Component 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. 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. 4-hydroxyphenylacetate + H(+) = 4-methylphenol + CO2 3,4-dihydroxyphenylacetate + H(+) = 4-methylcatechol + CO2 2-hydroxy-2-(4-hydroxyphenyl)acetate + H(+) = 4-hydroxybenzyl alcohol + CO2 Binds 2 [4Fe-4S] clusters per subunit. 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. 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. Belongs to the HPA decarboxylase small subunit family.