Index >> Cellular Components >> Biosynthesis of Capsular Materials

Earlier the biosynthesis of this peptide was believed to involve a membrane associated polyglutamyl synthetase which required ATP and Mg++ but this has been now found to polymerize only L-glutamyl residues.

A great deal of information is available with regard to the biosyn­thesis of capsular polysaccharides in pneumococci and other bacteria. The biosynthesis of capsular polysaccharides generally occurs either via sugar nucleotide precursors or directly from sugars. The majority are however synthesized via sugar nucleotide precur­sors such as UDP glucose, UDP galactose, GDP fucose, GDP mannose etc. The formation of a polysaccharide chain involves succe­ssive transfer of glycosyl residues via a lipid carrier in the cell membrane.

Thus, a levan molecule (polymer of levulose) has a terminal glucosyl residue to which a chain of ß-glucosyl residue in 2-6 linkage are attached. The biosynthesis of dextran also occurs by a similar mechanism and dextrans contain a terminal fructosyl residue. No ATP is requir­ed for the polymerization reaction. The enzyme involved has been characterized as dextran sucrase. In L. mesentroldes chain elonga­tion occurs by transglycosylation. For this reason, these compounds are synthesized only when the bacteria grow in sucrose containing media. In Acetabacter (the vinegar bacteria), some species synthesize a capsule consisting of only cellulose. The biosynthesis of cellulose in these organisms appears to occur by a pathway similar to that in the plants.