Abstract
It has been proposed that a heme-peroxide complex is formed as an intermediate state in the enzymatic reaction. Since the axial position of the heme in tryptophan 2, 3-dioxygenase (TDO) is occupied with a nitrogenous ligand from histidine residue, we studied the heme-peroxide complexes with an axial nitrogenous ligand at the fifth position in a chemical model for TDO. Addition of N-methyl-imidazole to a ferric high-spin porphyrin exhibited ESR spectrum due to a formation of ferric low-spin complex (complex A). Following addition of tetramethylammonium hydroxide to the reaction mixture, the signal due to a different ferric low-spin complex (complex B) appeared. When 3-methylindole (skatole; SK) was added to the resulting solution followed by introduction of dry oxygen gas, a new ESR signal due to a ferric low-spin complex (complex C) was detectable. A similar ESR signal was observed when N-acetyl-tryptophan ethyl ester was used in place of Sk (complex D). Bohan plotting of the crystal-field parameters as a rhombicity-tetragonality diagram obtained from ESR g-values for the complexes (A, B, C, D) revealed that these heme iron complexes can be classified into three groups having N-Fe-N (complex A), N-Fe-O (complex B), and N-Fe-OO (complexes C, D) coordinations. Product analyses for the same reaction systems demonstrated the conversion of each substrate to the dioxygenated product.
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