Two immunologically unrelated cellobiohydrolases (I and II), isolated from the extracellular cellulase system elaborated by the fungus Penicillum pinophilum, acted in synergism to solubilize the microcrystalline cellulose Avicel; the ratio of the two enzymes for maximum rate of attack was approx. 1:1. A hypothesis to explain the phenomenon of synergism between two endwise-acting cellobiohydrolases is presented. It is suggested that the cellobiohydrolases may be two stereospecific enzymes concerned with the hydrolysis of the two different configurations of non-reducing end groups that would exist in cellulose. Only one type of cellobiohydrolase has been isolated so far from the cellulases of the fungi Fusarium solani and Trichoderma koningii. Only cellobiohydrolase II of P. pinophilum acted synergistically with the cellobiohydrolase of the fungi T. koningii or F. solani to solubilize Avicel. Cellobiohydrolase II showed no capacity for co-operating with the endo-1,4-beta-glucanase of T. koningii or F. solani to solubilize crystalline cellulose, but cellobiohydrolase I did. These results are discussed in the context of the hypothesis presented.

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