Chlamydophila pneumoniae AR39 is an obligate intracellular pathogen that causes human acute and chronic respiratory tract diseases. One protein from C. pneumoniae AR39 was assigned as 4-hydroxybenzoate decarboxylase (HBDC). Assays done with the purified oxygen-sensitive protein showed that the optimum pH and temperature were 7.5 and 30 degrees C, respectively. The Km and Vmax obtained for 4-hydroxybenzoate were approximately 0.21 mM and 11.9 nM min(-1) mg(-1), respectively. During the period of 4-hydroxybenzoate decarboxylation, overall activity of the thermal-sensitive protein was 5.06 nM min(-1) mg(-1) protein. The 4-hydroxybenzoate decarboxylation was promoted by Mg2+, Fe2+, Mn2+ stop, and Ca2+ but not by Cu2+ or Zn2+. The enzyme also slowly catalyzed the reverse reaction, which was phenol carboxylation.