Biomass Conversion Methods and Protocols
Biomass conversion research is a combination of basic science, applied science, and engineering testing and analysis. Conversion science includes the initial treatment (called pre-treatment) of the feedstock to render it more amenable to enzyme acti
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1. Introduction Lignins are the most recalcitrant of the biopolymers in vascular plant cell walls. They vary quite widely in cell-wall content (15– 35 % w/w) and macromolecular configuration (i.e., monomer-unit composition and substructure frequency). They are assembled through the dehydrogenative coupling of p-coumaryl alcohol and its 3-methoxy and 3,5-dimethoxy derivatives (coniferyl and sinapyl alcohol) in varying proportions. By these means, coumaryl, guaiacyl, and syringyl units are respectively linked together through six or more different substructures in native lignin macromolecules.
Michael E. Himmel (ed.), Biomass Conversion: Methods and Protocols, Methods in Molecular Biology, vol. 908, DOI 10.1007/978-1-61779-956-3_21, © Springer Science+Business Media, LLC 2012
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Softwoods (coniferous gymnosperms) possess lignins composed primarily of guaiacyl monomer residues, while hardwoods (deciduous angiosperms) contain lignins with guaiacyl and syringyl units in similar proportions. On the other hand, the lignins of grasses (monocots) are composed of monomer residues formed in varying ratios from all three monolignols (1). Generally speaking, lignin composition and structure can change profoundly during the course of plant growth and development (2, 3). Yet it is possible, by simple physicochemical means alone, to separate the lignins in plant cell walls into different welldefined fractions with contrasting macromolecular configurations (4). Moreover, lignins are covalently linked through about 3 % of their monomer units to hemicellulose chains (5); there is a relationship between the constituent monosaccharide residues and the substructures in the lignin macromolecules to which they are bound (Fig. 1). 1.1. Lignin-Degrading Enzyme Assays
As far as the efficacies of candidate-enzymes for lignin degradation are concerned, there is no consensus about the polymeric lignin substrate that should be used in depolymerase assays. The difficulty appeared already in 1983, when the activity of a putative lignindegrading enzyme was first detected in extracellular culture solutions of the white-rot fungus, Phanerochaete chrysosporium (7, 8). There were two problems with that inaugural study. The methylated aqueous acetone spruce wood extract employed as a substrate was not actually a polymeric lignin preparation. Then, just 3 years later, lignin peroxidase, the enzyme conspicuously present in the extracellular P. chrysosporium culture solution, was found to polymerize alkali-isolated straw lignin and spruce milled-wood lignin (9) in the presence of H2O2 at pH 4.0 (Fig. 2). Such apparent contradictions have confounded the field of lignin biodegradation for more than two decades. The types of enzymes given most attention as plausible agents of lignin degradation are indeed poised between cleavage and polymerization in their effects on the macromolecular substrate. For example, lignin peroxidase has a sufficiently high redox potential to oxidize nonphenolic aromatic rings of the veratryl type in li
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