Cellulolytic Activities of Clostridium thermocellum and Its Carbohydrate Metabolism
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概要
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The degradation of cellulose by an anaerobic and thermophilic bacterium, Clostridium thermocellum ATCC 27405,was studied. The cellulolytic enzymes and β-glucosidase have been found to be constitutive enzymes in C. thermocellum, and this organism is capable of degrading cellulose and accumulate glucose and reducing sugars. The ability of this organism to grow on glucose depended on yeast extract concentration and growth factors. Glucose was utillized only in the medium containing 0.5-0.7% yeast extract. The important growth factors found, in the order of their effect as measured by the cell growth in the glucose medium, were : biotin>thiamine・HCl>pyridoxal・HCl>pantothenic acid. When it was cultivated in the glucose medium containing 0.5% yeast extract, cell growth was observed after a lag period of 30-35 h, and the specific growth rate was about 0.16h^<-1>. The cellulolytic activity was expressed in the glucose medium. When this glucose-adapted culture was again transferred to cellulose medium, it retained the ability to utilize cellulose. When it was again transferred to glucose medium containing 2 g yeast extract per liter, it retained the ability to utilize glucose. When both glucose and cellobise as carbon source were present in the medium containing 0.5% yeast extract, the original culture utilizes cellobiose preferentially without a prolonged lag, but the glucose-adapted culture assimilated glucose preferenitally without an extended lag. It has been found that C. thermocellum was capable of converting cellulose directly to ethanol and other chemicals including sugars. The specific rate of this organism on Solka Floc was about 0.05h^<-1> and the final concentration of the accumulated reducing sugars was 3.4 g/l representing about 58% yield based on the degraded cellulose(5.84 g/l). In simple system where C. thermocellum was cultivated on cellobiose, it was utillized and glucose was accumulated during active growth phase, the molar ratio of the accumulated glucose to the degraded cellobiose(the glucose yield ratio) ranged between 0.16-0.59. But once the cell growth has ceased the glucose yield ratio ranged between 1.5-1.8. The cellulose degradation by the glucose-adapted culture and the orginal culture was compared. In case of the glucose-adapted culture, the specific growth rate of this culture on Solka Floc was about 0.16h^<-1> and 55% of cellulose was hydrolyzed. The residual reducing sugar was only 16% of the hydrolyzed xellulose(5.2 g/l). When theglucose-adapted culture was compared with the original culture, the specific growth rate was about three times greater, the amount of reducing sugar accumulated was only about 1/4,and the reducing sugar yield from cellulose was only about 1/4. But the amount of cellulose degraded, and ethanol yield were about the same for both cultures.
- 公益社団法人日本生物工学会の論文
- 1983-12-25