Purple acid phosphatase: A journey into the function and mechanism of a colorful enzyme

► Purple acid phosphatase (PAP) is a target for novel anti-osteoporotic drugs. ► PAP requires a mixed-valent metal center (Fe3+-M2+) for activity. ► PAP is bifunctional, operating as a hydrolase and peroxidase. ► The proposed reaction mechanism for hydrolysis involves eight steps. ► Two nucleophilic hydroxides are available to initiate hydrolysis.