Abstract
Cellulases are the key enzymes used in the biofuel industry. A typical cellulase contains a catalytic domain connected to a carbohydrate-binding module (CBM) through a flexible linker. Here we report the structure of an atypical trimodular cellulase which harbors a catalytic domain, a CBM46 domain and a rigid CBM_X domain between them. The catalytic domain shows the features of GH5 family, while the CBM46 domain has a sandwich-like structure. The catalytic domain and the CBM46 domain form an extended substrate binding cleft, within which several tryptophan residues are well exposed. Mutagenesis assays indicate that these residues are essential for the enzymatic activities. Gel affinity electrophoresis shows that these tryptophan residues are involved in the polysaccharide substrate binding. Also, electrostatic potential analysis indicates that almost the entire solvent accessible surface of CelB is negatively charged, which is consistent with the halophilic nature of this enzyme.
Highlights
With the increasing energy cost, dwindling oil fuel reserve and ever-worsening problem of pollution, the search for a replacement for the fossil fuels has become an urgent task
The overall structure of CelB consists of a typical (β/α)8 TIM barrel catalytic domain, a CBM_X domain and a CBM46 domain (Fig 1)
In many cellulases, extended linker sequences are found between the catalytic domains and the carbohydrate-binding module (CBM)[40, 51]
Summary
With the increasing energy cost, dwindling oil fuel reserve and ever-worsening problem of pollution, the search for a replacement for the fossil fuels has become an urgent task. Due to the abundant lignocellulosic substance in the biosphere, the production of biofuel with cellulose has emerged as a promising solution[1, 2]. The building blocks of cellulose are glucose molecules, which is a good raw material for fermentation. The cellulose consists of straight chain of glucose polymers. These polymers form rod-like structures which are strengthened by the multiple hydrogen-bonds between or within the polymers. In the cell wall of plant, PLOS ONE | DOI:10.1371/journal.pone.0142107. In the cell wall of plant, PLOS ONE | DOI:10.1371/journal.pone.0142107 November 12, 2015
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