title: Hemes and hemoproteins. 3. The reaction of microperoxidase-8 with cyanide: comparison with aquocobalamin and hemoproteins.
abstract: The monomeric heme octapeptide from cytochrome c, microperoxidase-8, (MP-8), coordinates CN- with log K = 7.55 +/- 0.04 at 25 degrees C in 20% (v/v) aqueous methanol. Log K values are independent of pH between 6 and 9. A spectrophotometric titration of cyanoMP-8 between pH 5.5 and 13.8 gave a single pKa greater than or equal to 13.5 ascribed to ionization of the proximal His ligand. A study of the kinetics of the reaction of MP-8 with cyanide between pH 5.5 and 12, at 25 degrees C and mu = 0.1, indicates that formation of cyanoMP-8 occurs via three routes: attack of CN- on Fe(III) (k1 = 6.0 +/- 0.3 X 10(5) M-1 sec-1); attack of HCN on Fe(III) (k2 = 4.8 +/- 2.0 X 10(3) M-1 sec-1), followed by deprotonation and isomerization to form the C-bound species; and displacement of OH- by CN- when the proximal His ligand is ionized (k5 = 1.8 +/- 0.1 X 10(5) M-1 sec-1). These results are compared with available data for the reaction of cyanide with aquocobalamin and with various hemoproteins.
mesh_terms: D000818:Animals; D003486:Cyanides / Q000378:metabolism*; D003574:Cytochrome c Group / Q000378:metabolism; D006420:Hemeproteins / Q000378:metabolism*; D006736:Horses; D006863:Hydrogen-Ion Concentration; D006879:Hydroxocobalamin / Q000378:metabolism*; D007700:Kinetics; D009206:Myocardium / Q000378:metabolism; D010544:Peroxidases / Q000378:metabolism*; D011485:Protein Binding
chemical_list: D003486:Cyanides; D003574:Cytochrome c Group; D006420:Hemeproteins; D010544:Peroxidases; C021799:microperoxidase; D006879:Hydroxocobalamin