Biosynthesis, cellulose

Aquifer bioremediation, defined, 3 758t Aquifer flow, 12 842 Aquifers, 12 838-839 Aquifer sparging, defined, 3 758t AraA [9-(P-D-arabinofuranosyl)-9H-purin-6-amine], 4 713 Arabidopsis, molecular genetics of cellulose biosynthesis, 5 366 Arabinanase, 10 300 Arabinans, classification by structure, 4 723t... 

Source Adapted from various sources including (i) Wood chemistry , E. Sjostrom, 2nd edition, 1993, p. 52 (ii) Cellulose Biosynthesis , D.F. Delmer, Ann. Rev. Plant Physiol., 1987, 38, 259-290 (iii) Biosynthesis in Plant Cell Walls , D.F. Delmer, in The Biochemistry of Plants , vol. 14, Academic Press, San Diego, 1988, pp. 373-420). 

Cellulose biosynthesis is a complex, sensitive, and not fully characterized process that occurs in organisms ranging from plants to bacteria to animals. Two fundamental approaches have been used to investigate cellulose biosynthesis one structural and the other biochemical. 

The terminal complex hypothesis proposes that the cellulose synthesizing enzyme complex can be visualized with electron microscopy. Terminal complex is the name given to collections of plasma membrane particles thought to represent the cellulose synthase. While direct evidence is still not available to support this hypothesis, the amount of indirect supporting evidence has grown dramatically in the past few years. The relationship between terminal complexes, cellulose physical structure and the biochemical events of cellulose biosynthesis will be discussed. 

Cellulose, a polysaccharide consisting of linear 1,4-/ -D-anhydroglucopyra-nose chains laterally associated by hydrogen bonds, is the most abundant and commercially important plant cell wall polymer (1). Consequently, cellulose is also one of the most thoroughly investigated plant cell wall polymers. However, it is enigmatic in the sense that significant elements of cellulose physical structure and the mechanism of cellulose biosynthesis still are not well understood. Since these subjects have been reviewed recently (2-10), this review will update topics covered previously and provide a new analysis of selected topics of contemporary interest. 

PF had been proposed as the terminal complex (23) and associated pores were reported on the outer membrane EF (24). Due to their proximity to the site of cellulose ribbon extrusion from the cell surface, these structures were assumed to be responsible for cellulose synthesis. A model was advanced in which cellulose synthase was localized on the outer membrane, which invoked adhesion sites between the outer and plasma membranes as a mechanism to explain the transfer of uridine-diphosphoryl-glucose (UDPG) from the cytoplasm to the cellulose synthases (25,26). However, when the outer and plasma membranes of Acetobacter were isolated separately by density-gradient centrifugation, the cellulose synthase activity was localized only in the plasma membrane fraction (27). Therefore, the linear structures observed on the Acetobacter outer membrane, while they may be associated in some manner with cellulose biosynthesis, are probably not the cellulose synthase terminal complexes. Since no ultrastructural evidence for adhesion sites between the outer and plasma membranes has been presented, a thorough investigation of the mechanism of / (1-4) glucan chain translocation from the cytoplasmic membrane to the outer membrane in Acetobacter xylinvm is now in order. 

The cellulose crystallite size is highest in ulvophycean and certain chlorophycean algae (114-169A), lowest in vascular plants (49-62A) and intermediate in Aceiobacter (70-84A) (1). It appears that cellulose crystalline dimensions are independent of the type of terminal cellulose synthesizing complex. The idea that cellulose biosynthesis is not exclusively responsible for determining its crystalline dimensions has been proposed previously by Marx-Figini (52). 

A characteristic feature of the SuSy isoforms is a conserved phosphorylated serine residue near the N-terminus [8-10]. In-vivo studies have demonstrated that phosphorylation and dephosphorylation direct the distribution of SuSy isoforms in the plant cell [10-12]. The soluble phosphorylated SuSy interacts with the actin cytoskeleton in the cytoplasm [13], and the dephosphorylated SuSy isoforms are targeted to the cell membrane to form complexes with other enzymes, e.g., glucan synthase, catalyzing cellulose biosynthesis from sucrose [4, 10, 14]. In this respect, recent studies on the dephosphorylated enzymes by cloning and expression of SMS genes in E. coli have shown differences in some biochemical features when compared to the enzymes isolated from the corresponding plant material. Recom-... 

The herbicide dichlobenil (Figure 2.27) is believed to exert its effect through the inhibition of cellulose biosynthesis of actively growing plant tissue, leading to a cessation of cell division and death. 

Delmer, D.P. (1999) Cellulose biosynthesis exciting times for a difficult field of study. Annu. Rev. Plant Physiol. Plant Mol. Biol. 50, 245-276. 

Cellulose biosynthesis in plants from genes to rosettes. Plant Cell Physiol. 43, 1407-1420. 

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