Substrate and Enzyme Characteristics that Limit Cellulose Hydrolysis

Abstract

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. IntroductionDespite the extensive amount of research undertaken over the last few decades, our understanding of how enzymes completely hydrolyze lignocellulosic substrates is still far from comprehensive or complete. Past work has indicated the complexity of the substrate and the need for many different enzymes before these substrates can be effectively and completely hydrolyzed. However, the deceptive simplicity of the repeating -1-4 linking cellobiose unit is not indicative of the complex arrangement of the substrate at the fibril, fiber, and wood/pulp levels. Similarly, although tools such as molecular biology and protein engineering have helped elucidate the role of some of the enzymes in the synergistic attack of lignocellulosic substrates, our understanding of basic mechanisms such as enzyme regulation, kinetics, and the extent of true synergism is still lacking. As a result, an important aspect of much of the research to date has been to ascertain the limiting factors involved in the decreased hydrolysis rate as time progresses. These factors have traditionally been divided into two groups: those which relate to the structure of the substrate and those related to the mechanisms and interactions of the cellulase enzymes.Various model cellulose substrates have been used for the purpose of studying the mechanism of action and interaction of individual cellulase enzymes and the effect of substrate characteristics, such as degree of polymerization and crystallinity, on the rate and efficiency of enzymatic hydrolysis. These include Avicel, solka floc, filter paper, cotton, valonia cellulose, phosphoric acid swollen cellulose, bacterial microcrystalline cellulose (BMCC) and solu...

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