Substrate and enzyme characteristics that limit cellulose hydrolysis

The ability and, consequently, the limitations of various microbial enzyme systems to completely hydrolyze the structural polysaccharides of plant cell walls has been the focus of an enormous amount of research over the years. As more and more of these extracellular enzymatic systems are being identified and characterized, clear similarities and differences are being elucidated. Although much has been learned concerning the structures, kinetics, catalytic action, and interactions of enzymes and their substrates, no single mechanism of total lignocellulosic saccharification has been established. The heterogeneous nature of the supramolecular structures of naturally occurring lignocellulosic matrices make it difficult to fully understand the interactions that occur between enzyme complexes and these substrates. However, it is apparent that the efficacy of enzymatic complexes to hydrolyze these substrates is inextricably linked to the innate structural characteristics of the substrate and/or the modifications that occur as saccharification proceeds. This present review is not intended to conclusively answer what factors control polysaccharide biodegradation, but to serve as an overview illustrating some of the potential enzymatic and structural limitations that invariably influence the complete hydrolysis of lignocellulosic polysaccharides.