Involved in the non-oxidative decarboxylation and detoxification of phenolic derivatives under anaerobic conditions (PubMed:10438791, PubMed:7744052, PubMed:24193968, PubMed:15979273). Oxygen-sensitive phenolic acid decarboxylase that catalyzes the reversible decarboxylation of 4-hydroxybenzoate and 3,4-dihydroxybenzoate (PubMed:10438791, PubMed:7744052, PubMed:15979273). 4-hydroxybenzoate + H(+) = CO2 + phenol 3,4-dihydroxybenzoate + H(+) = catechol + CO2 Binds 1 prenylated FMN (prenyl-FMN) per subunit. Inhibited by Zn(2+), (2,3,4)-trihydroxybenzoate and (3,4,5)-trihydroxybenzoate (PubMed:7744052). Ammonium and rubidium ions decrease the activity of the carboxylation of 3,4-dihydroxybenzoate by about 20% (PubMed:7744052). kcat is 3300 min(-1) with 4-hydroxybenzoate as substrate. kcat is 1100 min(-1) with 3,4-dihydroxybenzoate as substrate. Optimum pH is between 5.6-6.2. Optimum temperature is 50 degrees Celsius. Homohexamer. By 4-hydroxybenzoate and 3,4-dihydroxybenzoate. It is not known, if phenolic acid decarboxylase forms a complex composed of ShdB, ShdC and ShdD. The term subunit is often used in reference to the operon, however there is no experimental evidence to prove the existence of the complex. Belongs to the UbiD family. YclC subfamily.