Metalloporphycene is an Attractive Cofactor for Hemoproteins

Abstract

Porphycene, a constitutional isomer of porphyrin, is an attractive ligand, and its metal complexes have been investigated as alternative metal cofactors for hemoproteins such as myoglobin. Iron, cobalt and manganese complexes of porphycene are smoothly inserted into apomyoglobin after removal of native heme b, resulting in stable reconstituted proteins. Myoglobin reconstituted with iron porphycene exhibits extremely high O2 affinity compared to native myoglobin (nMb). In addition, the reconstituted protein also shows catalytic activity toward one-electron oxidation of phenol derivatives and sulfoxidation of thioanisole, although the natural function of nMb is O2 storage. Furthermore, myoglobin reconstituted with manganese porphycene can promote H2O2-dependent hydroxylation of inert alkane species as seen with cytochrome P450s. Myoglobin reconstituted with iron porphycene can act as a catalyst for an abiological reaction such as cyclopropanation with ethyl diazoacetate. These results clearly indicate that replacement of heme with metalloporphycenes can dramatically alter the function of hemoproteins. Myoglobin reconstituted with iron porphycene, a constitutional isomer of iron porphyrin, exhibits significantly higher O2 affinity with unique O2/CO discrimination ability compared to native myoglobin. Furthermore, myoglobin, an O2 storage protein, can be converted to artificial metalloenzymes with catalytic activities such as peroxidase, hydroxylase and carbene transferase, by replacing the native heme cofactor with iron porphycene or manganese porphycene.