title: Kinetics and activation parameters of the reaction of cyanide with free aquocobalamin and aquocobalamin bound to a haptocorrin from chicken serum.
abstract: 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.
authors.affiliation: Department of Chemistry, University of Texas, Arlington 76019.
authors.forename: H M
authors.initials: HM
authors.lastname: Marques
authors.forename: K L
authors.initials: KL
authors.lastname: Brown
authors.forename: D W
authors.initials: DW
authors.lastname: Jacobsen
mesh_terms: D000818:Animals; D002645:Chickens; D003486:Cyanides / Q000378:metabolism*; D006856:Hydrogen Cyanide / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D006879:Hydroxocobalamin / Q000378:metabolism*; D007700:Kinetics; D013053:Spectrophotometry; D013696:Temperature; D013816:Thermodynamics; D014155:Transcobalamins / Q000378:metabolism*
chemical_list: D003486:Cyanides; D014155:Transcobalamins; D006856:Hydrogen Cyanide; D006879:Hydroxocobalamin