Protein involved in the biosynthesis of purine, a nitrogenous heterocyclic base, e.g. adenine, guanine, hypoxanthine and xanthine. De novo synthesis involves a complex, energy-expensive pathway that yields inosine 5'-monophosphate (IMP), a purine ribonucleotide. AMP and GMP are then formed from IMP in separate pathways. Adenine and guanine are found in both DNA and RNA. Hypoxanthine and xanthine are important intermediates in the synthesis and degradation of the purine nucleotides.