Polysaccharide intermediate forms

Figure 3 Biosynthetic pathways. (A) In the terpenoid coupling reaction, isomers of isopentenyl pyrophosphate are joined with the loss of pyrophosphate, leading to a linear intermediate that is cyclized to a terpenoid skeleton, as shown for the diterpene taxol. (B) In the polysaccharide coupling reaction, hexose and pentose monomers are joined with the loss of a nucleoside diphosphate, as shown for the epivancosaminyl-glucose disaccharide of vancomycin. (C) In the first step of the nonribosomal peptide coupling reaction, an aminoacyl adenylate is transferred to a carrier protein or thiolation domain (denoted T ) with loss of adenosine monophosphate. In the second step, this carrier protein-tethered aminoacyl group is coupled to the amine of an aminoacyl cosubstrate, forming a peptide bond, as shown for two residues in backbone of vancomycin. (D) In the polyketide coupling reaction, the loss of carbon dioxide from a two or three-carbon monomer yields a thioester enolate that attacks a carrier protein-tethered intermediate, forming a carbon-carbon bond as shown for the polyketone precursor of enterocin.

In concentrated sulfuric acid, polysaccharides are hydrolyzed to their constituent monosaccharides, which are dehydrated to reactive intermediates. In the presence of phenol, these intermediates form yellow products, such as that shown in Eq. 1.6,28 with a combined maximal absorbance at 492 nm. 

The biosynthesis of poly-2 -> 8 or -2 9-(A-acetyl-D-neuraminic acid) capsular polysaccharide (colominic acid) from E. coli or N. meningitidis (see Chapter 6, Fig. 6.20 for the structures) has also been shown to involve a polyprenol phos-phoryl A-acetyl-D-neuraminic acid lipid intermediate formed from CMP-NeuNAc [67,68]. This is an example of the biosynthesis of a homopolysaccharide that requires a polyprenol phosphate coenzyme lipid carrier. 

Gum ghatti is the calcium and magnesium salt of a complex polysaccharide which contains L-arabinose, D-galactose, D-mannose, and D-xylose and D-glucuronic acid (48) and has a molecular weight of approximately 12,000. On dispersion in water, gum ghatti forms viscous solutions of viscosity intermediate between those of gum arabic and gum karaya. These dispersions have emulsification and adhesive properties equivalent to or superior to those described for gum arabic. 

We may return now to the polysaccharides present in the peanut for a brief consideration of the relationship of the other components present in the pectic materials to the araban constituent. All the evidence indicates that the pectic acid portion of the peanut is identical with normal pectic acid and, as was indicated in the previous section, this material, which is very stable to acid hydrolysis and possesses a high positive rotation contains a main chain which is built up of D-galac-turonic acid residues of the pyranose type. If, therefore, the araban associated with the pectic acid had been derived directly from the pectic acid by decarboxylation without intermediate hydrolysis of the poly-galacturonide, the sugar residues in the araban should also be in the pyranose form. The experimental evidence shows clearly, however, that the arabinose residues in araban are furanose in type and it follows that any hypothesis concerning the direct conversion of pectic acid into the araban by decarboxylation is untenable. 

Clearly, in biosynthesis the ketoses play a major role as active intermediates in the intertransformation of monosaccharides and as precursors of aldoses which usually appear as stable end-products and are stored in the form of polysaccharide or glycosides. An exception is the novel D-glu-... 

Metal ions of transition and other elements of variable valency, e.g. Ce, Co, Fe, V, Mn, etc., are known to oxidize polysaccharides rather selectively, producing macroradicals as intermediates which are capable of adding vinyl monomers and form graft copolymers. These initiators are redox systems which differ from those previously described by not producing free radicals of low molecular weight. Only macroradicals on the substrate are formed in the redox reaction. Some homopolymer may still be formed in the process, e.g. due to oxidation of monomer or other side reactions. ... 

Phosphoric acid esters of the ketopentose D-ribulose (2) are intermediates in the pentose phosphate pathway (see p.l52) and in photosynthesis (see p.l28). The most widely distributed of the ketohexoses is D-fructose. In free form, it is present in fruit juices and in honey. Bound fructose is found in sucrose (B) and plant polysaccharides (e.g., inulin). 

A number of lower volume chemicals can be obtained from wood hydrolysis. Furfural is formed from the hydrolysis of some polysaccharides to pentoses, followed by dehydration. This process is still used in the Soviet Union. Furfural is used in small amounts in some phenol plastics it is a small minor pesticide and an important commercial solvent. It can be converted into the common solvent tetrahydrofuran (THF) and an important solvent and intermediate in organic synthesis, furfuryl alcohol. 

One of the most impressive findings has been the discovery of lipid intermediates in the biosynthesis of polysaccharides (see Refs. 2 and 465.) At least two structurally different types of these compounds exist the intermediate may be an isoprenoid alcohol ester of the glycosyl pyrophosphate or the analogous derivative of the glycosyl phosphate. Derivatives of the first type are formed by reaction between the sugar nucleotide and the alcohol phosphate, for example, undecaprenyl phosphate (120), which participates in the biosynthesis of Salmonella lipopolysaccharide.466... 

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