Structural Considerations Relevant to Biosynthesis

No critical discussion of biosynthesis can be attempted without consideration of the information available on the structure of native cellulose. As for studies on biosynthesis, much controversy has existed in the field of structural analyses of cellulose. There seem to be only two points on which all workers are in agreement, namely, that (a) native cellulose is a composite of linearly extended chains of (l- 4)-j6-D-glu-can (1), and (b) strong intrachain and interchain hydrogen-bonding between D-glucosyl residues occurs in such a way as to create a highly insoluble, and partially crystalline, fibrillar structure. 

Another problem for biosynthesis concerns the fact that certain other, native plant-polysaccharides, most notably the xyloglucans found in higher-plant cell-walls,6 contain backbone structures of (1— 4)-/3-D-glucan, and it is necessary to consider whether separate enzymes synthesize these structures, and, if so, whether these enzymes could be confused with enzymes specifically involved in the synthesis of cellulose. 

Another structural consideration relevant to biosynthesis is the fact that, within a chain, each D-glucosyl residue is rotated approximately 180° with respect to its neighbor, as depicted in 1. Thus, cellobiose may actually be considered to be the true repeating-unit within the chain, and it is necessary to consider whether some activated form of the disaccharide (rather than of D-glucose) might be the donor for chain elongation this point will be considered again in Section V,2. 

Finally, the question of the average degree of polymerization (d.p.) of the gluean chains must be considered. Determinations of the d.p. of polysaccharides are usually made by viscosity measurements, but native cellulose, being highly insoluble and often in close association with other cell-wall polysaccharides and lignin, presents special prob- 

Acetobacter xylinum51,52 Gossypum hirsutum (cotton fiber)53 2,000-3,700 5,700 

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