Triazole pyrimidine nucleosides as inhibitors of Ribonuclease A. Synthesis, biochemical, and structural evaluation

Five ribofuranosyl pyrimidine nucleosides and their corresponding 1,2,3-triazole derivatives have been synthesized and characterized. Their inhibitory action to Ribonuclease A has been studied by biochemical analysis and X-ray crystallography. These compounds are potent competitive inhibitors of RNase A with low μM inhibition constant (K_i) values with the ones having a triazolo linker being more potent than the ones without. The most potent of these is 1-[(β-d-ribofuranosyl)-1,2,3-triazol-4-yl]uracil being with K_i = 1.6 μM. The high resolution X-ray crystal structures of the RNase A in complex with three most potent inhibitors of these inhibitors have shown that they bind at the enzyme catalytic cleft with the pyrimidine nucleobase at the B₁ subsite while the triazole moiety binds at the main subsite P₁, where P-O5′ bond cleavage occurs, and the ribose at the interface between subsites P₁ and P₀ exploiting interactions with residues from both subsites. The effect of a susbsituent group at the 5-pyrimidine position at the inhibitory potency has been also examined and results show that any addition at this position leads to a less efficient inhibitor. Comparative structural analysis of these RNase A complexes with other similar RNase A - ligand complexes reveals that the triazole moiety interactions with the protein form the structural basis of their increased potency. The insertion of a triazole linker between the pyrimidine base and the ribose forms the starting point for further improvement of these inhibitors in the quest for potent ribonucleolytic inhibitors with pharmaceutical potential.