Bifunctional enzyme that catalyzes the last two steps of purine biosynthesis.Acts as a transformylase that incorporates a formyl group to the AMP analog AICAR (5-amino-1-(5-phospho-beta-D-ribosyl)imidazole-4-carboxamide) to produce the intermediate formyl-AICAR (FAICAR).Can use both 10-formyldihydrofolate and 10-formyltetrahydrofolate as the formyl donor in this reaction.Also catalyzes the cyclization of FAICAR to IMP.Is able to convert thio-AICAR to 6-mercaptopurine ribonucleotide, an inhibitor of purine biosynthesis used in the treatment of human leukemias.Promotes insulin receptor/INSR autophosphorylation and is involved in INSR internalization.The de novo purine synthesis pathway includes 10 sequential steps, beginning with phosphoribosyl pyrophosphate and ending with inositol monophosphate (IMP), the first purin compound of the pathway.