Glutamine synthetase that catalyzes the ATP-dependent conversion of glutamate and ammonia to glutamine (PubMed:19895308). When expressed in liver, it may be involved in detoxifying intramitochondrially generated ammonia (PubMed:4401992). Also acts as glutamate decarboxylase by catalyzing the production of 4-aminobutanoate (gamma-aminobutyric acid, GABA) in a pyridoxal phosphate-independent manner (PubMed:19895308). ATP + L-glutamate + NH4(+) = ADP + H(+) + L-glutamine + phosphate H(+) + L-glutamate = 4-aminobutanoate + CO2 Glutamate to glutamine ratio influences catalytic activity (PubMed:19895308). At glutamate to glutamine ratios greater than 4, decarboxylase activity ceases (PubMed:19895308). In the presence of manganese, synthetase activity is limited to concentrations between 10 mM and 20 mM, whereas decarboxylase activity is not affected (PubMed:19895308). Both catalytic activities are inhibited by avidin (PubMed:19895308). Homooctamer and homotetramer. In the liver found in mitochondria, in brain and retina in the cytoplasm (PubMed:4401992, PubMed:12965260). In retinal cells found in the outer part of the inner nuclear layer and in the bacillary layer of the photoreceptor, and is probably associated with the cell membrane (PubMed:4401992, PubMed:12965260). Expressed in retina, brain and liver (PubMed:1356223). Little or no detectable expression in breast muscle, pancreas and spleen (PubMed:1356223). Weakly expressed in retina on embryonic day 18, with levels increasing until day 6 after hatching and then remaining high until day 21 (at protein level). In the retina, down-regulated upon application of glutamate concentrations of 15 umol/eye or higher. Belongs to the glutamine synthetase family.