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14 декабря, 2021
The requirement for high level cellulase synthesis in order for microbes to grow on cellulose raises the question: how does an anaerobic cellulolytic bacterium generate enough ATP for cellulase synthesis? We determined that C. thermocellum assimilated cellodextrins with a mean degree of polymerization (DP) of about four during growth on cellulose and that these cellodextrins were subsequently cleaved by intracellular phosphorolytic enzymes (44). The process is presented in Figure 16.4.
Soluble cellodextrins (G„) from cellulose hydrolysis were transported across the cell membrane via the ABC transportation system with one ATP expenditure per molecule (45),
Gn + ATP ^ Gn + ADP + Pi
Although the sugar ABC transportation seems bioenergetically costly, it could be very important for (thermophilic) bacteria competing for low concentrations of hydrolysis products. These ABC transportation systems are widely observed in thermophilic or hyperthermophilic microorganisms with very high affinities of sugar
After sugar assimilation, intracellular cellodextrin and cellobiose phosphorylases, rather than p-glucosidase, cleave the (3-(1-4) bonds of cellodextrins via substrate phosphorylation (49),
Gn + Pi + H2O ^ G„-1 + G — 1 — P
This phosphorylation process is important because it conserves energy stored in p-glucosidic bonds to generate one molecule of G-1-P (convertible to ATP) per cleavage and avoids energy dissipation resulting from hydrolysis mediated by p -glucosidase. Our bioenergetic analysis clearly indicated that by assimilating cellodextrins of average DP of 4, C. thermocellum has more ATP available for cell synthesis when growing on cellulose than on cellobiose (44). Hydrolysis products longer than cellobiose are supported by the supramolec — ular structure of the cellulosome and the distance between two adjacent catalytic subunits in the C. thermocellum cellulosome has been estimated to be eight glucosidic bonds (30). Thus, simultaneous catalytic events mediated by adjacent catalytic components would be expected to result in an insoluble G8 fragment, and any subsequent inter-cleavage of this G8 fragment would result in two soluble products with mean chain length 4. Low solubility (50) and/or a tendency to bind to cellulose may well prevent yet longer cellodextrins from being assimilated, even though this would in theory offer bioenergetic benefits. In summary, assimilation of longer cellodextrins means more energy generated from substrate phosphorylation and less energy expenditure for sugar transportation.