FIGURE 7.

Proposed mechanism of action of PG O-acetyltransferases and O-acetyl-PG esterases. (i) The reaction is initiated with the binding of an acetyl donor (transferases) or substrate (esterases), depicted in red, into the active site of the respective enzyme. The H-bonding network between the catalytic triad residues increases the nucleophilicity of the catalytic Ser hydroxyl which attacks the carbonyl center of the bound acetyl group. (ii) The putative transient tetrahedral oxyanion intermediate is stabilized by the oxyanion hole residues (backbone amide of the Ser438 and side-chain amide of Asn491 for SpOatA_C). (iii) An acetyl-enzyme intermediate forms with the departure of the donor group (transferase) or de-acetylated reaction product (esterase). (iv) A MurNAc residue on PG (transferase) or water (esterase) binds, and (v) the catalytic His serves as a base to abstract the proton from either the C-6 hydroxyl group or water to permit its nucleophilic attack of the resulting oxyanion on the carbonyl center of the acetyl-enzyme. (vi) The second transient oxyanion formed is stabilized by the oxyanion hole residues prior to its (vii) collapse releasing either the 6-O-acetyl-PG or free acetate product.