19448: Contribution of accessory enzymes to the degradation of recalcitrant polysaccharides by filamentous fungi

Enzymatic hydrolysis of lignocellulose by cellulases is a slow and expensive process which must be improved for the economical production of cellulosic biofuels.  In addition to cellulases, many fungi secrete cellobiose dehydrogenases during growth on lignocellulose.  Here we report on the deletion of cdh-1, the gene encoding the major cellobiose dehydrogenase (CDH) of Neurospora crassa.  Deletion of cdh-1 reduced cellulase activity in the culture filtrate of the Dcdh-1 strain substantially compared to the wild-type strain.  Addition of purified CDHs to the Dcdh-1 strain culture filtrate restored cellulase activity to wild-type levels, while addition of CDH to a mixture of purified cellulases showed no stimulatory effect.  Domain truncations of a full length CDH showed that only one domain was required for the stimulatory effect.  The stimulatory effect of CDH requires the presence of molecular oxygen and other metalloproteins.  These metalloproteins were identified as members of the enigmatic glycosyl hydrolase family 61.  The discovery that cellobiose dehydrogenase plays an integral role in degradation of pure cellulose in conjunction with GH61 family proteins is a significant shift from previous models centered on the degradation of cellulose by mixtures of canonical glycosyl hydrolases.