PHOSPHOMONOESTERASE ENZYMES THAT UTILIZE HISTIDINE OR CYSTEINE AS NUCLEOPHILES IN S(N)2(P) REACTIONS

The past decade has seen a striking development in studies of the mech anisms used by a group of phosphomonoesterase enzymes that have been b roadly defined as 'acid'' phosphatases on the basis of the pH optima t hat they exhibit with certain substrates. There is now convincing evid ence that this apparent kinetic similarity masks the mechanistic use, by these enzymes, of at least two quite different nucleophilic residue s in order to achieve catalysis in the hydrolysis of phosphate monoest ers. The general properties of these enzymes are reviewed together wit h a summary of the extensive range of mechanistic information that is now available. This includes kinetic data that implicates the occurren ce of a common rate limiting step, burst titration kinetics, stereoche mical studies with phosphomonoesters that are chiral at phosphorus, tr apping of covalent phosphoenzyme intermediates, and the identification of enzyme nucleophilic residues by covalent modification, trapping, s pectroscopic and other experimental means. Finally, experiments are de scribed that illustrate the use of active site-directed mutagenesis to explore the mechanisms of these enzymes. This research was supported by U.S. DHHS NIH Grant GM27003.