Transforms homoprotocatechuic acid (HPC) into 5-carboxymethyl-2-hydroxy-muconic semialdehyde (CHMS). 3,4-dihydroxyphenylacetate + O2 = 2-hydroxy-5-carboxymethylmuconate semialdehyde + H(+) Aromatic compound metabolism; 4-hydroxyphenylacetate degradation; pyruvate and succinate semialdehyde from 4-hydroxyphenylacetate: step 2/7.