Glycyl radical subunit of the HPA decarboxylase that decarboxylates phenylacetates with a hydroxyl group in the p-position. Active toward 4-hydroxyphenylacetate and 3,4-dihydroxyphenylacetate, forming 4-methylphenol and 4-methylcatechol, respectively. Is likely involved in the catabolism of aromatic amino acids such as tyrosine fermentation. 4-methylphenol (p-cresol) formation provides metabolic toxicity, which allows an active suppression of other microbes and may provide growth advantages for the producers in highly competitive environments (PubMed:16878993). The large subunit is the catalytic subunit that binds the substrate (PubMed:21823587). 4-hydroxyphenylacetate + H(+) = 4-methylphenol + CO2 3,4-dihydroxyphenylacetate + H(+) = 4-methylcatechol + CO2 kcat is 135 sec(-1) for the decarboxylation of 4-hydroxyphenylacetate. kcat is 67 sec(-1) for the decarboxylation of 3,4-hydroxyphenylacetate. Heterooctamer consisting of 4 large (HpdB) subunits and 4 small (HpdC) subunits, arranged as a tetramer of heterodimers (PubMed:16878993, PubMed:21823587). Also forms a catalytically inactive homodimer (PubMed:16878993). Requires the activating protein CsdA to generate the key active site glycyl radical that is involved in catalysis. Phosphorylated on serine. Phosphorylation may trigger the formation of the active heterooctamers and thereby regulates enzyme activity. Belongs to the glycyl radical enzyme (GRE) family. HPAD subfamily.