4-hydroxyphenylacetate decarboxylase (HPA) is a kind of glycyl radical enzymes (GREs), which catalyze the formation of p-cresol or skatole. It is characterized by a small subunit (HpdC), and shows unique properties that have not been observed in other GREs. Thus HPA decarboxylase is considered as a new kind of GRE. The existing researches show the importance of HpdC in the decarboxylase family, but its importance hasn't been experimentally verified. To elucidate the functions of HpdC, a series of over-expression plasmids for the hybrid decarboxylases were established and the corresponding encoded proteins were purified and analyzed. The final physio-chemical analysis and the enzyme activity test results prove that the small subunit is directly involved in the regulation of the enzymes' oligomeric state and activity.