Phosphorolysis, glycogen

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. 

Simplified mecharusms of peptide hydrolysis and glycogen phosphorolysis arc outlined as follows ... 

This results in activation of cAMP-dependent protein kinase (protein kinase A), with consequent phosphorylation of target proteins, such as phosphorylase b kinase in cells that activate glycogen phosphorolysis. 

Fig. 3.8. The role of cAMP as an intermediate to environmental stimulus (hormone secretion) and the onset of physiological response. Adrenaline and GTP bind to a specific membrane bound regulatory subunit of adenyl cyclase. This elicits a conformational change such as to activate the catalytic subunit and stimulate cAMP synthesis. cAMP activates a protein kinase by causing its dissociation into an active catalytic subunit (R = regulatory subunit C = catalytic subunit). This in turn catalyzes the phosphorylation of two serine residues of an inactive phosphorylase b enzyme. Two dimers combine to form an active tetramer phosphorylase a. Phosphorylase catalyzes the breakdown of the sugar storage form glycogen (phosphorolysis) to glucose 1-...

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...

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 ... 

Some phosphoric acid derivatives of 2-desoxy-D-ribose have been obtained by enzymic methods of preparation. A reaction analogous to the phosphorolysis of glycogen to D-glucose 1-phosphate241 has been effected with either hypoxanthine- or guanine-D-riboside, both of which could be split by enzymic phosphorolysis with the formation of D-ribose 1-phosphate.242 The successful conclusion of these experiments prompted similar investigations with desoxyribonucleosides. 

IMP is cleaved by phosphorolysis of the nucleoside to yield hypoxanthine and ribose-1-phosphate. (This reaction is similar to the phosphorolysis of glycogen by glycogen phosphorylase.)... 

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

Glycogen phosphorylase catalyzes the sequential phosphorolysis of glycogen to release glucose 1-phosphate it is thus the key enzyme in the utilization of tissue glycogen reserves. 

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