Electron transfer principles in amperometric biosensors: Direct electron transfer between enzymes and electrode surface

The most promising approach for the development of reagentless enzyme electrodes is to establish a direct electrical communication between the enzyme and the electrode surface. We could demonstrate for monolayer-immobilized enzymes catalyzing the reduction of H2O2. (e.g. cytochrome c, microperoxidase MP-11 and horseradish peroxidase) that their catalytic activity in solution is not correlated with their abilities for direct electrochemical communication with the electrode when immobilized at thio-monolayers. In this case, the distance between the active site of the enzyme and the electrode surface is by far more important for electron-transfer processes with high rate constant. To achieve the smallest possible distance it is advantageous to use the biocatalyst with the best access to its active site and hence the smallest molecular weight. Using monolayer-immobilized microperoxidase MP-11 instead of horseradish peroxidase, the current caused by the electrocatalytic reduction of H2O2 could be increased by a factor of about 18 000 compared with the enzymatic activity in solution. Consequently, in this special electrode arrangement allowing as the only electron-transfer pathway the direct electrochemical communication between monolayer-immobilized biocatalyst and electrode surface, the most effective biocatalysts should be the smallest molecule which still shows the envisaged catalytic activity. These structures are called 'minizymes' (minimized enzymes).