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 (PubMed:16878993). 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.