There are two possibilities as far as the loop movement is concerned: It could move in a rigid manner or it could come from a disordered structure that acquires order. The second scenario seems more likely to occur in OPRTase. There must be an energy balance between the peptide new order and hydrogen bond formation in the loop, between the loop and the rest of the protein, and between the loop and the ligands. There is a 30:1 equilibrium between the close and open structures in the enzyme-Mg-PRPP complex, which suggests that the close conformation is favored.