Polysaccharides phosphorolysis

Assuming that the pyrophosphate formed is hydrolyzed, the equation for the incorporation of the glucosyl residue of glucose-1 -phosphate into starch is the reverse of the equation for the phosphorolysis of a polysaccharide, except for the conversion of ATP to ADP and phosphate ion. 

It has also been shown that the C-1—O—C-4 bond in the polysaccharide is split, on phosphorolysis, between C-1 and O. This was demonstrated for glycogen and KH2P 04, and, at equilibrium, was demonstrated in the D-glucosyl phosphate. 

Glycosidic linkages in polysaccharides may also be cleaved by phosphorolysis. The phosphorolysis reaction is shown in Structure 1. 

An additional problem in limit-dextrin analysis requires comment. Assuming (a) complete phosphorolysis and /3-amylolysis and (b) the presence of equal numbers of A- and B-chains, the difference A between the CL of a d>-dextrin and a S-dextrin of various polysaccharides should be constant. This is not the case. With rabbit-liver glycogen, wheat amylopectin, and maize amylopectin, A was 1.8, 2.6, and 2.7 n-glucose residues, and for seventeen other polysaccharides, A varied between 1.1 and 2.6. If the ( -dextrin had single-unit A-chains, this variation could be ascribed to differences in the ratio of A-chains to B-chains. This conclusion is probably incorrect, and no ready explanation for the variation in A is available as, in all the experiments, enzymic degradation was essentially complete. 

See also Phosphorolysis, Glycogen phosphorylase. Figure 13.18, Kinase Cascade, Figure 13.16, Figure 13.17, Polysaccharides, Glycogen Breakdown, Hydrolysis vs Phosphorolysis, Glycogen Breakdown Regulation... 

This conclusion was confirmed by kinetic measurements. Preparations of purified cytosolic and plastidic phosphorylase isozymes from Pisum sativum were adjusted to an approximately equal activity concentration (as determined at saturating levels of soluble starch). Using these isozyme preparations kinetic measurements were performed at varying levels of the polysaccharide fraction. Orthophosphate or glucose 1-phosphate levels were saturating. The initial rates of phosphorolysis (Fig. 2A) or of polysaccharide biosynthesis (Fig. 2B) were determined. 

FIGURE 2A and 2B. Polysaccharide-dependent phosphorolysis (A) or biosynthesis (B) catalyzed by purified cytosolic (I) or plastidic (II) isozymes from Pisum sativum. Rates are given as percent of the values obtained at saturatin concentrations os soluble starch. Quantification of the polysaccharide concentrations as in Fig. 1... 

The polysaccharide fraction functioned as substarte for phosphorolysis and as primer for biosynthesis. However, for the cytosolic isozyme by far higher initial rates were obtained than with the plastid-specific enzyme form. 

Phosphorolysis entails the depolymerisation of polysaccharides. In this process (which is the reverse of glycogenesis and uses a different enzyme, namely glycogen phosphorylase), polysaccharides are successfully depolymerised by the action of orthophosphoric acid to glucose-l-phosphate (11.61). This is the form required for the process of glycolysis described below. 

Oligo- and polysaccharides are degraded in organisms by specific enzymatic hydrolysis (by hydrola ) or phosphorolysis (by phosphorylases). 

In the case of polysaccharides and sucrose both phosphorolytic and hydrolytic types of cleavage have been recognized. It appeared for a time as if phosphorolysis were the only mechanism for nucleoside splitting. Now, however, two other reactions involving cleavage at the N-glycosidic linkage are known. They are (1) hydrolysis and (2) base transfer reactions. 

Phosphorylases have been described for the polysaccharides, glycogen and amylose, and for sucrose. Phosphorylases from both muscle and plants are able to cleave the 1, 4 -qj linkages of glycogen and starch to yield glucose-1-phosphate. Sucrose phosphorylase is a separate enzyme obtained from bacteria which produces glucose-l-phosphate and fructose from sucrose. Phosphorolysis has also been involved in the cleavage of the pentosidic... 

Phosphorylases are key enzymes of carbohydrate metabolism and catalyze the phosphorolysis of glycosidic bonds in oligo- and polysaccharide substrates. This type of reaction afforded amylose via in vitro polymerization of D-glucosyl... 

When either starch or glycogen is utilized, the same number of bonds are formed as with glucose but since the bond energy of the gluco-sidic link of the polysaccharide is preserved by phosphorolysis in the phosphate bond of glucose-l-phosphate, the only bond utilized is at the fructose-6-phosphate fructose diphosphate level. A net synthesis of three is thus observed. 

The mass-law equation (a) or (b) appears to be in perfect agreement with Hanes observation that the equilibrium of the phosphorolysis reaction is not affected by the concentration of the polysaccharide, provided a certain minimum concentration is exceeded. 

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