Purple acid phosphatases catalytic mechanism

Stereochemistry is another powerful tool for determining the net reaction pathway of phosphatases and sulfatases. These enzymes catalyze the net transfer of a phosphoryl or sulfuryl group to water from a monoester, producing inorganic phosphate or sulfate. Inversion results when the reaction occurs in a single step (Scheme 2, pathway a). Phosphatases that transfer the phosphoryl group directly to water with inversion typically possess a binuclear metal center and the nucleophile is a metal-coordinated hydroxide. Examples of phosphatases that follow this mechanism are the purple acid phosphatases (PAPs) and the serine/threonine phosphatases (described in Sections 8.09.4.3 and 8.09.4.4.1). Net retention of stereochemistry occurs when a phosphorylated or sulfiirylated enzyme intermediate is on the catalytic pathway, which is hydrolyzed by the nucleophilic addition of water in a subsequent step (Scheme 2, pathway b). 

The two-metal mechanisms have been known for most phosphotransferases e.g., alkaline phosphatase (7), inositol monophosphatase (35), serine/threonine phosphatase-1 (36), and purple acid phosphatase (30). The catalytic function of the metals in these multinuclear metallo-enzymes may be rather electrostatic and seems insensitive to the nature of the metal ions. 

Figure 14 Catalytic mechanisms proposed for the purple acid phosphatases (1) attack of a terminal hydroxide on the Fe on a monodentate phosphate ester substrate coordinated to the divalent metal site (2) attack of the bridging hydroxide on a bridging phosphate ester (3) attack of a hydroxide ion generated in the second coordination sphere of the Fe on a monodentate phosphate ester.

Wynne CJ, Hamilton SE, Dionysius DA, Beck JL, De Jersey J. 1995. Studies on the catalytic mechanism of pig purple acid-phosphatase. Arch Biochem Biophys 319 133-141. 

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