4-Oxalocrotonate tautomerase (4-OT) catalyzes the conversion of 2-oxo-4E-hexenedioate to 2-oxo-3E-hexenedioate through the intermediate, 2-hydroxy-2,4E-hexadienedioate. 4-OT and a homologue found in Bacillus subtilis (designated YwhB) share sequence identity and two key catalytic groups, Pro-1 and Arg-11, with the two subunits comprising trans-3-chloroacrylic acid dehalogenase (CaaD). 4-OT and YwhB have now been found to display a low-level hydratase activity, resulting in the dehalogenation of 3E-haloacrylates. The enzymes are highly selective for the (E)-isomer, and Pro-1 is critical for the activity while an arginine is likely required. Two mechanisms are proposed in which Pro-1 functions as a general base or a general acid catalyst and, along with the arginine, facilitates the Michael addition of water. Both mechanisms suggest an intriguing route for the evolution of the CaaD activity. One or more mutations could decrease the hydrophobic environment of the active site, which would make it more favorable for a hydrolytic reaction, thereby raising the pKa of Pro-1 and increasing the concentration of enzyme in the reactive form.