InterPro domain: IPR029033

The larger clade-1 contains a wide variety of catalytic functions, the best known being fructose 2,6-bisphosphatase (found in a bifunctional protein with 2-phosphofructokinase) and cofactor-dependent phosphoglycerate mutase. The latter is an unusual example of a mutase activity in the superfamily: the vast majority of members appear to be phosphatases. The bacterial regulatory protein phosphatase SixA is also in clade-1 and has a minimal, and possible ancestral-like structure, lacking the large domain insertions that contribute to binding of small molecules in clade-1 members.

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 [ 1 , 2 ].

The histidine phosphatase superfamily is a large and functionally diverse group of proteins. They share a conserved catalytic core centred on a histidine which becomes phosphorylated during the course of the reaction. The superfamily is mainly composed of phosphatases, but the best-studied member is dPGM (cofactor-dependent phosphoglycerate mutase). The superfamily contains two branches sharing very limited sequence similarity: histidine phosphatase clade-1 and clade-2 [ 3 ].

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