Acid phosphatases (EC 3.1.3.2) are a heterogeneous group of proteins that hydrolyze phosphate esters, optimally at low pH. It has been shown [1] that a number of acid phosphatases, from both prokaryotes and eukaryotes, share two regions of sequence similarity, each centered around a conserved histidine residue. These two histidines seem to be involved in the enzymes' catalytic mechanism [2,3]. The first histidine is located in the N-terminal section and forms a phosphohistidine intermediate while the second is located in the C-terminal section and possibly acts as proton donor. Enzymes belonging to this family are called 'histidine acid phosphatases' and are listed below:

• Escherichia coli pH 2.5 acid phosphatase (gene appA).
• Escherichia coli glucose-1-phosphatase (EC 3.1.3.10) (gene agp).
• Yeast constitutive and repressible acid phosphatases (genes PHO3 and PHO5).
• Fission yeast acid phosphatase (gene pho1).
• Aspergillus phytases A and B (EC 3.1.3.8) (gene phyA and phyB).
• Mammalian lysosomal acid phosphatase.
• Mammalian prostatic acid phosphatase.
• Caenorhabditis elegans hypothetical proteins B0361.7, C05C10.1, C05C10.4 and F26C11.1.