Involved in the non-oxidative decarboxylation and detoxification of phenolic derivatives under anaerobic conditions, however the precise biochemical function of ShdD in metabolism of phenolic acid is unknown. 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.