['Mutation of Trp-40 in the Cel7A cellobiohydrolase from Trichoderma reesei (TrCel7A) causes a loss of crystalline cellulose-degrading ability.', 'Mutant W40A showed reduced specific activity for crystalline cellulose and diffused the cellulose chain from the entrance of the active site tunnel.'] Is the tryptophan residue at the active site tunnel entrance of Trichoderma reesei cellobiohydrolase Cel7A important for initiation of degradation of crystalline cellulose?