Polysaccharide synthase

Dhugga, K.S. (2001) Building the wall genes and enzyme complexes for polysaccharide synthases. Curr. Opin Plant Biol. 

All of the enzymes responsible for formation of cell-wall polysaccharide appear to be membrane-bound, and are usually recovered in the particulate, cell-wall fraction sedimenting at 20,000g none have been purified significantly. Several names have been given to these complex enzyme-systems, which may possess more than one activity, including polysaccharide synthase and glycosyltransferase. The term polysaccharide synthase is preferred for use in this article. 

The general equation for single steps in the reaction catalyzed by a polysaccharide synthase is... 

The parallels among chitin, cellulose, and HA structures, all being j3 -chains of hexose polymers are reflected in the striking similarity in sequence between the HAS from vertebrates, cellulose synthases from plants, and chitin synthases from fungi. A primordial ancestral gene must have existed from which all of these enzymes evolved that are involved in the biosynthesis of all polymers that contain -glycoside linkages, an ancient j3-polysaccharide synthase. 

The biosynthesis of oligomers and polysaccharides at the plasma membrane and ER-Golgi apparatus (i.e., polysaccharide synthases and glycosyl transferases)... 

Fairweather J.K., Lai Kee Him J., Heux L., Driguez H., and Bulone V. 2004 Structural characterization by C-NMR spectroscopy of products synthesized in vitro by polysaccharide synthases using C-enriched glycosyl donors apphcation to a UDP-glucose (l->3)-P-D-glucan synthase from blackberry (Rubus fruticosus). Glycobiology 14 775-781. 

Polysaccharides (e.g., glucans, chitin) are synthesized in the cytoplasm by integral polysaccharide synthases that are uniformly distributed in the plasma membrane, while the synthesis of protein-bormd polysaccharides (e.g., mannoproteins) occurs in... 

Faijes, M. and Planas, A. (2007) In vitro synthesis of artificial polysaccharides by glycosidases and glyco-synthases. Carbohydrate Research, 342, 1581-1594. 

Synthesis. The synthases are present at the endomembrane system of the cell and have been isolated on membrane fractions prepared from the cells (5,6). The nucleoside diphosphate sugars which are used by the synthases are formed in the cytoplasm, and usually the epimerases and the other enzymes (e.g., dehydrogenases and decarboxylases) which interconvert them are also soluble and probably occur in the cytoplasm (14). Nevertheless some epimerases are membrane bound and this may be important for the regulation of the synthases which use the different epimers in a heteropolysaccharide. This is especially significant because the availability of the donor compounds at the site of the transglycosylases (the synthases) is of obvious importance for control of the synthesis. The synthases are located at the lumen side of the membrane and the nucleoside diphosphate sugars must therefore cross the membrane in order to take part in the reaction. Modulation of this transport mechanism is an obvious point for the control not only for the rate of synthesis but for the type of synthesis which occurs in the particular lumen of the membrane system. Obviously the synthase cannot function unless the donor molecule is transported to its active site and the transporters may only be present at certain regions within the endomembrane system. It has been observed that when intact cells are fed radioactive monosaccharides which will form and label polysaccharides, these cannot always be found at all the membrane sites within the cell where the synthase activities are known to occur (15). A possible reason for this difference may be the selection of precursors by the transport mechanism. 

In addition to its catalytic subunit, cellulose synthase may have subunits that mediate extrusion of the polysaccharide chain (the pore subunit) and crystallization of the polysaccharide chains outside the cell... 

It serves as a membrane anchor for the growing polysaccharide. We have already discussed one example in the hypothetical cellulose synthase mechanism of Fig. 20-5. For some polysaccharides the mechanism is better established. The synthetic cycles all resemble that of Fig. 20-5 and can be generalized as in Eq. 20-20. Here NDP-Glx is a suitable nucleotide disphosphate derivative of sugar Glx, and Z-Glx is the repeating unit of the polysaccharide formed by the action of glycosyl-transferases and other enzymes. 

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