SYNTHESIS OF CELLULOSE BY ACETOBACTER XYLINUM: VIII. On the Formation and Orientation of Bacterial Cellulose Fibrils in the Presence of Acidic Polysaccharides.
The transfer of the glucosyl moiety from uridine diphosphate glucose in the presence of Acetobacter xylinum cell-free extracts led to the formation of a mixture of alkali-soluble and -insoluble cellodextrins. Typical cellulose fibrils could not be detected by electron microscopy in this product. Immediately after release into the medium, cellulose formed by whole cells is in a "prefibrous" form which passes through Millipore filters of 0.45 and 0.8 micro pore diameter. Non-filtrable fibrils arise from this material probably by a process of crystallization involving no extracellular enzymes. Fibrils formed in shaken cell suspensions intertwine and form aggregates visible to the naked eye. In quiet suspensions pellicles are formed which float on the surface. Soluble Na-carboxymethylcellulose (CMC) is incorporated into cellulose fibrils formed in its presence, probably by a process of co-crystallization. Aggregation of fibrils containing CMC is delayed because of electrostatic repulsion between carboxylic groups. The aggregation time depends on the amount of CMC incorporated, its degree of substitution, the pH of the medium, and the ionic strength. The amount of CMC incorporated depends on the relative concentration CMC/cellulose and on the similarity of the CMC and the cellulose molecules i.e. in molecular weight and the number of carboxyl substitutions. Cellulose pellicles formed in the presence of CMC by unshaken cell suspensions consist of crossed, superimposed layers of parallel oriented cellulose fibrils. The same phenomenon is observed when phosphomannan, but not levan, is substituted for CMC. The biogenesis of oriented cellulose fibrils is envisaged as a process comprising the following steps: polymerization of the monomeric precursor, diffusion of the molecule to crystallization sites, crystallization, and orientation. It is proposed that charged polysaccharides play a role similar to that of CMC in affecting the orientation of cellulose fibrils in the plant cell wall.
Department of Biological Chemistry, The Hebrew University of Jerusalem, Israel.
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Ben-Hayyim
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Ohad