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Structure of hyperthermophilic endocellulase from Pyrococcus horikoshii

Cellulase is a key enzyme involved in the degradation of b-glucan cellulose biomass acting through hydrolysis of the unbranched b-1,4-linked glucose homopolymer. The major form of cellulose is a tightly packed crystalline structure. A hyperthermophilic cellulase activity toward crystalline cellulose would be useful in industrial applications because a high temperature is required for the degradation of biomass to remove bacterial contamination and to increase substrate solubility. A hyperthermophilic archaeon, Pyrococcus sp., was isolated from a hydrothermal volcanic vent in the ocean.1,2 Hyperthermophilic b-1,4 endocellulases (endotype cellulase) have been identified in the genome sequences of several hyperthermophilic archaea. The first hyperthermophilic endotype cellulase (family 12) was found in Pyrococcus furiosus. However, this enzyme exhibited no activity toward crystalline cellulose.3 We recently isolated a membrane-related unique hyperthermophilic endocellulase from Pyrococcus horikoshii (EGPh: family 5-subfamily 1) that can hydrolyze crystalline cellulose.4,5 Some kinds of hyperthermophilic b-1.4 glucosidases (exotype cellulase) have also been identified in archaea.6–10 The crystal structure of a family 5-subfamily 1 endoglucanase (EGAc) from Acidothermus cellulolyticus has been reported. However, the structure and substrate-recognition mechanism of hyperthermophilic endocellulase (Topt 5 1008C) from archaea are currently unknown. The structure of hyperthermophilic endocellulase from archaea seems to be primitive and fundamental in this class of enzymes. Crystallization of the wild-type EGPh had been unsuccessful, but the truncated mutant of EGPh, lacking five amino-acid residues from the C-terminal end, could make to prepare the crystal sufficient for the structure determination.11 Using this protein crystal, we determined the X-ray crystal structure of EGPh.