The cellobiose-oxidizing enzymes CBQ and CbO as related to lignin and cellulose degradation — a review

Abstract: In this review properties of cellobiose:quinone oxidoreductase (CBQ) and cellobiose oxidase (CbO) are presented and their possible involvement in lignin and cellulose degradation is discussed. Although these enzymes are produced by many different fungi, their importance for wood-degrading fungi is the topic here. CBQ is a FAD enzyme, while CbO also contains a heine group of the cytochrome b type. Protease activity is reported to convert CbO to CBQ. During oxidation of cellobiose (emanating from cellulose) to cellobiono-l,5-lactone, both enzymes reduce quinones produced by laccase and peroxidase during lignin degradation to the corresponding phenols. Many phenoxy and cation radicals are also reduced. Quinone reduction is more rapid than oxygen reduction, although oxygen is slowly reduced to superoxide and/or hydrogen peroxide. Thus, a more appropriate name for CbO is cellobiose dehydrogenase. CbO also reduces Fe(III) and together with hydrogen peroxide produced by the enzyme Fenton's reagent may be formed, resulting in hydroxyl radical production. This radical can degrade both lignin and cellulose, possibly indicating that cellobiose oxidase has a central role in degradation of wood by wood-degrading fungi.