TRANSITION-STATE SELECTIVITY FOR A SINGLE HYDROXYL GROUP DURING CATALYSIS BY CYTIDINE DEAMINASE

Cytidine deaminase binds transition-state analog inhibitors similar to 10(7) times more tightly than corresponding 3,4-dihydro analogs conta ining a proton in place of the Lt-hydroxyl group. X-ray crystal struct ures of complexes with the two matched inhibitors differ only near a ' 'trapped'' water molecule in the complex with the 3,4-dihydro analog, where contacts are substantially less favorable than those with the hy droxyl group of the transition-state analog. The hydrogen bond between the hydroxyl group and the Glu104 carboxylate shortens in that comple x, and may become a ''low-barrier'' hydrogen bond, since at the same t ime the bond between zinc and the Cys132 thiolate ligand lengthens. Th ese differences must therefore account for most of the differential bi nding affinity related to catalysis. Moreover, the trapped water molec ule retains some of the binding energy stabilizing the hydroxyl group in the transition-state analog complex. To this extent, the ratio of b inding affinities for the two compounds is smaller than the true contr ibution of the hydroxyl group, a conclusion with significant bearing o n interpreting difference free energies derived from substituent effec ts arising from chemical modification and/or mutagenesis.