IGPS catalyzes the conversion of PRFAR and glutamine to IGP, AICAR and glutamate. The HisH subunit provides the glutamine amidotransferase activity that produces the ammonia necessary to HisF for the synthesis of IGP and AICAR. 5-[(5-phospho-1-deoxy-D-ribulos-1-ylimino)methylamino]-1-(5-phospho-beta-D-ribosyl)imidazole-4-carboxamide + L-glutamine = 5-amino-1-(5-phospho-beta-D-ribosyl)imidazole-4-carboxamide + D-erythro-1-(imidazol-4-yl)glycerol 3-phosphate + H(+) + L-glutamate H2O + L-glutamine = L-glutamate + NH4(+) Amino-acid biosynthesis; L-histidine biosynthesis; L-histidine from 5-phospho-alpha-D-ribose 1-diphosphate: step 5/9. Heterodimer of HisH and HisF.