InterPro domain: IPR000560

The histidine phosphatase superfamily is so named because catalysiscentres on a conserved His residue that is transiently phosphorylatedduring the catalytic cycle. Other conserved residues contribute to a'phosphate pocket' and interact with the phospho group of substratebefore, during and after its transfer to the His residue. Structure andsequence analyses show that different families contribute differentadditional residues to the 'phosphate pocket' and, more surprisingly,differ in the position, in sequence and in three dimensions, of acatalytically essential acidic residue. The superfamily may be dividedinto two main branches. The relationship between the two branches isnot evident by (PSI-)BLAST but is clear from more sensitive sequencesearches and structural comparisons [ 1 ].

The smaller clade-2 is composed mainly of acid phosphatases and phytases. Acid phosphatases are a heterogeneous group of proteins that hydrolyse phosphate esters, optimally at low pH. The catalytic functions of these proteins include phytase, glucose-1-phosphatase and multiple inositol polyphosphate phosphatase. Fungal phytases are histidine acid phosphatases that catalyse the hydrolysis of phytate (myo-inositol hexakisphosphate) to myo-inositol and inorganic phosphate [ 2 , 3 ].

Included in this group are:

• Escherichia coli pH 2.5 acid phosphatase (gene appA).
• E. coli glucose-1-phosphatase ( 3.1.3.10 ) (gene agp).
• Yeast constitutive and repressible acid phosphatases (genes PHO3 and PHO5).
• Schizosaccharomyces pombe acid phosphatase (gene pho1).
• Aspergillus awamori phytases A and B ( 3.1.3.8 ) (gene phyA and phyB).
• Mammalian lysosomal and prostatic acid phosphatase.
• Several Caenorhabditis elegans hypothetical proteins.

1. The histidine phosphatase superfamily: structure and function. Biochem. J. 409, 333-48
2. Cloning, characterization and overexpression of the phytase-encoding gene (phyA) of Aspergillus niger. Gene 127, 87-94
3. Fungal phytases: characteristics and amelioration of nutritional quality and growth of non-ruminants. J Anim Physiol Anim Nutr (Berl) 99, 646-60