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 cel l 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 ident ified and characterized, clear similarities and differences are being eluci dated. Although much has been learned concerning the structures, kinetics, catalytic action, and interactions of enzymes and their substrates, no sing le mechanism of total lignocellulosic saccharification has been established . The heterogeneous nature of the supramolecular structures of naturally oc curring lignocellulosic matrices make it difficult to fully understand the interactions that occur between enzyme complexes and these substrates. Howe ver, it is apparent that the efficacy of enzymatic complexes to hydrolyze t hese substrates is inextricably linked to the innate structural characteris tics of the substrate and/or the modifications that occur as saccharificati on proceeds. This present review is not intended to conclusively answer wha t factors control polysaccharide biodegradation, but to serve as an overvie w illustrating some of the potential enzymatic and structural limitations t hat invariably influence the complete hydrolysis of lignocellulosic polysac charides.