A critical review of cellobiose dehydrogenases

Cellobiose dehydrogenase (CDH) is an extracellular enzyme produced by vario us wood-degrading fungi. It oxidizes soluble cellodextrins, mannodextrins a nd lactose efficiently to their corresponding lactones by a ping-pong mecha nism using a wide spectrum of electron accepters including quinones, phenox yradicals, Fe3+, Cu2+ and tiiodide ion. Monosaccharides, maltose and molecu lar oxygen are:poop substrates. CDH that adsorbs strongly and specifically to cellulose carries two prosthetic groups; namely, an FAD and a heme in tw o different domains that can be separated after limited proteolysis. The FA D-containing fragment carries all known catalytic and cellulose binding pro perties. One-electron accepters, like ferricyanide, cytochrome c and phenox y radicals, are, however, reduced more slowly by the FAD-fragment than by t he intact enzyme, suggesting that the function of the heme group is to faci litate one-electron transfer. Non-heme forms of CDH have been found in the culture filtrate of some fungi (probably due to the action of fungal protea ses) and were for a long time believed to represent a separate enzyme (cell obiose:quinone oxidoreductase, CBQ). The amino acid sequence of CDH has bee n determined and no significant homology with other proteins was detected f or the heme domain. The FAD-domain sequence belongs to the GMC oxidoreducta se family that includes, among others, Aspergillus niger glucose oxidase. T he homology is most distinct in regions that correspond to the FAD-binding domain in glucose oxidase. A cellulose-binding domain of the fungal type is present in CDH from Myceliophtore thermophila (Sporotrichum thermophile), but in others an internal sequence rich in aromatic amino acid residues has been suggested to be responsible for the cellulose binding. The biological function of CDH is not fully understood, but recent results support a hydr oxyl radical-generating mechanism whereby the radical can degrade and modif y cellulose, hemicellulose and lignin. CDH has found technical use in highl y selective amperometric biosensors and several other applications have bee n suggested. (C) 2000 Elsevier Science B.V. All rights reserved.