Cytoplasm polysaccharide precursor

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. 

Coumarin reduced labeled glucose incorporation and carbohydrate and protein content of the cytoplasm of Pythium, but enzymes related to the metabolism of cell wall polysaccharides were not affected [296]. It has been reported that moulds are more sensitive to coumarin when they are cultured on the simple synthetic nutrient medium than on the yeast water [140]. This suggests that coumarin blocks the synthesis of an unknown metabolite of ndamental importance. Coumarin was found to affect the level of a variety of free amino acids present in a cell. In addition, some nucleic acid precursors partially reverse the coumarin-induced inhibition of maize mesocotyle [131]. The stimulation of germination and growth caused by coumarin in low concentrations may be assumed as a supercompensation mechanism. 

Undecaprenol (bactoprenol) from Salmonella contains eleven isoprene units, and two irons and nine cis double bonds In the form of undecaprenyl phosphate, it acts as a carrier of carbohydrate residues in the biosynthesis of bacterial antigenic polysaccharides synthesis of Murein (see) also depends on undecaprenyl phosphate. In eukaryotes the Dolichol phosphates (see) function in the transfer of carbohydrate residues in the synthesis of glycoproteins and glycoli-pids. Probably the long lipid chains of these P serve to anchor them in membranes, while the phosphate group acts as a carrier by protruding into the cytoplasm. It is not known whether all P. function as carbohydrate carriers. The structural relationship between solanesol and plastoquinone-9 and ubiquinone-9, and the joint occurrence of these compounds suggest a precursor role for P. Biosynthesis of P. proceeds from mevalonic acid and the conformation of all double bonds is predetermined in early precursors. 

It has been well established that the enzymes required for the biosynthesis of the polymers of the outer membrane are localized in the inner membrane. Most importantly, the lipid-carrier molecules (poly-isoprenoid-phosphates) are found in this membrane. These molecules transfer newly synthesized, activated precursor molecules from the hydrophilic cytoplasmic environment into the lipophilic environment of the membrane, where the assembly into polymeric structures takes place. This assembly process is used for lipopolysaccharides, peptidoglycans, and capsular polysaccharides. In a subsequent step, the membrane-carrier molecules transfer the assembled polymers from the inner to the outer membrane. ... 

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