Cellobiose dehydrogenase (CDH) is an extracellular fungal enzyme with two domains, one containing flavin adenine dinucleotide (FAD) and one containing heme, The electrochemistry of CDH, as well. as its cleaved FAD- and heme-subunits, was studied using a membrane electrode, i.e. the enzyme was trapped under a permselective membrane on a cystamine or 3-mercaptopropionic acid modified gold electrode. Direct un-mediated electron transfer (ET) between the heme of CDH and thiol modified gold electrodes was demonstrated using cyclic voltammetry. At low sweep rate (10 mV s(-1)) and low pH (pH 4.3) up-hill ET from heme to FAD in CDH was observed. The formal potential of the heme in CDH and in the cleaved heme-subunit was found to be the same and equal to -41 mV versus Ag \ AgCl at pH 5.1. The dependence of the formal potential on the pH (in the pH range 3.6-6.0) indicates the presence of one redox-linked ionisable functional group. Entropy and enthalpy changes were determined in variable temperature experiments as follows, DeltaS(o') = -194 +/- 14 J mol(-1) K-1 and DeltaH(o') = -74 +/- 6 kJ mol(-1). The electrocatalytic behaviour of the CDH electrodes was demonstrated by addition of the enzyme substrate, cellobiose. The catalytic current was shown to decrease upon increased pH, in accordance with previous kinetic data in solution. The model of electron transport from the substrate (cellobiose) to FAD, and then through the heme domain to the electrode was confirmed in the experiments. (C) 2000 Elsevier Science B.V. All rights reserved.