• binuclear metal centre;
  • metalloenzyme;
  • purple acid phosphatase;
  • protein crystallization.

Purple acid phosphatase from sweet potato is a homodimer of 110 kDa. Two forms of the enzyme have been characterized. One contains an Fe–Zn centre similar to that previously reported for red kidney bean purple acid phosphatase. Another isoform, the subject of this work, is the first confirmed example of an Fe–Mn-containing enzyme. Crystals of this protein have been grown from PEG 6000. They have unit-cell parameters a = b = 118.4, c = 287.4 Å and have the symmetry of space group P6522, with one dimer per asymmetric unit. Diffraction data collected using a conventional X-­ray source from a cryocooled crystal extend to 2.90 Å resolution. The three-dimensional structure of the enzyme will provide insight into the coordination of this novel binuclear metal centre.