Humicola insolens cellobiose dehydrogenase:: cloning, redox chemistry, and logic gate-like dual functionality

Cellobiose dehydrogenase is a hemoflavoenzyme that catalyzes the sequential electron-transfer from an electron-donating substrate (e.g. cellobiose) to a flavin center, then to an electron-accepting substrate (e.g. quinone) either directly or via a heme center after an internal electron-transfer from the flavin to heme. We cloned the dehydrogenase from Humicola insolens, which encodes a protein of 761 amino acid residues containing an N-terminal heme domain and a C-terminal Ravin domain, and studied how the catalyzed electron transfers are regulated. Based on the correlation between the rate and redox potential, we demonstrated that with a reduced flavin center, the enzyme, as a reductase, could export electron from its heme center by a outer-sphere mechanism. With the resting flavin center, however, the enzyme could have a peroxidase-like function and import electron to its heme center after a peroxidative activation. The dual functionality of its heme center makes the enzyme a molecular logic gate, in which the electron how through the heme center can be switched in direction by the redox state of the coupled flavin center. (C) 2001 Elsevier Science Inc. All rights reserved.