Purple acid phosphatases (PAPs) catalyse the hydrolysis of a wide range of phosphomonoester and amide substrates. These enzymes have been identified and characterised from numerous plant and animal sources, and it is likely that a limited number of bacterial organisms also utilize this catalyst. In plants, the biological role of this enzyme is versatile, mainly due to the occurrence of a multitude of isoforms. Sequence database searches have indicated that Arabidopsis thaliana may contain up to 29 different genes encoding PAP-like proteins. Various isoforms have either been cloned or purified from red kidney beans, sweet potato, soy bean, tomato, spinach, rice and onion. The majority of plant PAPs can be separated into two groups, a larger 55-60 kDa form, and a smaller 35 kDa form. The latter is homologous to the animal enzyme, but common to all PAPs is a high degree of conservation in their catalytically relevant active site, characterised by five short, highly conserved sequence motifs. While this active site homology suggests similar chemical reactions, it has emerged that the functionality of PAPs may depend on the selection of the metal ions present at this site. Specifically, in plants, redox-inactive Fe³⁺-Zn²⁺/Mn²⁺ may partake in hydrolytic reactions maintaining phosphate homeostasis, while redox active Fe³⁺-Fe^2+/3+ centres are also implicated in the reactive oxygen species metabolism.