The efficiency of a crude cell-free cellulase system from C. thermocellum for the biodegrada­tion of crystalline cellulose was first reported by Johnson etal. (140). These authors claimed that the activity of this cellulase system toward cotton was at least 50 times higher than that of the extracellular cellulase system from T. reesei. This level of disparity has since been tempered somewhat, and elevated (~4-fold) levels of cellulose degradation have been estimated in favor of the cell-surface cellulases from C. thermocellum over the free cellulase system of T. reesei (16, 141). It is in fact very hard to assess this difference, since the equili­bration and estimation of equivalent crude or isolated preparations of cellulases from two

different species are difficult in themselves to determine. Nevertheless, such attempts have consistently shown that the C. thermocellum cellulosome is superior in its cellulolysis of recalcitrant cellulosic substrates when compared to the free fungal cellulase systems (Figure 13.8). The principal family-48 processive cellulase is decisive to the observed decomposition of the substrate, since cellulosome preparations that are deficient in this enzyme display re­duced levels of hydrolysis on recalcitrant forms of cellulose (142). In any case, it is important

Figure 13.8 Cellulase activity of the C. thermocellum cellulosome versus those of the free cellulase sys­tems from the fungi, T. reesei (Hypocrea) and Humicola insolens. With one exception, the cells were grown on microcrystalline cellulose (Avicel), and crude preparations of the extracellular enzymes (cellulosome or free cellulases) were produced. The curve (filled circles) denoting C. thermocellum (-Cel48S) represents a cellulosome preparation, derived from cells grown on cellobiose instead of cellulose, under conditions that result in highly reduced quantities of the Cel48S cellobiohydrolase in the cellulosome (27, 53, 142). Avicel was employed as a substrate and subjected to treatment using equivalent amounts of the bacterial cellulosome or fungal cellulase preparations.

to note that although bacterial cellulosomes seem to exhibit enhanced activity compared to that of the fungal enzymes, anaerobic bacteria produce much less cellulolytic enzymes (< 1 g/L) than do the fungi (~ 100 g/L). In view of this imbalance, industry has consistently turned to the economically favorable fungal enzymes, which are thus preferred in all current industrial applications of cellulases.

The cellulolytic potential of the cellulosome on Avicel was in fact demonstrated many years ago (143). For these experiments, cellulosome action was enhanced by inclusion of the Aspergillus niger p-glucosidase, which served to counteract the inhibitory effect of cel­lobiose on the cellulosomal enzymes. The p-glucosidase severs the p-1,4 bond of cellobiose to produce two molecules of the non-inhibitory glucose product. Without this added en­zyme, the course of cellulose hydrolysis by the cellulosome is rapidly impeded. In its pres­ence, however, facile degradation of relatively low concentrations of cellulose in suspension (20 g/L) is achieved to completion in a relatively short time period (Figure 13.9). Com­plete digestion of concentrated cellulose suspensions (200 g/L) are also attained, provided that optimal amounts of cellulosome complex are included in the reactor (Figure 13.9, arrow).