Biochemical and structural characterisation of a family GH5 cellulase from endosymbiont of shipworm P. megotara

Cellulases play a key role in enzymatic conversion of plant cell-wall polysaccharides into simple and economically relevant sugars. The discovery of novel cellulases from exotic biological niches is of interest as they may present properties that are valuable in biorefining of lignocellulose. We have characterized a glycoside hydrolase 5 (GH5) domain of a bi-catalytic GH5-GH6 multidomain enzyme from the unusual bacterial endosymbiont Teredinibacter waterburyi of the wood-digesting shipworm Psiloteredo megotara. The cellulase enzyme, TwCel5, was produced with and without a native C-terminal family 10 carbohydrate-binding module belongs to GH5, subfamily 2. Both variants showed hydrolytic endo-activity on soluble substrates such as, {beta}-glucan, carboxymethylcellulose and konjac glucomannan. However, low activity was observed towards a crystalline form of cellulose. Interestingly, when co-incubated with a cellulose active LPMO, a clear synergy was observed that boosted hydrolysis of crystalline cellulose. The crystal structure of the GH5 catalytic domain was solved to 1.0 [A] resolution and revealed a substrate binding cleft containing a putative +3 subsite, which is uncommon in this enzyme family. The enzyme TwCel5 was active in a wide range of pH and temperatures and showed high tolerance for NaCl. This study provides an important advance on discovery new enzymes from shipworm and shed new light on biochemical and structural characterization of cellulolytic cellulase and showed boost in hydrolytic activity of cellulase on crystalline cellulose when co-incubated with cellulose active LPMO. These findings will be relevant for the development of future enzyme cocktail that may be useful for the biotechnological conversion of lignocellulose.