Cellulose synthesis, enzymic

The FP cellulose per unit (ml) volume and enzyme yield per unit (g) cellulose or substrate obtained on wheat straw, wood, and CTMP in SSF were higher than those obtained in LSF on wheat straw and wood (Tables I, II, and III). And wheat straw proved to be a better substrate than wood for cellulose production in SSF. This could be attributed to the polysaccharides (cellulose and hemicelluloses) of wheat straw being more readily available for the organism s growth and cellulose synthesis than those of wood. The hemicelluloses and cellulose were presumably not as available in wood, because of its high lignin content and high cellulose crystallinity, as in wheat straw. 

It seems likely that the enzyme complexes for hemicelluloses, pectins and cellulose are constructed, at least in part, on the endoplasmic reticulum and then transferred to the Golgi apparatus, where they are modified and sorted so that they can be segregated within the compartments of the Golgi cisternae (30,31). The complex for cellulose synthesis is not normally active within the Golgi apparatus and it is transported to active sites at the plasma membrane (1). The hemicelluloses and pectins are formed within vesicles and cisternae of the Golgi apparatus and the vesicles are transported to the plasma membrane, where fusion occurs and the polysaccharides are packed into the wall (1). It is not known whether particular polysaccharides such as the xylans of the hemicellulose and the arabinogalactans of the pectins are transported in separate vesicles or together in one vesicle. Nor is it known if the complex for cellulose synthesis is transported by vesicles which carry hemicellulose and pectin polysaccharides. 

However, an enzyme that uses GDP-D-glucose as a substrate in order to synthesize a cellulose-like product has been reported to be present in developing cotton fibers during the period when primary-wall cellulose synthesis normally occurs, but is absent from older fibers undergoing active deposition of secondary-wall cellulose.303... 

The situation is complicated by the fact that the efficiency of cellulose synthesis in the cell-free system is low, and the same enzyme preparation catalyzes the incorporation of D-glucosyl groups from UDP-Glc into alkali-soluble (1 - 2)-/3-D-glucans.370 A similar process was reported to occur with a membrane preparation of Rhizobium meliloti,371... 

It has also been shown that mung beans, peas, and other plants contain a pyrophosphorylase which forms GDP-D-glucose from a-D-glucose 1-P and GTP. Based on the data obtained with enzymic plant preparations, we proposed the following mechanism for cellulose synthesis ... 

In the past few years, some impressive observations have been made by microscopists studying cells actively engaged in cellulose synthesis. It is now generally accepted by most workers that, in the bacterium A. xylinum, in most algae, and in all of the higher plants, cellulose is synthesized at the cell surface by an enzyme system localized in the plasma membrane. The notable exception to this conclusion concerns those algae which synthesize a cell wall composed of cellulosic scales such scales are synthesized intracellularly by way of the Golgi apparatus (see Ref. 57 and references cited therein). 

I. Tsekos, The sites of cellulose synthesis in algae Diversity and evolution of cellulose-synthesizing enzyme complexes, J. PhycoL, 35 (1999) 635-655. 

One strategy to the enhance CO2 fixation in woody plants is to enhance the expression of genes required for cellulose deposition, which should enhance plant growth either by cell wall or just cellulose deposition. We have already isolated the full length of cDNAs for three kinds of cellulases, XET and expansin as plant cell growth regulators, and for two kinds of cellulose synthases and suCTOse synthase as a system of cellulose synthesis, as summarized in Table 2. If cellulose deposition is increased by the increased activity of each enzyme in the transformants of Arabidopsis, the gene can be introduced to woody plants to determine the cellulose deposition. 

When the enzyme preparation was treated with eellulase and subsequently re-isolated by centrifuging, its ability to form radioactive cellulose was almost entirely lost. Cellulose synthesis was restored by the addition of a solution of cellodextrins which were shown to function as D-gluco-pyranosyl acceptors ( primers ). The reaction was, therefore, postulated to consist of repetitive D-glucopyranosyl transfers, as shown by the following equation. 

Despite some unexplained results concerning cellulose synthesis with a cell-free system in plants, the data seem, in general, to be consistent with the assumption that GDP-D-glucose is the sugar nucleotide from which the /3-D-(l — 4)-linked chain of cellulose originates. However, there is evidence for the presence of another enzyme system that, in some plants, produces from UDP-D-glucose a polymer that appears to be a mixed, )8-D-(l - 4), /3-D-(l - 3)-linked polysaccharide. - ... 

Aloni Y, Delmer D.P., and Benziman M. 1982. Achievement of high rates of in vitro synthesis of 1, 4-beta-D-glucan activation by cooperative interaction of the Acetobacter xylinum enzyme system with GTP, polyethylene glycol, and a protein factor. Proc Natl Acad Sci USA 79(21) 6448-6452. Amikam D. and Benziman M. 1989. Cyclic diguanylic acid and cellulose synthesis in Agrobacterium tumefaciens. J Bacterid 171(12) 6649-6655. 

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