Mechanism of cyanamide hydration catalyzed by carbonic anhydrase II suggested by cryogenic X-ray diffraction

The three-dimensional structure of a possible intermediate in the hydration reaction of cyanamide to urea catalyzed by human carbonic anhydrase II (hC AII) has been determined by cryocrystallographic techniques. The crystal st ructure shows that two different adducts are formed under the experimental conditions and that they have different occupancy in the crystal. The high occupancy form consists of a binary hCAII-cyanamide complex where the subst rate has replaced the zinc-bound hydroxide anion present in the native enzy me, maintaining the tetrahedral geometry around the metal ion. The second, low-occupancy form consists of a hCAII-cyanamide-water ternary complex wher e the catalytic zinc ion, still being bound to cyanamide, is approached by a water molecule in a five-coordinate adduct. While the first form can be c onsidered a nonproductive complex, the second form may represent an interme diate state of the catalyzed reaction where the water molecule is about to perform a nucleophilic attack on the zinc-activated cyanamide substrate. Th e structural evidence is consistent with the kinetic data previously report ed about this recently described hydrolytic reaction catalyzed by hCAII, an d indicates that a different mechanism with respect to that generally accep ted for the physiologic carbon dioxide hydration reaction may be adopted by the enzyme, depending on the substrate chemical properties.