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) ei ther directly or via a heme center after an internal electron-transfer from the flavin to heme. We cloned the dehydrogenase from Humicola insolens, wh ich encodes a protein of 761 amino acid residues containing an N-terminal h eme domain and a C-terminal Ravin domain, and studied how the catalyzed ele ctron transfers are regulated. Based on the correlation between the rate an d redox potential, we demonstrated that with a reduced flavin center, the e nzyme, as a reductase, could export electron from its heme center by a "out er-sphere" mechanism. With the "resting" flavin center, however, the enzyme could have a peroxidase-like function and import electron to its heme cent er after a peroxidative activation. The dual functionality of its heme cent er makes the enzyme a molecular "logic gate", in which the electron how thr ough 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.