Phosphorylase action mechanism

Despite continued and intensive research, the mechanism of phosphorylase action is not known, although the formation of an intermediary n-glucosyl-enzyme complex is now considered to be imlikely the role of pyridoxal 5-phosphate and the mode of activation by adenosine 5-phosphate are also unknown. The fact that adenosine 5-phosphate lowers the energy of activation of phosphorylase a suggests that it may be situated near to the active center, and further, that each molecule of phosphorylase a may contain four active centers. ... 

Stimulation of glycogen breakdown involves consumption of molecules of ATP at three different steps in the hormone-sensitive adenylyl cyclase cascade (Figure 15.19). Note that the cascade mechanism is a means of chemical amplification, because the binding of just a few molecules of epinephrine or glucagon results in the synthesis of many molecules of cyclic / MP, which, through the action of c/ MP-dependent protein kinase, can activate many more molecules of phosphorylase kinase and even more molecules of phosphorylase. For example, an extracellular level of 10 to 10 M epinephrine prompts the for-... 

Fig. 14.3 5 -FU catabolism, anabolism and mechanism of action. 5-FUH2, 5-fluoro-5,6-dihydrouracil 5-FdUMP, 5-fluorodeoxyuridine monophosphate TP, thymidine phosphorylase TK, thymidine kinase TS, thymidylate synthase CH2THF, 5,10-methylenetetrahydrofolate.

Niridazole exhibits schistosomicide and amebicidal action. The mechanism of action is not known. It seems likely that it is concentrated in the parasite organism, which causes inhibition of phosphorylase activation, which is expressed in the depletion of glycogen reserves. 

The hormonal mechanisms of action to regulate glycogenesis and glycogenol-ysis involve reversible phosphorylation of the critical enzymes glycogen synthase and phosphorylase, respectively (Figure 6—6). 

The mechanism of action of spleen exonuclease is similar to that seen for venom exonuclease (19-21) but different from the processive type of attack exhibited by E. coli RNase II, sheep kidney exonuclease, and polynucleotide phosphorylase (22, 23), in which cases each polynucleotide molecule is completely degraded before the enzymes attack a new molecule. The results of Bernardi and Cantoni (12) contradict the previous beliefs that the enzyme has an intrinsic, though weak, endonucleolytic activity (5) and that a phosphate group in a terminal 5 position makes a polynucleotide chain completely resistant to the enzyme (15, 24, 25). 

The coordinated mechanism proposed here for debrancher action on phosphorylase limit dextrin, although hypothetical, is consistent with the evidence that the debrancher has two distinct catalytic sites and a polymer binding site or sites that overlap or interact strongly. It accounts for the observed inhibition patterns with limit dextrin and glucose incorporation and simul-... 

The polysaccharide phosphoTylases and the other depolymerizing enzymes differ in both mechanism of action and structure, although it is generally accepted that phosphorylases share the general acid catalysis. 

The phosphate group of pyridoxal 5-phosphate is not believed to play a direct part in the reaction, as no exchange can be found between it and inorganic phosphate. However, two detailed mechanisms for the action of muscle phosphorylase have been proposed these directly involve the p)oidoxal 5-phosphate. ... 

Phosphonomannans, fungal, 394,417 Phosphorus, in yeast mannan, 391 Phosphorylases active sites of, 346 amino acids of, 345 kinetics, 289,353 mechanism of action of, 356 properties of, 347... 

Action patterns Adhesion domains Affinity and mechanism-based inhibitors Catalytic domains Classification schemes Hydrolases Lyases Mechanism of action Phosphorylases Proteinaceous inhibitors... 

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