RECENT ADVANCES IN FUNGAL CELLOBIOSE OXIDOREDUCTASES

When grown on cellulose, the white-rot fungus Phanerochaete chrysosporium (Sporotrichum pulverulentum), produces two cellobiose oxidoreductases, i.e., cellobiose:quinone oxidoreductase (CBQ) and cellobiose oxidase (CBO). Similar cellobiose-oxidizing enzymes, capable of utilizing a wide variety of electron acceptors, have been detected in many other fungi. However, the role of the cellobiose oxidoreductases in white-rot fungi, or in any fungi for that matter, is still not known. The original role ascribed to CBQ was as a link between cellulose and lignin degradation. CBQ has been shown to reduce quinones and phenoxyradicals released during lignin degradation concomitantly oxidizing cellobiose and other cellodextrins released during cellulose degradation. Thus, one function proposed for the cellobiose oxidoreductases is to prevent repolymerization of phenoxyradicals formed when phenoloxidases (peroxidases and laccases) attack lignin and lignin degradation products. However, evidence obtained so far indicates that the presence of CBO/CBQ with lignin peroxidases and laccases actually reduces the rate of oxidation of lignin degradation products. CBQ has a molecular mass of about 60 kD and contains an FAD cofactor. CBO contains both heme and FAD, and has a mass of about 90 kD. It has recently been demonstrated that CBO can be proteolytically cleaved into FAD and heme domains. The FAD domain of CBO seems to have all the properties of CBQ, suggesting that CBQ is a cleavage product of CBO. Whether CBO is a precursor of CBQ is not yet known. CBO and CBQ can be distinguished not only by the differences in their spectral properties, but also by the ability of CBO, but not CBQ, to reduce cytochrome c. Both CBO and CBQ have a cellulose-binding domain (CBD), as do a large number of endoglucanases and cellobiohydrolases. The induction-repression patterns regulating cellobiose oxidoreductase genes are not known in any detail. Most reports point to induction during cellulose degradation, but repression has not been studied. Induction has also been suggested to occur by addition of lignosulfonate to the medium.