PurH is bifunctional: IMP cyclohydrolase (EC 3.5.4.10); phosphoribosylaminoimidazolecarboxamide formyltransferase (EC 2.1.2.3) Involved in purine ribonucleotide biosynthesis. The IMP cyclohydrolase activity is in the N-terminal region. [Purines, pyrimidines, nucleosides, and nucleotides, Purine ribonucleotide biosynthesis]