By comparing the average RMSD for the last 5 ns of each replica to the R409 phosphate group distance, we observe a clear requirement that this conformation has to be adopted in order for productive loop closure to be possible. It is important to note that the correct conformation of R409 did not guarantee productive loop closure, in all instances, however. This observation helps to explain the population shift towards the closed state that is known to occur once a phosphate group is bound (covalently or non-covalently) to the active site. That is, the phosphate groups helps to place the R409 side-chain in the correct position for productive loop closure, as also observed in our earlier study of wild-type PTP1B and YopH.