We have previously performed molecular dynamics (MD) simulations of protein tyrosine phosphatase (PTP1B) in the phospho-enzyme intermediate state, which is the reactant state for the second, rate limiting cleavage step of the catalytic mechanism (Figure ). In this study, we used our previously prepared structures (based on PDB ID: 6B90 ) to simulate both the closed and open conformations of the WPD-loop with the AMBER18 59 simulation package.