Enzymatic Depolymerization of Plant Cell Wall Hemicelluloses

Abstract

Hemicellulose is somewhat loosely defined as the non-cellulose, non-pectin polysaccharide fraction of plant cell walls. In contrast to the insoluble, highly crystalline, homogeneous, and unbranched cellulose polymer, hemicellulose is more often than not highly branched, substituted, and heterogeneous, composed of a wide variety of subunits, including sugars, sugar acids, and non-carbohydrate moieties. This complex structure distinguishes the hemicelluloses from other plant cell wall polysaccharides, rendering them soluble, hygroscopic, and highly cross-linked to a variety of other plant cell wall components. Hemicelluloses are reported to be linked to lignin through cinnamate acid ester linkages, to cellulose through interchain hydrogen bonding, and to other hemicelluloses via covalent and hydrogen bonds. Though pectins have properties similar to hemicellulose, they are distinguished from the hemicelluloses by being more acidic, having longer side chains, more numerous branches, and by their lack of covalent cross-links to other cell wall components. In a general sense, hemicellulose and pectin have similar water retaining functions while differing in their cross-linking abilities. Lignin, the third major component of plant cell walls, is a large, three-dimensional polymer of various phenylpropanoid derivatives. While cellulose provides structural support and hemicelluloses cross-link and retain water, lignin acts as the overall binder, filling in gaps and providing adhesion between the various cell wall components.The structural and compositional heterogeneity of hemicellulose spans across plant families, cell types, and even cell wall subsections. It is well established that different plant families have different ratios of the various hemicelluloses. Herbaceous plants contain mainly xylans, especially arabinoxylan, while hardwood xylans are predominantly glucuronoxylan. Softwoods, in contrast, are dominated by glucomannan, galactomannan, and galacto(gluco)mannan. In addition, other hemicelluloses are present in many plant cell walls, including xyloglucan and ␤-glucans (both 1→3, 1→4 and mixed 1→3, 1→4 linkages). Different hemicelluloses may occur in specific plants or as specific storage polysaccharides. Though often cited in the literature as a result of destructive compositional analysis, arabinan, galactan, and mannan occur relatively infrequently in a linear, unbranched form Biomass Recalcitrance: Deconstructing the Plant Cell Wall for Bioenergy. Edited by Michael. E. Himmel

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