ON THE ROLE OF THE AXIAL LIGAND IN HEME-BASED CATALYSIS OF THE PEROXIDASE AND P450 TYPE

The present commentary focusses on the role of the axial ligand in per oxidase- and P450-type catalysis. Based on molecular orbital calculati ons and the experimental evidence available, it is argued that the lig and of a heme-containing enzyme may be a factor in setting the relativ e chance, although not the intrinsic capability, of the enzyme to cata lyse a specific type of heme-based reaction chemistry. The ligand can do so by influencing the electrophilicity, i.e. the redox potential of the high-valency iron-ore complex, and also by influencing the energy barrier for a reaction pathway through delocalization of valence elec trons along the axial ligands, thereby, in the case of a cysteinate bu t not a histidine axial ligand, stabilizing oxygen transfer pathways.