Glycyl radical subunit of the HPA decarboxylase that decarboxylates phenylacetates with a hydroxyl group in the p-position. Active toward 4-hydroxyphenylacetate, 3,4-dihydroxyphenylacetate and to a lesser extent p-hydroxymandelate (2-hydroxy-2-(4-hydroxyphenyl)acetate), forming 4-methylphenol, 4-methylcatechol and 4-hydroxybenzylalcohol, respectively. Is likely involved in the catabolism of aromatic amino acids such as tyrosine fermentation. 4-methylphenol (p-cresol) formation provides metabolic toxicity, which may benefit the pathogen C.difficile by suppression of the endogenous gastrointestinal microflora, allowing the development of gastrointestinal infections (PubMed:11231288, PubMed:16878993). The large subunit is the catalytic subunit that binds the substrate (By similarity). 4-hydroxyphenylacetate + H(+) = 4-methylphenol + CO2 3,4-dihydroxyphenylacetate + H(+) = 4-methylcatechol + CO2 2-hydroxy-2-(4-hydroxyphenyl)acetate + H(+) = 4-hydroxybenzyl alcohol + CO2 Enzyme activity catalyzed by the HPA decarboxylase complex is rapidly and irreversibly inactivated by oxygen. Competitively inhibited by p-hydroxyphenylacetamide. Not inhibited by m- or o-hydroxyphenyl-acetate, p-hydroxybenzoate or p-hydroxyphenylpropionate. kcat is 110 sec(-1) for the decarboxylation of 4-hydroxyphenylacetate. kcat is 65 sec(-1) for the decarboxylation of 3,4-hydroxyphenylacetate. Optimum pH is 7.0. Has a half-life of 15 minutes at 30 degrees Celsius. Heterooctamer consisting of 4 large (HpdB) subunits and 4 small (HpdC) subunits. Also forms a catalytically inactive homodimer. Phosphorylated on serine. Phosphorylation may trigger the formation of the active heterooctamers and thereby regulates enzyme activity. Requires the activating protein HpdA to generate the key active site glycyl radical that is involved in catalysis. Belongs to the glycyl radical enzyme (GRE) family. HPAD subfamily.