Search

Direct and mediated electron transfer (DET and MET) between the enzyme and electrodes were compared for cellobiose dehydrogenase (CDH) from the basidiomycete Phanerochaete sordida (PsCDH). For DET, PsCDH was adsorbed at pyrolytic graphite (PG) electrodes while for MET the enzyme was covalently linked to a low potential Os redox polymer. Both types of electrodes were prepared in the presence of single walled carbon nanotubes (SWCNTs). DET requires the oxidation of the heme domain, while MET occurs partially via the heme and the flavin domain at pH 3.5. At pH 6 MET occurs solely via the flavin domain. Most probably, the interaction of the domains decreases from pH 3.5 to 6.0 due to electrostatic repulsion of deprotonated amino acid residues, covering the surfaces of both domains. MET starts at a lower potential than DET. The midpoint potentials at pH 3.5 for the flavin (40 mV) and the heme domain (170 mV) were determined with spectroelectrochemistry. The electrochemical and spectroelectrochemical measurements presented in this work are in conformity. The pH dependency of DET and MET was investigated for PsCDH. The optimum was observed between pH 4 and 4.5 pH for DET and in the range of pH 5-6 for MET. The current densities obtained by MET are 1 order of magnitude higher than by DET. During multicycle cyclic voltammetry experiments carried out at different pHs, the PsCDH modified electrode working by MET turned out to be very stable. In order to characterize a PsCDH modified anode working by MET with respect to biofuel cell applications, this electrode was combined with a Pt-black cathode as model for a membraneless biofuel cell. In comparison to DET, a 10 times higher maximum current and maximum power density in a biofuel cell application could be achieved by MET. While CDH modified electrodes working by DET are highly qualified for applications in amperometric biosensors, a much better performance as biofuel cell anodes can be obtained by MET. The use of CDH modified electrodes working by MET for biofuel cell applications results in a less positive onset of the electrocatalytic current (which may lead to an increased cell voltage), higher current and power density, and much better long-term stability over a broad range of pH.

Cellobiose dehydrogenase (CDH) is capable of direct electrontransfer(DET) on various carbon and thiol-modified gold electrodes. As a result,these systems have been utilized as biocatalyst in biosensors andbiofuel cell anodes. Class I CDHs, from basidiomycetous fungi, arehighly specific to cellulose or lactose, and DET is only observedat pH values below 5.5. To extend the applicability of CDH-based electrodes,the catalytic properties and the behavior on electrode surfaces ofascomycetous class II CDHs from Chaetomium attrobrunneum,Corynascus thermophilus, Dichomera saubinetii, Hypoxylon haematostroma,Neurospora crassa, and Stachybotrys bisbyiwere investigated. We found that class II CDHs have diverse propertiesbut generally show a lower substrate specificity than class I CDHsby converting also glucose and maltose. Intramolecular electron transfer(IET) and DET at neutral and alkaline pH were observed and elucidatedby steady-state kinetics, pre-steady-state kinetics, and electrochemicalmeasurements. The CDHs ability to interact with the electron acceptorcytochrome cand to communicate with electrode surfacesthrough DET at various pH conditions was used to classify the investigatedenzymes. In combination with stopped-flow measurements, a model forthe kinetics of the pH-dependent IET is developed. The efficient glucoseturnover at neutral/alkaline pH makes some of these new CDHs potentialcandidates for glucose biosensors and biofuel cell anodes. [ABSTRACT FROM AUTHOR]