Iodide modulation of the EDTA-induced iodine reductase activity of horseradish peroxidase by interaction at or near the EDTA-binding site

Abstract

Horseradish peroxidase (HRP) catalyses the reduction of iodinium ion (I⁺) to iodide by H₂O₂ in the presence of EDTA. I⁺ reduction occurs optimally at pH 6 whereas the enzyme catalyses iodide oxidation optimally at pH 3.5. Thus the two activities reside on the same enzyme with two characteristic pH optima. Iodide modulates the expression of the reductase activity by EDTA. Higher concentrations of iodide inhibit the reductase activity by EDTA. Nitrite, an electron donor, acts similarly to iodide. Both EDTA and nitrite competitively inhibit iodide oxidation, indicating that they compete with iodide for the same binding site for electron flow to the haem iron group. However, unlike iodide, EDTA converts compound I, not into the native enzyme, but into a compound absorbing at 416 nm which reduces I⁺ and then returns to the native form. The apparent equilibrium dissociation constant, K_D, for the formation of the EDTA-HRP complex (15 mM) is doubled in the presence of iodide, indicating interference with EDTA binding by iodide. EDTA binds away from the haem iron centre and not through intramolecular Ca²⁺. The pH-dependence of EDTA binding indicates that an ionizable group of the enzyme with pK_a 5.8, presumably a distal histidine, controls the binding. The data suggest that iodide competes with EDTA for compound I and modulates the iodine reductase activity by limiting the formation of the 416 nm-absorbing active compound.