RECENT ADVANCES IN FUNGAL CELLOBIOSE OXIDOREDUCTASES

When grown on cellulose, the white-rot fungus Phanerochaete chrysospor ium (Sporotrichum pulverulentum), produces two cellobiose oxidoreducta ses, i.e., cellobiose:quinone oxidoreductase (CBQ) and cellobiose oxid ase (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-r ot fungi, or in any fungi for that matter, is still not known. The ori ginal 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 (perox idases and laccases) attack lignin and lignin degradation products. Ho wever, evidence obtained so far indicates that the presence of CBO/CBQ with lignin peroxidases and laccases actually reduces the rate of oxi dation of lignin degradation products. CBQ has a molecular mass of abo ut 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 tha t 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 dif ferences in their spectral properties, but also by the ability of CBO, but not CBQ, to reduce cytochrome c. Both CBO and CBQ have a cellulos e-binding domain (CBD), as do a large number of endoglucanases and cel lobiohydrolases. The induction-repression patterns regulating cellobio se oxidoreductase genes are not known in any detail. Most reports poin t to induction during cellulose degradation, but repression has not be en studied. Induction has also been suggested to occur by addition of lignosulfonate to the medium.