edit ] Structural studies In some bacteria , IGPS is a single chain]] [[enzyme. In others, such as Escherichia coli , it is the N-terminal domain of a bifunctional enzyme that also catalyses N-(5'-phosphoribosyl)anthranilate isomerase ( EC 5.3.1.24 ) (PRAI) activity, the third step of tryptophan biosynthesis. In fungi , IGPS