Biosynthesis of cellulose

The preceding, brief discussion about the complex structure of the cell walls and the fine features of the cellulosic microfibrils clearly indicates that the formation and functioning of this system must involve not only the chemical synthesis of the raw materials, but also other fundamental processes, such as the packing of the cellulose molecules in the microfibrils and the orientation of the microfibrils within the cell walls. These complex processes are so closely related in biological processes that a discussion of one aspect without consideration of others would be rather meaningless. However, for the sake of convenience and simplicity, the biosynthesis of cellulose, or the process of polymerization of the D-glucose residues, will be considered first, irrespective of the physical structure of the final product. 

Early experiments on the biosynthesis of cellulose mainly involved 

The mechanism of the biosynthesis of polysaccharides has been reviewed in authoritative articles by Hassid and coworkers in this Volume and an earlier Volume of this Series. The present discussion will include only the biosynthesis of cellulose, because of its significance in the composition of plant cell-walls and because of the complexities that have been revealed by recent investigations. 

Ordin and found that particulate preparations from oat 

Villemez and coworkers have also reported that an enzyme preparation from mung beans could produce an alkali-insoluble D-glucan from UDP-D-glucose that contains both )8-D-(l — 3)- and )8-D-(l — 4)-links. However, it was not determined whether the product was a mixture of cellulose and callose or a single polysaccharide having mixed linkages. 

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Figure 2.7 Biosynthesis of cellulose from D-glucopyranose in land plants.

The biosynthesis of cellulose is less well understood than that of glycogen or starch. As a major component of the plant cell wall, cellulose must be synthesized from intracellular precursors but deposited and assembled outside the plasma membrane. The enzymatic machinery for initiation, elongation, and export of cellulose chains is more complicated than that needed to syn-... 

On the basis of these findings, Pont Lezica and co workers341 proposed a scheme, involving glucolipids and a glucoprotein as intermediates, for the biosynthesis of cellulose in Prototheca zopfii this is shown in Scheme 2. 

Comment on unresolved questions about the biosynthesis of cellulose, amylose, and amylopec-tin. What glycosyl carrier groups are required ... 

Scheme 2.—Proposed Pathway for the Biosynthesis of Cellulose in Algae.82...

Tt is a widely recognized fact that true cellulolytic microorganisms A produce three basic cellulase components IS), and that these enzyme components act in concert to hydrolyze crystalline cellulose to glucose (6). Many research laboratories have undertaken the task to purify cellulose components from various cellulolytic microorganisms and to study the mechanisms of cellulose hydrolysis. Much information has accumulated concerning the mode of action of cellulose hydrolysis since Reese et al. first proposed the Ci-C concept (7). In spite of this, however, conflicting reports still flourish concerning the composition of the "cellulase complex, the multiplicity of cellulase components, the biosynthesis of cellulose, and the mechanisms of cellulose hydrolysis. 

The addition of water-soluble compounds affects the biosynthesis of cellulose chains (polymer formation), and they may also be adsorbed by single fibers as the nanofiber network is built (crystallization). Moreover, the bacterial cells themselves can be modified. 

The topic of the biosynthesis of cellulose was discussed by Shafiza-deh and McGinnis1 in this Series about a decade ago, and since then, several reviews have appeared in the literature.2-7 One theme com-... 

Such a situation, although frustrating, is also fascinating. Are we supplying the correct substrates in vitro If so, what is the nature of the extraordinary lability of the cellulose-synthesizing system What essential features of the intact system are lost when cells are broken, or even just rendered permeable to substrates The key to unraveling the complexities of the biosynthesis of cellulose will only come when full answers to such questions have been obtained. 

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