2-hydroxymuconic acid semialdehyde can be converted to 2-hydroxypent-2,4-dienoate either directly by the action of 2-hydroxymuconic semialdehyde hydrolase (HMSH) or by the action of three sequential enzymes, the first of which is HMSD. (2Z,4E)-2-hydroxy-6-oxohexa-2,4-dienoate + H2O + NAD(+) = (2Z,4E)-2-hydroxyhexa-2,4-dienedioate + 2 H(+) + NADH Aromatic compound metabolism; benzoate degradation via hydroxylation. Belongs to the aldehyde dehydrogenase family.