Towards new enzymes for biofuels: lessons from chitinase research

Enzymatic conversion of structural polysaccharides in plant biomass is a key issue in the development of second generation ('lignocellulosic') bioethanol. The efficiency of this process depends in part on the ability of enzymes to disrupt crystalline polysaccharides, thus gaining access to single polymer chains. Recently, new insights into how enzymes accomplish this have been obtained from studies on enzymatic conversion of chitin. First, chitinolytic microorganisms were shown to produce non-hydrolytic accessory proteins that increase enzyme efficiency. Second, it was shown that a processive mechanism, which is generally considered favorable because it improves substrate accessibility, might in fact slow down enzymes. These findings suggest new focal points for the development of enzyme technology for depolymerizing recalcitrant polysaccharide biomass. Improving substrate accessibility should be a key issue because this might reduce the need for using processive enzymes, which are intrinsically slow and abundantly present in current commercial enzyme preparations for biomass conversion. Furthermore, carefully selected substrate-disrupting accessory proteins or domains might provide novel tools to improve substrate accessibility and thus contribute to more efficient enzymatic processes.