Properties of cellulolytic enzyme systems.

Cellulolytic enzymes are synthesized by a large number of micro-organisms which include fungi, actinomycetes, gliding bacteria (myxobacteria) and true bacteria. However, only the fungi appear to excrete large amounts of cellulase enzymes in active form into culture media and not surprisingly these are the enzyme systems that have been most extensively studied. Bacterial cellulases are cell-wall bound, in the main. While many cellulases are known to solubilize the amorphous, easily hydrated areas of the cellulose, only a few appear to have the capacity for solubilizing native cellulose to an extent that denotes that the most highly ordered crystalline areas are being degraded. Notable in this regard are the cellulases of the fungi Trichoderma viride (Ogawa & Toyama, 1972), T. reesei (Mandels & Reese, 1964), T. koningii (Halliwell, 1965; Wood, 1968), Fusarium solani (Wood, 1969), Penicillium funiculosum (Wood & McCrae, 1978), Sporotrichum pulverulentum (Eriksson, 1975) and Talaromyces emersonii (Folan & Coughlan, 1979). Of the cellulolytic bacteria studied, only the anaerobe Clostridium thermocellum appears to liberate an extracellular enzyme system that effects the extensive degradation of crystalline cellulose (Johnson et al., 1982). Little is known about the properties of the cellulases of the bacteria as yet, but significant advances have been made recently in the understanding of the enzymes and their modes of operating in the fungi. It is well established, for example, that rapid dissolution of native cellulose requires the independent and cooperative action of a mixture of enzymes. The enzymes present in the mixture have been well characterized, but it is not known yet how they interact and what factors control the interactions on the face of the cellulose crystallite. These uncertainties have resulted in much speculation regarding the mechanism of cellulase action.