CelE, a Multidomain Cellulase from Clostridium cellulolyticum : a Key Enzyme in the Cellulosome?

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Abstract
CelE, one of the three major proteins of the cellulosome of Clostridium cellulolyticum , was characterized. The amino acid sequence of the protein deduced from celE DNA sequence led us to the supposition that CelE is a three-domain protein. Recombinant CelE and a truncated form deleted of the putative cellulose binding domain (CBD) were obtained. Deletion of the CBD induces a total loss of activity. Exhibiting rather low levels of activity on soluble, amorphous, and crystalline celluloses, CelE is more active on p -nitrophenyl–cellobiose than the other cellulases from this organism characterized to date. The main product of its action on Avicel is cellobiose (more than 90% of the soluble sugars released), and its attack on carboxymethyl cellulose is accompanied by a relatively small decrease in viscosity. All of these features suggest that CelE is a cellobiohydrolase which has retained a certain capacity for random attack mode. We measured saccharification of Avicel and bacterial microcrystalline cellulose by associations of CelE with four other cellulases from C. cellulolyticum and found that CelE acts synergistically with all tested enzymes. The positive influence of CelE activity on the activities of other cellulosomal enzymes may explain its relative abundance in the cellulosome.