Catalyzes the pyridoxal-dependent decarboxylation of glutamate to produce 4-aminobutanoate. Has weak activity with aspartate, but cannot complement an E. coli panD deletion mutant. H(+) + L-glutamate = 4-aminobutanoate + CO2 kcat is 0.055 sec(-1) at 37 degrees Celsius. Belongs to the group II decarboxylase family.