Phosphorolysis

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. 

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PNP catalyzes the phosphorolysis of purine nucleosides and deoxynucleosides in mammalian cells. The absence of PNP interferes with the proper degradation of 2 -deoxyguanosine, leading to an imbalance of cellular levels of deoxynucleotides... 

Fig. 7.8 Two examples of important reactions in the thioester world a the phosphorolysis of thioesters leads to acylphosphates and b a-ketoadds are formed in the reductive cleavage of thioesters with accompanying carbonylation (de Duve, 1991)...

According to de Duve, thioesters on the young Earth were capable of a broad spectrum of chemical reactions for example, of phosphorolysis leading to acylphos-phates (Fig. 7.8a) or of reductive thioester cleavage, which (after carbonylation) made possible the synthesis of ketoacids (Fig. 7.8b). 

The first group is comprised that of compounds which potentially could be degraded to form G-l-P by phosphorolysis by beta-linkage specific enzymes. They include IPTGlu, cellobiose, sophorose, salicin, and sucrose. Addition of these compounds, or exogenous G-l-P, to Solka Floe fermentations improved maximum cellulase yields from 171 to 309%, and the time period for enzyme synthesis was reduced from 95 to 59% compared with using Solka Floe only. 

I. Glycogen phosphorylase catalyzes glycogen phosphorolysis (bond cleavage of the a-(I 4) bonds by the substitution of a phosphate group) to yield glucose-1-phosphate ... 

The glycogen phosphorolysis of phosphorylase can be reverted, which makes it possible to enzymatically polymerize amylose as well as hybrid structures with amylose as outlined in the following section. 

Fig. 5 Catalytic mechanism of glycogen phosphorylases. The reaction scheme accounts for the reversibility of phosphorolysis of oligosaccharides (R) in the presence of orthophosphate (upper half) and primer-dependent synthesis in the presence of glucose-l-phosphate (lower half). PL enzyme-bound pyridoxal BH-f a general base contributed by the enzyme protein. Reprinted with permission from [109]. Copyright 1990 American Chemical Society...

Phosphorolysis of the glycosyl esters of sugar nucleotides leads to the corresponding nucleoside 5 -pyrophosphate and glycosyl phosphate. A wheat-germ enzyme specific for adenosine 5 -(a-D-glucopy-ranosyl pyrophosphate),462 and a yeast enzyme specific for guanosine 5 -(a-D-mannopyranosyl pyrophosphate),463,464 are known. 

Arsenate similarly replaces phosphate in various phosphorolysis reactions, so that sucrose phosphorylase catalyzes the hydrolysis of sucrose in its presence (23), potato phosphorylase can hydrolyze amylose and amylopectin (24), nucleoside phosphorylase can hydrolyze inosine... 

Glycogen phosphorylase sequentially cleaves the cx(1 -h>4) glycosidic bonds between the glucosyl residues at the nonreducing ends of the glycogen chains by simple phosphorolysis until four glucosyl units remain on each chain before a branch point (Figure 11.7). [Note ... 

Phosphopantothenylcysteine decarboxylases 753, 755 Phosphophoryns 442 Phosphoporin (PhoE) 411, 412s Phosphoric acid in lipids 380 pKa value of 99 Phosphorolysis, 593 Phosphorothioate... 

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