Cellulose biosynthesis, model

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. 

M. R. Brown and I. M. J. Saxena, Cellulose biosynthesis A model for understanding the assembly of biopolymers, Plant Physiol. Biochem., 38 (2000) 51-67. 

If the cellulose biosynthesis outside the cell walls is delayed or repressed, the polymer can accumulate as latex particles within the cells. This supports the association-crystallization hypothesis [55] for cellulose biosynthesis. According to this model the cellulose is synthesized as individual chains within the cytoplasm, which then adsorb onto the cell walls. This is in contrast to other models where it is presumed that the enzyme is only active at the microfibril ends embedded in cell walls. If the former model is correct, and it is supported experimental evidence, then the biosynthesis of cellulose can be considered, in part, as an emulsion polymerization. 

By sequence similarity in combination with molecular modeling, the GGDEF domain has been suggested to confer nucleotide cyclization activity (Pei and Grishin 2001), but this function has to be experimentally proven. However, it is not very far fetched to speculate that the GGDEF domain might synthesize the cyclic nucleotide c-di-GMP, which has been identified as the allosteric activator for cellulose biosynthesis in G. xylinus (Ross et al. 1991). 

Model for regulation of cellulose biosynthesis in A. xylinum (reprinted from Polymer Degradation and Stability, vol. 59, E.J. Vandamme, S. De Baets, A. Vanbaelen, K. Joris and P. De Wulf, Improved production of bacterial cellulose and its application potential, 93-99, copyright (1998), with permission from Elsevier). 

Brown, R. M., Herth, W., Franke, W. W., Romanovicz, D. The role of the Golgi apparatus in the biosynthesis and secretion of a cellulosic glycoprotein in Pleurochrysis a model system for the synthesis of structural polysaccharides. In Biogenesis of plant cell wall polysaccharides, pp. 207. Loewus, F. (ed.). New York Academic Press 1973... 

Many difficulties have been encountered in the study of the biosynthesis of cellulose, chief among them being the apparent lability of the cellulose-synthetase system. Based on evidence accumulated to date, a current model of cellulose synthesis184 can be envisaged as indicated in Fig. 6. 

Fig. 6.—Hypothetical Model for the Biosynthesis of Cellulose.184 [Numbers refer to reactions catalyzed by the following enzymes 1, invertase (EC 3.2.1.26) 2, sucrose synthetase 3, hexokinase (EC 2.7.1.1) 4, phosphoglucomutase (EC 2.7.5.1) 5, UDP-glucose pyrophosphorylase and 6, 7, and 8, hypothetical reactions in the pathway to cellulose.]...

Harada H and Cote WA (1985) Structure of wood. In Higuchi T (ed). Biosynthesis and biodegradation of wood components. Academic Press, London, 1-42 Harrington 11 (2002) Hierarchical modelling of softwood hygro-elastic properties. Ph.D thesis. University of Canterbury, Christchurch, New Zealand Harris IF (1976) Acid hydrolysis and dehydration reactions for utilizing plant carbohydrates. In Timell TE (ed). Proceedings of 8th cellulose conference. Wiley, New York, Vol. 1 131-44... 

When the models incorporating antiparallel arrangement of the chains are extended to native cellulose, they pose serious questions concerning proposed mechanisms for the biosynthesis of cellulose. 

The size of the cellulose molecule is normally expressed in terms of their degree polymerization (DP), i.e, the number of anhydroglucose units present in a chain. However, the conformational analysis of cellulose indicates that cellobiose (4-0-a-D-glucopyranosyl-a-D-glucopyranose, Fig. 10.2a) is its basic structural unit [10]. The conformation of the repeating unit of cellulose can be explained if we consider the model proposed for the biosynthesis of glucose [11]. 

G. xylinus was first described by Brown in 1886 [2, 3] who identified a jelly-like film formed over the surface of a vinegar broth fermentation. A microscopic analysis revealed the presence of bacteria distributed within the whole film. Still today, G. xylinus is taken as the model microorganism in the research on the biosynthesis, crystallization and structural properties of bactraial cellulose [4]. The cellulose produced by G. xylinus contains approximately... 

---