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 () (PRAI) activity, the third step of
tryptophan biosynthesis. In
fungi, IGPS is the central domain of a trifunctional enzyme that contains a PRAI
C-terminal domain and a
glutamine amidotransferase () (GATase) N-terminal domain. A
structure of the IGPS domain of the bifunctional enzyme from the
mesophilic bacterium
E. coli (eIGPS) has been compared with the
monomeric indole-3-glycerol phosphate synthase from the
hyperthermophilic archaeon Sulfolobus solfataricus (sIGPS). Both are single-domain (beta/alpha)8 barrel proteins, with one (eIGPS) or two (sIGPS) additional
helices inserted before the first
beta strand. As of late 2007, 11
structures have been solved for this class of enzymes, with
PDB accession codes , , , , , , , , , , and . ==References==