Structure, properties and reactivity of the (FeFeIII)-Fe-II and (ZnFeIII)-Fe-II purple acid phosphatases

This microreview describes the structure, properties and mechanisms of the purple acid phosphatases (PAP). The enzyme is isolated from mammalian, plan t and bacterial sources. X-ray structural information is now available for the enzyme from pig (uteroferrin), rat and kidney beans. Features of the me chanism are the concerted action of a labile M-II centre (Fe-II or Zn-II) a longside a more inert Fe-III. The latter is effective as a conjugate-base F eOH2+, which initiates hydrolysis at the M-II-bound phosphate ester by a pr ocess involving OH- replacement of OR- at the PV. Histidine residues near t o the active site help bind the phosphate and are involved in the release o f OR-. Effects of replacement of the Fe-II by Mn-II, Co-II, Ni-II, Cu-II an d Zn-II, and of Fe-III by Ga-III,Al-III and In-III have been studied. The m echanistic role of the (ZnZnII)-Zn-II combination in alkaline phosphatases, and other related dinuclear centres is also considered.