Enhancement of enzymatic activity and catalytic current of cellobiose dehydrogenase by calcium ions

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Cellobiose dehydrogenase (CDH) has recently become a redox enzyme at focus in bioelectrochemistry especially for the construction of sugar biosensors and biofuel cell anodes. The present study shows that an increase in the CaCl2 concentration to up to 100mM led to an increase in the maximal catalytic current generated by two different Ascomycete and one Basidiomycete CDH immobilised on a spectroscopic graphite electrode. For the Ascomycete Myriococcum thermophilum CDH the catalytic current was increased 5.1 fold, whereas Ascomycete Humicola insolens CDH showed a four-fold increase and Basidiomycete Phanerochaete chrysosporium CDH showed an increase by a factor of 2.4. On the other hand, the addition of a monovalent cation salt, KCl (up to 100mM), to the buffers increased the catalytic currents only up to 2-fold for Myriococcum thermophilum CDH. Activity assays in solution with cyt c accepting solely the electrons from the CYTCDH domain also revealed an increased activity in the presence of CaCl2. Experiments with the isolated DHCDH domain from Humicola insolens have shown that the catalytic turnover is totally independent on the addition of KCl or CaCl2 to the solution. The results indicate a positive effect of metal cations, particularly Ca2+, on the electron transfer between the DHCDH and the CYTCDH domains or between the CYTCDH domain and the final electron acceptor, whereas the first hypothesis is favoured. These findings are of interest both for the construction of 3rd generation biosensors and biofuel cell anodes, and also for a deeper understanding of the electron transfer mechanism in CDH.