Kinetic study of the slow cyanide binding to Glycera dibranchiata monomer hemoglobin components III and IV.

Compared to other monomeric heme proteins and the heme peroxidases, the Glycera dibranchiata monomer hemoglobin components III and IV exhibit very slow cyanide binding kinetics. This is agreement with the previously reported behavior of component II. Similar to component II, components III and IV have been studied under pseudo-first-order conditions at pH 6.0, 7.0, 8.0, and 9.0 by using a 100-250-fold excess of potassium cyanide at each pH. At 20 degrees C with micromolar protein concentrations, kobs for component III varies between 7.08 x 10(-5) s-1 at pH 6.0 and 100-fold cyanide excess and 1.06 x 10(-2) s-1 at pH 9.0 and 250-fold cyanide excess. For component IV, the values are 2.03 x 10(-4) s-1 for 100-fold cyanide excess at pH 6.0 and 4.13 x 10(-2) s-1 for 250-fold cyanide excess at pH 9.0. In comparison to other heme proteins, our analysis shows that the bimolecular rate constant (klapp) is small. For example, at pH 7.0, it is 3.02 x 10(-1) M-1 s-1 for component III and 1.82 M-1 s-1 for component IV, compared to 400 M-1 s-1 for sperm whale metmyoglobin, 692 M-1 s-1 for soybean metleghemoglobin a, 111 M-1 s-1 for guinea pig methemoglobin, and 1.1 x 10(5) M-1 s-1 for cytochrome c peroxidase. Our results also show that the dissociation rates (k-lapp) are extremely slow and no larger than 10(-6) s-1.(ABSTRACT TRUNCATED AT 250 WORDS)