The heme group for a peroxidase monomer is embed deeply in the crevice between two domains . A helix contains the fifth iron ligand for the heme group (green).
For Horseradish Peroxidase (HRP) the heme iron is covalently bound to a nitrogen in Histidine 170's imidazole ring . The heme porphyrin plane is perpendicular to the imidazole ring. This Histidine is a conserved residue that is present in almost all types of peroxidases.
Aside from Histidine, there are 3 other residues that are conserved for most peroxidases at the heme's distal side . These three function as a ligand pocket for H2O2. They are responsible for catalyzing the heterolytic cleavage of H2O2. For HRP, these amino acids are Arg38 (cyan), Phe41 (purple), His42 (magenta) .
The heme, together with these residues, forms the peroxidase's active site .