Type 1 glutamine amidotransferase (GATase1) domain found in Anthranilate synthase
Type 1 glutamine amidotransferase (GATase1) domain found in Anthranilate synthase (ASase). This group contains proteins similar to para-aminobenzoate (PABA) synthase and ASase. These enzymes catalyze similar reactions and produce similar products, PABA and ortho-aminobenzoate (anthranilate). Each enzyme is composed of non-identical subunits: a glutamine amidotransferase subunit (component II) and a subunit that produces an aminobenzoate products (component I). ASase catalyses the synthesis of anthranilate from chorismate and glutamine and is a tetrameric protein comprising two copies each of components I and II. Component II of ASase belongs to the family of triad GTases which hydrolyze glutamine and transfer nascent ammonia between the active sites. In some bacteria, such as Escherichia coli, component II can be much larger than in other organisms, due to the presence of phosphoribosyl-anthranilate transferase (PRTase) activity. PRTase catalyses the second step in tryptophan biosynthesis and results in the addition of 5-phosphoribosyl-1-pyrophosphate to anthranilate to create N-5'-phosphoribosyl-anthranilate. In E.coli, the first step in the conversion of chorismate to PABA involves two proteins: PabA and PabB which co-operate to transfer the amide nitrogen of glutamine to chorismate forming 4-amino-4 deoxychorismate (ADC). PabA acts as a glutamine amidotransferase, supplying an amino group to PabB, which carries out the amination reaction. A third protein PabC then mediates elimination of pyruvate and aromatization to give PABA. Several organisms have bipartite proteins containing fused domains homologous to PabA and PabB commonly called PABA synthases. These hybrid PABA synthases may produce ADC and not PABA.