Cellobiose dehydrogenase (CDH) is an extracellular enzyme produced by many white rot fungi (Henriksson et al. 2000). It oxidizes soluble cellodextrins, mannodextrins, and lactose efficiently to their corresponding lactones by a ping-pong mechanism using a wide spectrum of electron acceptors including quinones, phenoxy radicals, Fe3+, Cu2+, and triodide ion. The function of CDH is not obvious, but P. chrysosporium produces relatively high levels of the enzyme, approximately 0.5% of the secreted protein on a mass basis, suggesting it plays an important role. Many functions have been suggested for CDH, some of which are related to cellulose degradation, but in the current context only those that are relevant to lignin biodegradation will be discussed. It has been suggested that CDH reduces aromatic radicals formed by ligninolytic enzymes, thereby preventing their repolymerization and supporting lignin degradation (Ander et al. 1990; Temp and Eggert 1999). In addition, it has been shown that CDH can generate hydroxyl radicals (OH*) in a fenton type reaction, which can modify cellulose, hemicellulose, and lignin (Henriksson et al. 1995; Wood
1994). The modification of lignin by OH* radicals produced by CDH may result in hydroxylation of nonphenolic structures to phenolic ones, facilitating MnP and LAC action and suggesting that the enzymes may form a pathway in lignin biodegradation (Hilden et al. 2000). It has also been suggested that CDH may support MnP by reducing precipitated MnO2, which is common in rotten wood to Mn2+ or Mn3+. It might also aid MnP by producing cellobionic acid, which should complex Mn3+ (Roy et al. 1994).
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