Crystal structure of dihydropyrimidine dehydrogenase, a major determinant of the pharmacokinetics of the anti-cancer drug 5-fluorouracil

Dihydropyrimidine dehydrogenase catalyzes the first step in pyrimidine degr adation: the NADPH-dependent reduction of uracil and thymine to the corresp onding 5,6-dihydropyrimidines. Its controlled inhibition has become an adju nct target for cancer therapy, since the enzyme is also responsible for the rapid breakdown of the chemotherapeutic drug 5-fluorouracil, The crystal s tructure of the homodimeric pig liver enzyme (2 x 111 kDa) determined at 1. 9 Angstrom resolution reveals a highly modular subunit organization, consis ting of five domains with different folds. Dihydropyrimidine dehydrogenase contains two FAD, two FMN and eight [4Fe-4S] clusters, arranged in two elec tron transfer chains that pass the dimer interface twice. Two of the Fe-S c lusters show a hitherto unobserved coordination involving a glutamine resid ue. The ternary complex of an inactive mutant of the enzyme with bound NADP H and 5-fluorouracil reveals the architecture of the substrate-binding site s and residues responsible for recognition and binding of the drug.