Docking Ligands using FlexX

I am using Sybyl FlexX.  I downloaded the protein 1A7T from the
 Protein Data Bank (  It is called Metallo-B-Lactamase
 with the ligand MES docked into it.  When trying to reproduce the
 experimental results of docking MES into the active site,
 containing ZN251 and ZN252, FlexX orients the ligand with the SO3
 group oriented towards the two zincs.  But, the experimentally
 docked ligand of MES has the SO3 group oriented towards the N of
 the LYS167 group.  I tried including the LYS167 as part of the
 active site, but this did not change the orientation of the docked
 ligand.  What forces cause the ligand to orient with the SO3 group
 near the LYS167?  How can I use FlexX to duplicate this experimental
 Amanda Henkes
 Southern Methodist University
 aehenkes |-at-|