Converts glutamate to gamma-aminobutyrate (GABA), consuming one intracellular proton in the reaction. The gad system helps to maintain a near-neutral intracellular pH when cells are exposed to extremely acidic conditions. The ability to survive transit through the acidic conditions of the stomach is essential for successful colonization of the mammalian host by commensal and pathogenic bacteria (By similarity). H(+) + L-glutamate = 4-aminobutanoate + CO2 Homohexamer composed of three dimers. By acidic conditions. Expression is regulated by a complex system involving RpoS, cAMP, CRP, EvgAS, H-NS, GadE, GadW and GadX. The level of involvement for each regulator varies depending upon the growth phase and the medium (By similarity). Belongs to the group II decarboxylase family.