PHOTOOXIDATION OF HORSE AND SPERM-WHALE MYOGLOBIN SENSITIZED BY THE HEME GROUP

Abstract —The irradiation of horse and sperm-whale Fe 3 * or Fe 2* myoglobins with visible light showed that axial ligands that render the heme diamagnetic (e.g. O₂, CO or CN^-) endow the hemoproteins with a marked photosensitivity. In contrast, high-spin myoglobins are unaffected by visible light. These findings appear to be of general validity for all hemo-proteins and are in agreement with the involvment of the triplet state of the heme as the reactive intermediate. In all cases, the overall photoprocess occurs within a very narrow spatial range, leading to specific modification of these photooxidizable side chains adjacent to the chromophore. Therefore, this technique can be used to probe the environment of the prosthetic group in hemoproteins. In particular, our data suggest that, in horse myoglobin, histidines-93 and -64 represent the heme-linked and the distal imidazole groups, respectively; moreover, the thioether function of methionine-131 must be nearer the heme in horse than in sperm-whale myoglobin. The selectivity of the photoreaction can be further enhanced by a suitable choice of the sixth ligand, and/or by controlling the pH of the irradiated solution. For example, for both proteins, irradiation of the cyanide derivative results in specific photooxidation of the proximal histidine, whereas irradiation of horse CO-ferromyoglobin at pH values below 6 causes specific photooxidation of methionine-131. Consequently, this photooxidative procedure can also be utilized to monitor conformational changes upon binding of the heme with different ligands, as well as to achieve the selective modification of amino acid residues, which are usually buried inside the protein molecule.