Crystal structure of a mammalian purple acid phosphatase

Tartrate-resistant acid phosphatase (TRAP) is a mammalian di-iron- containing enzyme that belongs to the family of purple acid phosphatases (PAP). It is highly expressed in a limited number of tissues, predominantly in bone-resorbing osteoclasts and in macrophages of spleen. We have determined the crystal structure of rat TRAP in complex with a phosphate ion to 2.7 A resolution. The fold resembles that of the catalytic domain of kidney bean purple acid phosphatase (KBPAP), although the sequence similarity is limited to the active site residues. A surface loop near the active site is absent due to proteolysis, leaving the active-site easily accessible from the surrounding solvent. This, we believe, gives a structural explanation for the observed proteolytic activation of TRAP. The current structure was determined at a relatively high pH and without any external reducing agents. It is likely that it represents an oxidized and therefore catalytically inactive form of the enzyme.