Unusual cyanide bindings to a heme-regulated phosphodiesterase from Escherichia coli: Effect of Met95 mutations

In order to understand heme environment of a heme-regulated phosphodiesterase (Ec DOS), the binding behavior of cyanide to the Fe (III) complex was examined. Interestingly, the rate of cyanide binding to full-length Ec DOS was unusually slow with k_on = 0.0022mM^-1 s^-1, while the rate for the isolated heme domain of Ec DOS (0.045mM^-1 s^-1) was 20-fold higher. Ala and Leu mutations at Met95, which has been suggested to be a heme axial ligand, increased the k_on rate 11- and 8-fold, respectively, and dramatically decreased the cyanide dissociation rate from the isolated heme domain. His mutation at Met95, on the other hand, caused a 17-fold decrease in the k_on value. We discuss the unusual cyanide binding behavior and the role of Met95 in controlling cyanide binding.