Selective Inhibition of Trypanosoma brucei 6-Phosphogluconate Dehydrogenase by High-Energy Intermediate and Transition-State Analogues

Abstract

Two series of compounds were designed to mimic the transition state and high-energy intermediates (HEI) of the enzymatic reaction of 6-phosphogluconate dehydrogenase (6PGDH). Sulfoxide analogues (7-11) were designed to mimic the transition state during the oxidation of the substrate to 3-keto-6-phosphogluconate, an enzyme-bound intermediate of the enzyme. Hydroxamate and amide derivatives of d-erythronic acid were designed to mimic the 1,2-cis-enediol HEI of the 6PGDH reaction. These two series of compounds were assayed as competitive inhibitors of the Trypanosoma brucei and sheep liver enzymes, and their selectivity value (ratio sheep/parasite) was calculated. The sulfoxide transition-state analogues showed weak and selective inhibition of the T. brucei enzyme. The hydroxamic derivatives showed potent and selective inhibition of the T. brucei 6PGDH with a Ki in the nanomolar range.

Inhibitors of Trypanosoma brucei 6-Phosphogluconate Dehydrogenase

“…This hydroxamate, with a K i = 10 nM and selectivity of 254-fold for the parasite enzyme over the sheep liver enzyme, is the compound with the highest affinity for the T. brucei 6PGDH reported to date and it also shows the highest selectivity for the parasite over the sheep liver enzyme [27]. [27]. From the mechanistic point of view, the fact that compounds 1-6 are strong 6PGDH inhibitors suggests that they bind to the enzyme in a similar way to the high-energy reaction intermediates adding support to the widely held view that these are 3-keto 6PG and the 1,2-enediol of Ru5P [1,2,26,27,31,32].…”

“…(2)", compound 3)), synthesized specifically to mimic the high-energy intermediates produced following the second (decarboxylation) step of the catalyzed reaction, shown in "Scheme (1)". This hydroxamate, with a K i = 10 nM and selectivity of 254-fold for the parasite enzyme over the sheep liver enzyme, is the compound with the highest affinity for the T. brucei 6PGDH reported to date and it also shows the highest selectivity for the parasite over the sheep liver enzyme [27]. [27].…”

“…[27]. From the mechanistic point of view, the fact that compounds 1-6 are strong 6PGDH inhibitors suggests that they bind to the enzyme in a similar way to the high-energy reaction intermediates adding support to the widely held view that these are 3-keto 6PG and the 1,2-enediol of Ru5P [1,2,26,27,31,32]. The selectivity of compound 3 for the T. brucei enzyme compared to the sheep liver one correlates to the higher turnover number of the parasite 6PGDH (27 s -1 against 21 s -1 for the sheep liver 6PGDH) and the higher affinity for 3-keto 6PG (K i on 6PG are 0.41 μM and 7.4 μM for the T. brucei and the sheep liver 6PGDH, respectively) [30].…”

“…Many compounds have been found with higher affinity for the parasite enzyme compared to the sheep liver 6PGDH [15,[25][26][27], despite their crystallographic structures having a similar overall fold and many residues which are nearest neighbours to the substrate being conserved [28]. Interestingly the affinity for the coenzyme NADP + in the T. brucei 6PGDH (K m = 1 μM) is also 7 times higher than in the sheep enzyme and the compound 2',5'-ADP representing a portion of the coenzyme lacking the nicotinamide group, has an affinity for the parasite enzyme some 40-fold higher than for the mammalian one (K i values being 20 and 800 μM, respectively) [15].…”

“…Since then, new better inhibitors have been found, which mimic the transition-state and high-energy intermediates of the enzymatic reaction of 6PGDH [27]. Hydrophobic analogues of these also revealed some anti-parasite activity.…”