Peroxidase is one of the most used enzymes for biosensors. In general, peroxidase biosensors can be used for the detection of hydrogen peroxide, organic hydroperoxides, phenols, aromatic amines and inhibitors. Combined with hydrogen peroxide producing oxidases they can also be used for monitoring the concentration of the substrates of oxidases, e.g., glucose, alcohols, glutamate, and choline. Peroxidase biosensors were reviewed in 1996 by Ruzgas et al. [1]. To improve the properties of these biosensors one needs to establish efficient direct or mediated electron transfer (ET) between the active site of the surface-immobilised peroxidase and the electrode.

The aim of the present paper is to contribute to the understanding of direct ET between the electrode and the heme cofactor of peroxidases. One of the aims has been to study how and how fast the electrons are transferred between the electrode and the enzyme, and find out whether all active enzyme molecules are in direct ET contact with the electrode. Another aim has been to find correlations between the structure of the peroxidase and its ET characteristics, since it might be possible to find or make new peroxidases more suitable for their applications.