InterPro domain: IPR006221

This entry represents the anthranilate synthase/para-aminobenzoate synthase domain, which share sequence similarity to the glutamine amidotransferase domain IPR017926 . Anthranilate synthase play a role in the tryptophan-biosynthetic pathway, while the para-aminobenzoate synthase is involved in the folate biosynthetic pathway. In at least one case, a single polypeptide from Bacillus subtilis was shown to have both functions.

This entry 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. [ 1 , 2 , 3 , 4 , 5 , 6 ].

1. Nucleotide sequence of the trpD gene, encoding anthranilate synthetase component II of Escherichia coli. J. Mol. Biol. 156, 245-56
2. Localization of two functions of the phosphoribosyl anthranilate transferase of Escherichia coli to distinct regions of the polypeptide chain. J Bacteriol 117, 502-8
3. Immunological study of anthranilate synthetase. J Bacteriol 123, 620-30
4. Folate synthesis in plants: the p-aminobenzoate branch is initiated by a bifunctional PabA-PabB protein that is targeted to plastids. Proc Natl Acad Sci U S A 101, 1496-501
5. Para-aminobenzoate synthase gene of Saccharomyces cerevisiae encodes a bifunctional enzyme. Yeast 9, 669-75
6. Evolution of aminobenzoate synthases: nucleotide sequences of Salmonella typhimurium and Klebsiella aerogenes pabB. Mol Biol Evol 5, 531-48