The Met243 sulfonium ion linkage is responsible for the anomalous magneticcircular dichroism and optical spectral properties of myeloperoxidase

The heme group of myeloperoxidase shows anomalous optical properties, and t he enzyme possesses the unique ability to catalyze the oxidation of chlorid e. However, the nature of the covalently bound heme macrocycle has been dif ficult to identify. In this work, the electronic and magnetic properties of the heme groups in oxidized and reduced forms of wildtype and Met243Thr mu tant myeloperoxidase and wildtype lactoperoxidase have been investigated us ing variable-temperature (1.6-273 K) magnetic circular dichroism (MCD) spec troscopy along with parallel optical absorption and electron paramagnetic r esonance studies. The results provide assessment of the spin state mixtures of the oxidized and reduced samples at ambient and liquid helium temperatu res and show that the anomalous MCD properties of myeloperoxidase, e.g. red -shifted and inverted signs for bands in the high-spin ferric and low-spin ferrous forms compared to other heme peroxidases and heme proteins in gener al, are a direct consequence of a novel electron-withdrawing sulfonium ion heme linkage involving Met243.