Starch 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. 

Figure 12-1 Phosphorylase-catalyzed phosphorolysis of glycogen or starch to glucose-1-phosphate.

Potato phosphorylase has successfully been employed for the enzymatic synthesis of 2-deoxyglucosyl 1-phosphate 135 (Scheme 45) [252]. D-glucal 136 was transferred onto maltotetrose 137 in the presence of the phosphorylase and catalytic amounts of phosphate to afford a modified maltooligosaccharide 138 that contained 2-deoxy-D-arabino-hexopyranosyl residues [253]. Formation of 2-deoxyglucosyl phosphate 135 can be explained by reverse phosphorolysis of the oligosaccharide 138. When soluble starch was employed as primer and an equimolar amount of phosphate was added, 2-deoxyglucosyl phosphate 135 was obtained in 50% isolated yield on a preparative scale. 

In this connection it may be mentioned that many ordinary yeasts ferment, generally very slowly, isomaltose and trisaccharides (limit dextrins) with one maltose and one isomaltose linkage. - Dried yeasts ferment starch and limit dextrins mth a fairly high velocity. This fermentation is probably not due to a hydrolysis of the saccharides to D-glucose or maltose, but to a phosphorolysis to the Cori ester (a-D-glucopyranose 1-phosphate). 

Maltodextrin phosphorylase and glycogen phosphorylase (Reaction (6)) occur in many bacteria and catalyzes the synthesis as well as the phosphorolysis of a-4-glucosidic linkages present in glycogen or starch. Maltodextrin phosphorylase however is only induced in the presence of maltodextrins, and for the microorganisms studied, glycogen phosphorylase activity is insufficient to account for their rate of glycogen accumulation. Moreover,... 

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... 

Glucose 1-phosphate, Corl esler the product of phosphorolysis of Glycogen (see) and Starch (see). It is converted to glucose 6-phosphate by phosphogluco-mutase (EC 2.7.5.1). 

Fig. 6.3 B). The initial, slow degradation of starch therefore appears to be a consequence of phosphorolysis but the later, and more rapid degradation also requires amylolysis. 

The free energy change in phosphorolysis compared to hydrolysis is much closer to zero, and thus the known phosphorolytic enzymes catalyze biosynthetic reactions as well as catabolic changes. They are implicated chiefly in the synthesis of starch by plants and glycogen by animals. Recent work by Kornberg and his associates has pointed to a pyro-phosphorylase in yeast and hog liver that catalyzes the following general reaction ... 

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... 

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. 

Phosphorolysis of Starch and Glycogen. Within cells, the storage carbohydrate is broken down by a different route, involving phosphorolysis. This holds true both for the breakdown of starch in plant cells and for that of glycogen in muscle and liver. 

The utilisation of starch (see Fig. 5.11 for structure) appears to involve two distinct mechanisms—one hydrol) ic and the other involving phosphorolysis. 

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