InterPro domain: IPR017932

A large group of biosynthetic enzymes are able to catalyse 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) [ 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) 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). An enzyme which catalyses 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. This enzyme catalyses 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). 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 catalysing the formation of glutamate from glutamine and 2-oxoglutarate. Glutamate synthase is a multicomponent iron-sulphur flavoprotein and three types occur which use a different electron donor: NADPH-dependent gltS (large chain), ferredoxin-dependent gltS and NADH-dependent gltS [ 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 stabilisation of the formed tetrahedral intermediate. An insert of ~120 residues can occur between the conserved regions [ 9 ]. In some class-II GATases (for example in Bacillus subtilis or chicken amido phosphoribosyltransferase) the enzyme is synthesised 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 addition to the propeptide that is excised on enzyme activation.

The 3-D structure of the GATase type 2 domain forms a four layer alpha/beta/beta/alpha architecture 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.

1. The amidotransferases. Adv. Enzymol. Relat. Areas Mol. Biol. 39, 91-183
2. Structural role for a conserved region in the CTP synthetase glutamine amide transfer domain. J. Bacteriol. 169, 3023-8
3. Sequence of the small subunit of yeast carbamyl phosphate synthetase and identification of its catalytic domain. J. Biol. Chem. 259, 9790-8
4. Glutamate synthase: a complex iron-sulfur flavoprotein. Cell. Mol. Life Sci. 55, 617-38
5. The glutamine-utilizing site of Bacillus subtilis glutamine phosphoribosylpyrophosphate amidotransferase. J. Biol. Chem. 258, 10582-5
6. The N-terminal cysteine of human asparagine synthetase is essential for glutamine-dependent activity. J. Biol. Chem. 264, 19475-7
7. The mechanism of glutamine-dependent amidotransferases. Cell. Mol. Life Sci. 54, 205-22
8. Glutamate synthase: a fascinating pathway from L-glutamine to L-glutamate. Cell. Mol. Life Sci. 61, 669-81