BIOSYNTHESIS OF CELLULOSE

摘要

Cellulose is synthesized by a large number of living organisms ranging from the bacterium Acetobacter xylinum to forest trees. A. xylinum produces abundant amounts of cellulose and this bacterium has been used as a model system for studies on cellulose biosynthesis and structure of the cellulose product. Cellulose is synthesized by the enzyme cellulose synthase, a membrane protein that catalyzes the direct polymerization of glucose from the substrate UDP-glucose into a cellulose product. Genes for cellulose synthases have been identified from many bacteria, Dictyostelium discoideum, and higher plants. Analysis of the predicted protein sequences has allowed identification of conserved residues in cellulose synthases from different organisms. The conserved residues are found in the globular region of the cellulose synthases. Using site-directed mutagenesis experiments we have shown that the conserved amino acid residues are required for cellulose synthase activity in A. xylinum. Although cellulose synthase activity can be monitored in vitro using membrane fractions from A. xylinum, it is not easy to monitor this activity when membrane fractions from plants are used. We have initiated experiments to analyze cellulose synthases from plants in A. xylinum in an effort to characterize the different cellulose synthases, for example the ones involved in cellulose biosynthesis during primary cell wall formation and those that are active during secondary wall synthesis. A general model describing the possible sequence of events in the cellulose synthase catalytic site will be presented to provide sufficient details not only into the biosynthesis of cellulose but also other polysaccharides.