Mechanism of Cellulose Synthesis
In straightforward terms, cellulose biosynthesis requires the enzyme cellulose synthase for catalyzing the polymerization of glucose residues from UDP-glucose into a β-1,4-linked glucan chain. This simple mechanism envisions direct polymerization without the need for any intermediates or a primer. Cellulose biosynthesis has been demonstrated in vitro using membrane and detergent-solubilized extracts from A. xylinum and a number of plants in the presence of only UDP-glucose (Kudlicka and Brown, 1997; Lai-Kee-Him et al., 2002; Lin and Brown, 1989; Okuda et al., 1993). The synthesis of cellulose in vitro with the minimal added components in the reaction mixture strongly supports the direct polymerization of glucose without any requirement for a primer. However, in the absence of purified cellulose synthases it is not possible to completely exclude the role of other proteins or components contributed by the membrane fraction or detergent extracts during cellulose synthesis. In 2002, Peng et al. proposed a model for cellulose biosynthesis in which they suggested that SG serves as a primer for synthesis of SCDs by CesA proteins (Peng et al., 2002). According to their model, a membrane-associated endoglucanase Kor (encoded by the Korrigan gene) cleaves SCDs giving rise to SG and cellodextrins (CDs). In the next step, the CDs undergo β-1,4-glucan chain elongation catalyzed by CesA proteins.