Synthesis of Homopolysaccharides

The a-D-glucopyranosyl residues are united by the same a-D-(1 — 4) linkages, forming linear chains a high-molecular-weight acceptor of the same type of linkage is required for the formation of the polysaccharide. When the linear chains become about ten resi- 

Studies by Lamer and coworkers showed that 3 5 -cyclic AMP is an important factor that regulates the interconversion of the two forms of glycogen synthetase. The conversion of the independent into the dependent form is catalyzed by a kinase requiring ATP and Mg +, the activity of which is increased by cyclic AMP. Huijing and Lamer and others showed that muscle kinase appears to be similar to, but not identical with, phosphorylase b kinase, - and that the two enzymes seem to be equally sensitive to stimulation by cyclic AMP. It has been suggested - that cyclic AMP may in this instance act by increasing the affinity of an allosteric site for magnesium on the kinases. As epinephrine and other hormones increase the level of 

Another hormone, namely, insulin, acts in the opposite direction, and produces a rapid rise in glycogen synthetase activity. It has been suggested that insulin acts by changing the glycogen synthetase kinase into a form having a low affinity for cyclic AMP, so that the conversion of the independent into the dependent form (that is, inactivation of the glycogen synthetase) is retarded. 

The dependent form can be reconverted into the independent form through dephosphorylation by a specific phosphatase that is inhibited by glycogen this can provide a mechanism for the feedback regulation of glycogen levels in tissues, through the inhibition of reactivation of glycogen synthetase by its own product. 

The independent form can be converted into the dependent form by a difiFerent mechanism. It has been found that addition of calcium ions to certain glycogen synthetase preparations produces a conversion of the independent into the dependent form. This conversion does not involve ATP, and is not aflFected by adenosine 3 5 -cyclic phosphate it requires a protein similar to that involved in the activation of inactive phosphorylase b kinase by Ca . Thus, calcium appears to exert an effect on the regulation of glycogen synthetase, and it is hypothesized that the inactivation of muscle glycogen synthetase after muscle contraction may be caused by this mechanism, mediated by an alteration of intracellular levels of Ca +. 

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In the synthesis of homopolysaccharides the monosaccharide units are activated by the formation of nucleoside diphosphate derivatives from which they are transferred to a nonactivated growing polymer. The following nucleotide coenzymes are used in the synthe of the most common polymers. 

This chapter focuses on the synthesis of complex carbohydrates. It begins with a consideration of the hexoses that are the building blocks of complex carbohydrates. Then some aspects of the synthesis of simple homopolysaccharides are examined followed by a brief consideration of heteropolymers that contain more than one hex-ose. A major portion of the chapter is concerned with glycoproteins that contain complex linear and branched carbohydrates attached to proteins. Finally the synthesis of the bacterial wall is examined. 

From this brief survey, it is seen that there were few features of carbohydrate metabolism in plants that escaped Hassid s touch, and much that we now know about the role of sugar nucleotides in the interconversion of carbohydrates in plants is a direct result of his persistent effort. From the incorporation of labelled precursors into monosaccharides, to the conversion of the monosaccharides into their glycosyl phosphates, to the action of the pyrophosphorylases in the synthesis of glycosyl esters of nucleoside pyrophosphates, to the interconversion of the resulting sugar nucleotides, to the polymerization of the activated monosaccharides to yield disaccharides and the homopolysaccharides, and, finally, to the modification of the polysaccharides by methylation—in summary, to almost every aspect... 

Scher and coworkers showed that an alkali-stable, acid-labile lipoid material was synthesized from GDP-D-mannose and an endogenous lipid in crude, cell-free preparations of Micrococcus lysodeik-ticus. This compound has been shown to be a polyisoprenyl (d-mannosyl phosphate) it can serve as a D-mannosyl donor in the enzymic synthesis of a homopolysaccharide of D-mannose. The poly-isoprenol was found to be a C55 compound containing two internal, trans double bonds. It was shown that only the D-mannosyl group of GDP-d -mannose is transferred to the polyisoprenyl phosphate ( undecaprenyl 1-phosphate ), and that the reaction is readily reversible. 

The synthesis of heteropolysaccharides is somewhat more complex than that of homopolysaccharides, because at each step a different primer is required and a different activated monosaccharide is added. 

For the configuration-conformational properties of simple carbohydrates, as well as their reactivities, comprehensive treatises should be consulted (Pigman and Horton, 1972). It has to be mentioned on closing, that recent achievements in macromolecular science have established the ways to the synthesis of linear homopolysaccharides and copolysaccharides (normally by cationic polymerization) of different suitably protected anhydrosugars (Schuerch, 1981). These syntheses open the route to the availability for our studies of a variety of tailor made carbohydrate polymers. A few synthetic polysaccharides have in fact already been used to investigate immunological and allergic reactions in experimental animals and the interaction of plant lectins with carbohydrates (Schuerch, 1981). 

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