PROSITE documentation PDOC00406

Glutamine amidotransferase type 2 domain profile

A large group of biosynthetic enzymes are able to catalyze the removal of the ammonia group from glutamine and then to transfer this group to a substrate to form a new carbon-nitrogen group. This catalytic activity is known as glutamine amidotransferase (GATase) (EC 2.4.2.-) [1]. The GATase domain exists either as a separate polypeptidic subunit or as part of a larger polypeptide fused in different ways to a synthase domain. On the basis of sequence similarities two classes of GATase domains have been identified [2,3]: class-I (also known as trpG-type or triad) (see <PDOC00405>) and class-II (also known as purF-type or Ntn). Class-II (or type 2) GATase domains have been found in the following enzymes:

• Amido phosphoribosyltransferase (glutamine phosphoribosylpyrophosphate amidotransferase) (EC 2.4.2.14). An enzyme which catalyzes the first step in purine biosynthesis, the transfer of the ammonia group of glutamine to PRPP to form 5-phosphoribosylamine (gene purF in bacteria, ADE4 in yeast).
• Glucosamine--fructose-6-phosphate aminotransferase (EC 2.6.1.16). This enzyme catalyzes a key reaction in amino sugar synthesis, the formation of glucosamine 6-phosphate from fructose 6-phosphate and glutamine (gene glmS in Escherichia coli, nodM in Rhizobium, GFA1 in yeast).
• Asparagine synthetase (glutamine-hydrolyzing) (EC 6.3.5.4). This enzyme is responsible for the synthesis of asparagine from aspartate and glutamine.
• Glutamate synthase (gltS), an enzyme which participates in the ammonia assimilation process by catalyzing the formation of glutamate from glutamine and 2-oxoglutarate. Glutamate synthase is a multicomponent iron-sulfur flavoprotein and three types occur which use a different electron donor: NADPH-dependent gltS (large chain) (EC 1.4.1.13), ferredoxin-dependent gltS (EC 1.4.7.1) and NADH-dependent gltS (EC 1.4.1.14) [4].

The active site is formed by a cysteine present at the N-terminal extremity of the mature form of all these enzymes [5,6,7,8]. Two other conserved residues, Asn and Gly, form an oxyanion hole for stabilization of the formed tetrahedral intermediate. An insert of ~120 residues can occur between the conserved regions [4]. In some class-II GATases (for example in Bacillus subtilis or chicken amido phosphoribosyltransferase) the enzyme is synthesized with a short propeptide which is cleaved off post-translationally by a proposed autocatalytic mechanism. Nuclear-encoded Fd-dependent gltS have a longer propeptide which may contain a chloroplast-targeting peptide in additon to the propeptide that is excised on enzyme activation [4].

The 3-D structure of the GATase type 2 domain forms a four layer α/β/β/α architecture (see <PDB:1LM1>) which consists of a fold similar to the N-terminal nucleophile (Ntn) hydrolases. These have the capacity for nucleophilic attack and the possibility of autocatalytic processing. The N-terminal position and the folding of the catalytic Cys differ strongly from the Cys-His-Glu triad which forms the active site of GATases of type 1 (see <PDOC00405>).

The profile we developed covers the entire GATase type 2 domain.