Hence, based on the active-
site geometry and catalytic mechanism, phytases have been grouped in four distinct
classes. The first and most extensively studied group of phytases belongs to the class of
histidine acid phosphatases. The other three groups of phytases are classified as
b-propeller phosphatase (BPP; also referred to as alkaline phytase), purple acid phos-
phatase (metalloenzyme), and protein tyrosine phosphatase (also referred to as dual-
specificity phosphatase or cysteine phytase) [6,7]. A study has been conducted in
which fingerprint motifs and relationships between the motifs and key active-site amino
acid residues were investigated and compared among the various classes of phytases [8].
Although phytases are widespread among animals, plants, and a variety of microbes,
microbial phytases are the most promising candidates for biotechnological applications.
Phytases from the microbes mostly belong to the histidine acid phosphatase and alkaline
phosphatase subfamilies and exhibit diversity in biochemical properties, stereospeci-
ficity, and kinetics. Extracellular phytate-degrading enzymes have been identified in the
molds and yeast, whereas in bacteria, these enzymes are mainly cell-associated, except
in Bacillus and Enterobacter. In Escherichia coli, these enzymes have been reported to be
periplasmic proteins, whereas phytate-degrading activity in Selenomonas ruminantium
and Mitsuokella multiacidus was associated with the outer membrane [9e11].