Kinetics and activation parameters of the reaction of cyanide with free aquocobalamin and aquocobalamin bound to a haptocorrin from chicken serum.

The kinetics of the reaction of cyanide with free aquocobalamin (H2OCbl) and with aquocobalamin bound to a vitamin B12-binding protein (haptocorrin) from chicken serum (HC-H2OCbl) have been investigated as a function of temperature and pH. The mechanism of replacement of H2O from protein-bound and free H2OCbl is apparently the same and involves attack of both CN- (k1) and HCN (k2). The reactions with HC-H2OCbl are somewhat slower than those with free H2OCbl, k1 being 22-fold smaller and k2 7-fold smaller at 25 degrees C. The relatively small effect of the protein on the rate constants supports the view that the metal in HC-H2OCbl is readily accessible to solvent. Activation parameters suggest that the transition states for ligand substitution are stabilized by nucleophilic participation (at least by CN-) and that ligand substitution on the protein-bound cobalamin proceeds through more ordered, concerted transition states. The latter effect suggests that the Co-O bond of H2OCbl is strengthened upon binding to the haptocorrin.