Structural homology of the glutamine amidotransferase subunits of the anthranilate synthetases of Eschierichia coil, Salmonella typhimurium and Serratia marcescens

Abstract

ANTHRANILATE synthetase (AS), the enzyme catalysing the initial reaction unique to tryptophan biosynthesis, chorismate + glutamine → anthranilate, has been characterised from a number of bacterial genera¹. In E. coli^2–4, Salmonella typhimurium^5–10, and Aerobacter aerogenes¹¹, this enzyme and anthranilate-5-phosphoribosylpyrophosphate phosphoribosyl-transferase (PRT) exist in a complex that also performs the second reaction of tryptophan biosynthesis, the conversion of anthranilate + 5-phosphoribosyl-1-pyrophosphate → phosphoribosyl anthranilate. The AS-PRT complex of these bacterial genera is composed of two non-identical polypeptide components, each with a molecular weight of approximately 60-65,000. Component I of the complex can convert chorismate + NH₃ → anthranilate but cannot use glutamine as amino donor. Component II of the complex is bifunctional; it provides the glutamine amidotransferase (GAT) and the PRT activities. In Serratia narcescens^12–14, Bacillus subtilis^15–16, Acinetobacter calcoaceticus^17–18, and various species of Pseudomonas^19–20, AS is not associated with PRT, which is a distinct protein. The AS of these bacteria consists of two non-identical subunits of different sizes. The large subunit, approximately 60–70,000 in molecular weight, functions as does component I of E. coli, S. typhimurium and A. aerogenes while the small subunit contributes the GAT activity. The molecular weights of the GAT subunit of S. marcescens^12,14, Pseudomonas putida^19,20, B. subtilis¹⁵, and A. calcoaceticus¹⁸ have been estimated to be 21,000, 18,000, 16,000 and 14,000 respectively. The PRT of S. marcescens has a molecular weight of 45,000, which is smaller than that of component II of E. coli and S. typhimurium by an amount equivalent to the molecular weight of a GAT subunit¹⁴.