A Trimodular Family 16 Glycoside Hydrolase From The Cellulosome Of Ruminococcus Flavefaciens Displays Highly Specific Licheninase (Ec 3.2.1.73) Activity

Cellulosomes are highly complex cell-bound multi-enzymatic nanomachines used by anaerobes to break down plant carbohydrates. The genome sequence of Ruminococcus flavefaciens revealed a remarkably diverse cellulosome composed of more than 200 cellulosomal enzymes. Here we provide a detailed biochemical characterization of a highly elaborate R. flavefaciens cellulosomal enzyme containing an N-terminal dockerin module, which anchors the enzyme into the multi-enzyme complex through binding of cohesins located in non-catalytic cell-bound scaffoldins, and three tandemly repeated family 16 glycoside hydrolase (GH16) catalytic domains. The DNA sequence encoding the three homologous catalytic domains was cloned and hyper-expressed in Escherichia coli BL21 (DE3) cells. SDS-PAGE analysis of purified His6 tag containing RfGH16_21 showed a single soluble protein of molecular size similar to 89 kDa, which was in agreement with the theoretical size, 89.3 kDa. The enzyme RfGH16_21 exhibited activity over a wide pH range (pH 5.0-8.0) and a broad temperature range (50-70 degrees C), displaying maximum activity at an optimum pH of 7.0 and optimum temperature of 55 degrees C. Substrate specificity analysis of RfGH16_21 revealed maximum activity against barley beta-d-glucan (257 U mg-1) followed by lichenan (247 U mg-1), but did not show significant activity towards other tested polysaccharides, suggesting that it is specifically a beta-1,3-1,4-endoglucanase. TLC analysis revealed that RfGH16_21 hydrolyses barley beta-d-glucan to cellotriose, cellotetraose and a higher degree of polymerization of gluco-oligosaccharides indicating an endo-acting catalytic mechanism. This study revealed a fairly high, active and thermostable bacterial endo-glucanase which may find considerable biotechnological potentials.