The bifunctional enzyme aminoimidazole carboxamide ribonucleotide transformylase/inosine monophosphate cyclohydrolase (ATIC) is responsible for catalysis of the last two steps in the de novo purine pathway. Gel filtration studies performed on human enzyme suggested that this enzyme is monomeric in solution. However, cross-linking studies performed on both yeast and avian ATIC indicated that this enzyme might be dimeric. To determine the oligomeric state of this protein in solution, we carried out sedimentation equilibrium analysis of ATIC over a broad concentration range. We find that ATIC participates in a monomer/dimer equilibrium with a dissociation constant of 240 +/- 50 nM at 4 degrees C. To determine whether the presence of substrates affects the monomer/dimer equilibrium, further ultracentrifugation studies were performed. These showed that the equilibrium is only significantly shifted in the presence of both AICAR and a folate analog, resulting in a 10-fold reduction in the dissociation constant. The enzyme concentration dependence on each of the catalytic activities was studied in steady state kinetic experiments. These indicated that the transformylase activity requires dimerization whereas the cyclohydrolase activity only slightly prefers the dimeric form over the monomeric form.