Structure of Escherichia Coli Aminodeoxychorismate Synthase: Architectural Conservation and Diversity in Chorismate-Utilizing Enzymes

Published

February 1, 2002

Author(s)

J F. Parsons, P Y. Jensen, A S. Pachikara, A J. Howard, Edward Eisenstein, Jane E. Ladner

Abstract

Aminodeoxychorismate synthase is part of a heterodimeric complex that catalyzes the two-step biosynthesis of 4-amino-4-deoxychorismate, a precursor of p-aminobenzoate and folate in microorganisms. In the first step, a glutamine amidotransferase encoded by the pabA gene generates ammonia as a substrate that, along with chorismate, is used in the second step, catalyzed by aminodeoxychorismate synthase, the product of the pabB gene. Here we report the X-ray crystal structure of Escherichia coli PabB solved in two different crystal forms at 2.0 resolution. The 453 residue monomeric PabB has a complex a/b fold which is similar to that seen in the structures of homologous, oligomeric TrpE subunits of several anthranilate synthases of microbial origin. A comparison of the structures of these two classes of chorismate utilizing enzymes provides a rationale for the differences in quaternary structures seen for these enzymes, and indicates that the weak, or transient association of PabB with PabA during catalysis stems at least partly from a limited interface for protein interactions. Additional analyses of the structures enabled the tentative identification of the active site of PabB, which contains a number of residues implicated from previous biochemical and genetic studies to be essential for activity. Differences in the structures determined from phosphate- and formate-grown crystals, and the location of an adventitious formate ion, suggest that conformational changes in loop regions adjacent to the active site may be needed for catalysis. A surprising finding in the structure of PabB was the presence of a tryptophan molecule deeply embedded in a binding pocket that is analogous to the regulatory site in the TrpE subunits of the anthranilate synthases. The strongly bound ligand, which cannot be dissociated without denaturation of PabB, may play a structural role in the enzyme since there is no effect of tryptophan on the enzymic synthesis of aminodeoxychorismate. Extensive sequence similarity in the tryptophan-binding pocket among several other chorismate utilizing enzymes, including isochorismate synthase, suggest that they too may bind tryptophan for structural integrity, and corroborate early ideas on the evolution of this interesting enzyme family.

Citation

Biochemistry

Volume

41

Issue

No. 7

Pub Type

Journals

Keywords

4-1mino-4-deoxychorismate synthase, chorismate, protein structure, X-ray crystallography

Citation

Parsons, J. , Jensen, P. , Pachikara, A. , Howard, A. , Eisenstein, E. and Ladner, J. (2002), Structure of Escherichia Coli Aminodeoxychorismate Synthase: Architectural Conservation and Diversity in Chorismate-Utilizing Enzymes, Biochemistry (Accessed April 25, 2024)