Binding of thiocyanate and cyanide to manganese(III)-reconstituted horseradish peroxidase: a ¹⁵N nuclear magnetic resonance study

Abstract

Binding of thiocyanate and cyanide ions to Mn(III) protoporphyrin-apohorseradish peroxidase complex [Mn(III)HRP] was investigated by relaxation rate measurements (at 50.68 MHz) of ¹⁵N resonance of SC¹⁵N^- and C¹⁵N^-. At pH = 4.0 the apparent dissociation constant (K_D) for thiocyanate and cyanide binding to Mn(III)HRP was deduced to be 156 and 42 mM, respectively. The pH dependence of the ¹⁵N line width as well as apparent dissociation constant for thiocyanate and cyanide binding were quantitatively analyzed on the basis of a reaction scheme in which thiocyanate and cyanide in deprotonated form bind to the enzyme in a protonated form. The binding of thiocyanate and cyanide to Mn(III)HRP was found to be facilitated by protonation of an ionizable group on the enzyme [Mn(III)HRP] with a pK_a = 4.0. From competitive binding studies it was shown that iodide, thiocyanate and cyanide bind to Mn(III)HRP at the same site; however, the binding site for resorcinol is different. The apparent dissociation constant for iodide binding deduced from competitive binding studies was found to be 117 mM, which agrees very well with the iodide binding to ferric HRP. The binding of thiocyanate and cyanide was shown to be away from the metal center and the distance of the ¹⁵N of thiocyanate and cyanide from the paramagnetic manganese ion in Mn(III)HRP was found to be 6.9 and 6.6 Å, respectively. Except for cyanide binding, these observations parallel with the iodide and thiocyanate ion binding to native Fe(III)HRP. Water proton relaxivity measurements showed the presence of a coordinated water molecule to Mn(III)HRP with the distance of Mn-H₂O being calculated to be 2.6 Å. The slow reactivity of H₂O₂ towards Mn(III)HRP could be attributed to the presence of water at the sixth coordination position of the manganese ion.