Catalyzes the synthesis of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) with pyridoxal 5'-phosphate as cofactor. Enzymatically inactive as glutamate decarboxylase. H(+) + L-glutamate = 4-aminobutanoate + CO2 Homodimer. Expressed in brain. Expressed in pancreatic islets, testis, adrenal cortex, and perhaps other endocrine tissues, but not in brain. The disease is caused by variants affecting the gene represented in this entry. Belongs to the group II decarboxylase family. Glutamate decarboxylase entry