In enzymology, an indole-3-glycerol-phosphate synthase (IGPS) () is an enzyme that catalyzes the chemical reaction
:1-(2-carboxyphenylamino)-1-deoxy-D-ribulose 5-phosphate 1-C-(indol-3-yl)-glycerol 3-phosphate + CO2 + H2O
Hence, this enzyme has one substrate, 1-(2-carboxyphenylamino)-1-deoxy-D-ribulose 5-phosphate, but 3 products: 1-C-(indol-3-yl)-glycerol 3-phosphate, CO2, and H2O.
This enzyme belongs to the family of lyases, to be specific, the carboxy-lyases, which cleave carbon-carbon bonds. The systematic name of this enzyme class is 1-(2-carboxyphenylamino)-1-deoxy-D-ribulose-5-phosphate carboxy-lyase (1-C-(indol-3-yl)glycerol-3-phosphate-forming ). Other names in common use include indoleglycerol phosphate synthetase, indoleglycerol phosphate synthase, indole-3-glycerophosphate synthase, 1-(2-carboxyphenylamino)-1-deoxy-D-ribulose-5-phosphate, and carboxy-lyase (cyclizing). This enzyme participates in phenylalanine, tyrosine and tryptophan biosynthesis and two-component system - general. It employs one cofactor, pyruvate.
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 .
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