Glutamine aminotransferase (GAT, glutaminase) domain of glutamine-dependent NAD synthetases (class 7 and 8 nitrilases)
Glutamine-dependent NAD synthetases are bifunctional enzymes, which have an N-terminal GAT domain and a C-terminal NAD+ synthetase domain. The GAT domain is a glutaminase (EC 220.127.116.11) which hydrolyses L-glutamine to L-glutamate and ammonia. The ammonia is used by the NAD+ synthetase domain in the ATP-dependent amidation of nicotinic acid adenine dinucleotide. Glutamine aminotransferases are categorized depending on their active site residues into different unrelated classes. This class of GAT domain belongs to a larger nitrilase superfamily comprised of nitrile- or amide-hydrolyzing enzymes and amide-condensing enzymes, which depend on a Glu-Lys-Cys catalytic triad. This superfamily has been classified in the literature based on global and structure based sequence analysis into thirteen different enzyme classes (referred to as 1-13), this subgroup corresponds to classes 7 and 8. Members of this superfamily generally form homomeric complexes, the basic building block of which is a homodimer. Mycobacterium tuberculosis glutamine-dependent NAD+ synthetase forms a homooctamer.