Malonate semialdehyde decarboxylase (MSAD) is a member of the tautomerase superfamily, a group of structurally homologous proteins that have a characteristic β−α−β-fold and a catalytic amino-terminal proline. In addition to its physiological decarboxylase activity, the conversion of malonate semialdehyde to acetaldehyde and carbon dioxide, the enzyme has now been found to display a promiscuous hydratase activity, converting 2-oxo-3-pentynoate to acetopyruvate, with a k_cat/K_m value of 6.0 × 10² M^-1 s^-1. Pro-1 and Arg-75 are critical for both activities, and the pK_a of Pro-1 was determined to be ∼9.2 by a direct ¹⁵N NMR titration. These observations implicate a decarboxylation mechanism in which Pro-1 polarizes the carbonyl oxygen of substrate by hydrogen bonding and/or an electrostatic interaction. Arg-75 may position the carboxylate group into a favorable orientation for decarboxylation. Both the hydratase activity and the pK_a value of Pro-1 are shared with trans-3-chloroacrylic acid dehalogenase, another tautomerase superfamily member that precedes MSAD in a bacterial degradation pathway for trans-1,3-dichloropropene. Hence, MSAD and CaaD could have evolved by divergent evolution from a common ancestral protein, retaining the necessary catalytic components for the conjugate addition of water.