Catalyzes the conversion of 2-hydroxypentadienoic acid (enolic form of 2-oxopent-4-enoate) to 4-hydroxy-2-ketopentanoic acid. (S)-4-hydroxy-2-oxopentanoate = (2Z)-2-hydroxypenta-2,4-dienoate + H2O Divalent metal ions. Optimum activity is obtained with Mn(2+). Inhibited by sodium oxalate. kcat is 450 sec(-1). Optimum pH is 5.5-8.0, but decreases linearly above pH 8.3, and shows an inflection at pH 5.5. Aromatic compound metabolism; 3-phenylpropanoate degradation. Belongs to the hydratase/decarboxylase family. MhpD subfamily. Extended N-terminus. Extended N-terminus.