Heme electron transfer in peroxidases: the propionate e-pathway.
Computational modeling offers a new insight about the electron transfer pathway in heme peroxidases. Available crystal structures have revealed an intriguing arrangement of the heme propionate side chains in heme-heme and heme-substrate complexes. By means of mixed quantum mechanical/molecular mechanics calculations, we study the involvement of these propionate groups into the substrate oxidation in ascorbate peroxidase and into the heme to heme electron transfer in bacterial cytochrome c peroxidase. By selectively turning on/off different quantum regions, we obtain the electron transfer pathway which directly involves the porphyrin ring and the heme propionates. Furthermore, in ascorbate peroxidase the presence of the substrate appears to be crucial for the activation of the electron transfer channel. The results might represent a general motif for electron transfer from/to the heme group and change our view for the propionate side chains as simple electrostatic binding anchors. We name the new mechanism "the propionate e-pathway".