The oxyanion hole and Asn170, which are located on opposite sides of the active site, are responsible for stabilization of the groups that change formal charge during the reaction: i.e., the carbonyl oxygen and Glu166 (See Scheme 1). During the reaction, negative charge is transferred from the carboxyl group of Glu166 to the deacylating water, and then onto the carbonyl oxygen when the TI is formed. For this reason, the residues of the oxyanion hole (the backbone amino groups of Ser70 and Ala/Ser/Thr237) are expected to stabilize the TI relative to the acyl-enzyme (and thereby lower the barrier for deacylation) . Asn170 is expected to have the opposite effect: it stabilizes the deprotonated state of Glu166 (present in the acyl-enzyme) by donating a hydrogen bond.