Component 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 (By similarity). The small subunit is essential for enzymatic activity of HPA decarboxylase, and seems also involved in the regulation of the enzyme oligomeric state and catalytic activity (By similarity). 4-hydroxyphenylacetate + H(+) = 4-methylphenol + CO2 3,4-dihydroxyphenylacetate + H(+) = 4-methylcatechol + CO2 Binds 2 [4Fe-4S] clusters per subunit. Heterooctamer consisting of 4 large (HpdB) subunits and 4 small (HpdC) subunits, arranged as a tetramer of heterodimers. Belongs to the HPA decarboxylase small subunit family.