DCoH, the dimerization cofactor of hepatocyte nuclear factor 1 (HNF-1), functions as both a transcriptional coactivator and a pterin dehydratase. To probe the relationship between these two functions, the X-ray crystal structures of the free enzyme and its complex with the product analogue 7,8-dihydrobiopterin were refined at 2.3 A resolution. The ligand binds at four sites per tetrameric enzyme, with little apparent conformational change in the protein. Each active-site cleft is located in a subunit interface, adjacent to a prominent saddle motif that has structural similarities to the TATA binding protein. The pterin binds within an arch of aromatic residues that extends across one dimer interface. The bound ligand makes contacts to three conserved histidines, and this arrangement restricts proposals for the enzymatic mechanism of dehydration. The dihedral symmetry of DCoH suggests that binding to the dimerization domain of HNF-1 likely involves the superposition of two-fold rotation axes of the two proteins.