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 Aromatic compound metabolism; 3-phenylpropanoate degradation. Belongs to the hydratase/decarboxylase family. MhpD subfamily.