sentences:  This is made possible by the two glutamate residues which hold the water in position directly adjacent to the α-β unsaturated double bond.This configuration requires that the active site for ECH is extremely rigid, to hold the water in a very specific configuration with regard to the acyl-CoA.The data for a mechanism for this reaction is not conclusive as to whether this reaction is concerted (shown in the picture) or occurs in consecutive steps.