Long chain
hydroxy acid oxidase (LCHAO) is responsible for the formation of
methylguanidine, a toxic compound with elevated serum levels in patients with
chronic renal failure. Its
isozyme glycolate oxidase (GOX), has a role in the formation of
oxalate, which can lead to pathological deposits of
calcium oxalate, in particular in the disease
primary hyperoxaluria. Inhibitors of these two
enzymes may have therapeutic value. These
enzymes are the only human members of the family of
FMN-dependent l-2-hydroxy
acid-oxidizing
enzymes, with yeast
flavocytochrome b(2) (Fcb2) among its well studied members. We screened a chemical library for inhibitors, using in parallel rat LCHAO, human GOX and the Fcb2 flavodehydrogenase domain (FDH). Among the hits was an inhibitor, CCPST, with an IC(50) in the micromolar range for all three
enzymes. We report here the crystal structure of a complex between this compound and LCHAO at 1.3 Å resolution. In comparison with a lower resolution structure of this
enzyme, binding of the inhibitor induces a conformational change in part of the TIM barrel loop 4, as well as protonation of the active site
histidine. The CCPST interactions are compared with those it forms with human GOX and those formed by two other inhibitors with human GOX and spinach GOX. These compounds differ from CCPST in having the
sulfur replaced with a
nitrogen in the five-membered ring as well as different hydrophobic substituents. The possible reason for the ∼100-fold difference in affinity between these two series of inhibitors is discussed. The present results indicate that specificity is an issue in the quest for therapeutic inhibitors of either LCHAO or GOX, but they may give leads for this quest.