The structure/function relationship of a dual-substrate (βα)₈-isomerase

Two structures of phosphoribosyl isomerase A (PriA) from Streptomyces coelicolor, involved in both histidine and tryptophan biosynthesis, were solved at 1.8 Å resolution. A closed conformer was obtained, which represents the first complete structure of PriA, revealing hitherto unnoticed molecular interactions and the occurrence of conformational changes. Inspection of these conformers, including ligand-docking simulations, allowed identification of residues involved in substrate recognition, chemical catalysis and conformational changes. These predictions were validated by mutagenesis and functional analysis. Arg¹⁹ and Ser⁸¹ were shown to play critical roles within the carboxyl and amino phosphate-binding sites, respectively; the catalytic residues Asp¹¹ and Asp¹³⁰ are responsible for both activities; and Thr¹⁶⁶ and Asp¹⁷¹, which make an unusual contact, are likely to elicit the conformational changes needed for adopting the active site architectures. This represents the first report of the structure/function relationship of this (βα)₈-isomerase.