Cellobiose dehydrogenase enhances Phanerochaete chrysosporium cellobiohydrolase I activity by relieving product inhibition

The interaction of cellobiose dehydrogenase (CDH) with cellobiohydrolase I (CBH I) in cellulose-grown cultures of Phanerochaete chrysosporium was investigated to clarify the role of CDH in cellulose degradation. Decomposition of bacterial microcrystalline cellulose by CBH I was enhanced significantly in the presence of the CDH/ferricyanide redox-system compared with CBH I alone. To explain this phenomenon, a model system, using p-nitrophenyl-beta-D-cellobioside as a substrate, was elaborated for measurement of CBH I activity with and without the CDH redox-system. The activity of CBH I for hydrolysis of p-nitrophenyl-beta-D-cellobioside was also enhanced in the presence of the redox system. It was found that K-m for hydrolysis of p-nitrophenyl-beta-D-cellobioside by CBH I was lower in the presence than in the absence of the CDH/ferricyanide redox-system, 142 mu M and 383 mu M, respectively. while no significant difference was observed between the k(cat) values. These results indicate that cellulase activity is enhanced by an increased affinity for p-nitrophenyl-beta-D-cellobioside, rather than by an increased hydrolysis rate. This shows that cellobiose, the hydrolysis product, acts as a competitive inhibitor of the interaction between CBH I and p-nitrophenyl-beta-D-cellobioside. This was confirmed by addition of cellobiose, which was found to competitively inhibit hydrolysis of p-nitrophenyl-beta-D-cellobioside by CBH I in the absence of the CDH redox system, and the K-i value for cellobiose inhibition was estimated to be 65 mu M. However, this inhibition did not occur if cellobiose was incubated with CDH before addition of CBH I. It was concluded from these results that the reason for the enhancement of CBH I activity in the presence of the CDH redox system was that it relieves competitive inhibition of cellobiose by its oxidation to cellobionolactone.