Structure Of Ribonuclease A Complexed With Inhibitors dCpdG, UpcU UpcA. L. Boque¹, A. Gustchina¹, J. N. Alexandratos¹, B. K. Sathyanarayana¹, K. Sh. Padykova², M. Ya. Karpeisky² and A. Wlodawer^{1 1}Macromolecular Structure Laboratory, ABL-Basic Research Program, NCI-FCRDC, Frederick, MD21702, ²V. Engelhardt Institute of Molecular Biology, Academy of Sciences of Russia, Vavilova Str, 32, Moscow, Russia

The structures of ribonuclease A co-crystalized with inhibitors dCpdG, UpcU and UpcA have been solved using uncomplexed ribonuclease A as a model,and refined to a resolution of 1.6, 1.5 and 1.4 Å, respectively, with a current R-factor below 17 % in all cases. While in the three structures the protein moiety is well orderered, none of the inhibitors appears to be bound in a standard, simple way. In the RNase-dCpdG co-crystal, the inhibitoris bound in a retroactive mode that has been previously reported for crystalsinto which the inhibitor was soaked. In the UpcU complex, where a carbon atom replaces the oxygen from the phosphoester bond that would normally be hydrolyzed, the first uridine occupies the B1 site while the second uridine in UpcU appears to be disordered. In UpcA, uridine and adenine occupy the B1 and B2 subsites, but a whole intact UpcA molecule cannot be easily modeled. Work is in progress to clarify the binding mode of these inhibitors and to relate it to the catalytic mechanism of RNase A.