Crystal Structures of Human Bifunctional Enzyme Aminoimidazole-4-carboxamide Ribonucleotide Transformylase/IMP Cyclohydrolase in Complex with Potent Sulfonyl-containing Antifolates

Aminoimidazole-4-carboxamide ribonucleotide (AICAR) transformylase/IMP cyclohydrolase (ATIC) is a bifunctional enzyme with folate-dependent AICAR transformylase and IMP cyclohydrolase activities that catalyzes the last two steps of purine biosynthesis. The AICAR transformylase inhibitors BW1540 and BW2315 are sulfamido-bridged 5,8-dideazafolate analogs with remarkably potent Ki values of 8 and 6 nm, respectively, compared with most other antifolates. Crystal structures of ATIC at 2.55 and 2.60 Å with each inhibitor, in the presence of substrate AICAR, revealed that the sulfonyl groups dominate inhibitor binding and orientation through interaction with the proposed oxyanion hole. These agents then appear to mimic the anionic transition state and now implicate Asn431′ in the reaction mechanism along with previously identified key catalytic residues Lys266 and His267. Potent and selective inhibition of the AICAR transformylase active site, compared with other folate-dependent enzymes, should therefore be pursued by further design of sulfonyl-containing antifolates.