-dimensional structure of β-propeller phytase is similar to a propeller with six blades. Current research suggests that β-propeller phytases are the major phytate-degrading enzymes in water and soil, and may play a major role in phytate-phosphorus cycling. Purple acid phosphatases A phytase has recently been isolated from the cotyledons of germinating soybeans that has the active site motif of a purple acid phosphatase (PAP). This class of metalloenzyme has been well studied and searches of genomic databases reveal PAP-like sequences in plants, mammals, fungi, and bacteria. However, only the PAP from soybeans has been found to have any significant phytase activity. The three-dimensional structure, active-site sequence motif and proposed mechanism of catalysis have been determined for PAPs. Protein tyrosine phosphatase-like phytases Only a few of the known phytases belong to a superfamily of enzymes called protein tyrosine phosphatases (PTPs). PTP-like phytases, a relatively newly discovered class of phytase, have been isolated from bacteria that normally inhabit the gut of ruminant animals. All characterized PTP-like phytases share an active site sequence motif (His-Cys-(X)5-Arg), a two-step, acid-base mechanism of dephosphorylation, and activity towards phosphrylated tyrosine residues, characteristics that are common to all PTP superfamily enzymes. Like many PTP superfamily enzymes, the exact biological substrates and roles of bacterial PTP-like phytases have not yet been clearly identified. The characterized PTP-like phytases from ruminal bacteria share sequence and structural homology with the mammalian PTP-like phosphoinositide/-inositol