Enzyme glycogen phosphorylase

Figure 3.29 Control of an enzyme activity by multiple allosteric regulators. The enzyme glycogen phosphorylase b in muscle is regulated by changes in the concentrations of AMP and inosine monophosphate (IMP) (which are activators) and ATP and glucose 6-phosphate (G6P), which are inhibitors.

Pyridoxine is present in food in the free form and as a glucoside, which may undergo partial hydrolysis in the gut lumen, or may be absorbed intact. Although pyridoxine is associated with the enzyme glycogen phosphorylase in muscles, it is not released in response to a dietary deficiency therefore it cannot be regarded as a storage form of the vitamin. 

Fmax of the enzyme glycogen phosphorylase from skeletal muscle is much greater than the I max of the same enzyme from liver tissue. 

Glycogen degradation requires two enzymes glycogen phosphorylase and glycogen-debranching enzyme. 

Fig. 6.8 Assuming a constant fluxJcNC from precursors to the pool of G6P, the hepatic glucose output HGO is more or less correlated to the changes in glycogen stores. The buildup of glycogen from G6P is controlled by the enzyme glycogen synthase, GgS, and the breakdown of glycogen is controlled by the enzyme glycogen phosphorylase, GgP.

Enzyme Activity and Physiological Function The Vmax of the enzyme glycogen phosphorylase... 

Adrenaline exerts its effect by binding to a receptor site on the cell surfaces of liver and muscle cells, where it initiates a series of signals that ultimately causes an inactive form of the enzyme glycogen phosphorylase to become active. This enzyme is the first in a sequence that leads to the breakdown of glycogen to glucose and other products. 

In the liver and muscle cells, another enzyme, glycogen phosphorylase, breaks down glycogen into glucose molecules, which then enter glycolysis. 

Epinephrine promotes the conversion of glycogen to glucose by activating adenylate cyclase, an enzyme whose product, cAMP, initiates a reaction cascade that activates the glycogen degrading enzyme glycogen phosphorylase. 

Inhibitors of the enzyme glycogen phosphorylase have been studied for their ability to lower blood glucose levels.35 37 In order to study the nature of the binding of glucose derived substrates to glycogen phosphorylase, the product shown in Scheme 1.4.438 was complexed to the enzyme and studied via x-ray crystallography.39 Aside from demonstrating reasonable inhibitors, the... 

Step 1. The enzyme glycogen phosphorylase catalyzes phosphorolysis of a... 

A difference Fourier map was calculated for the enzyme glycogen phosphorylase b using synchrotron Laue data (Hajdu et al (1987b, 1988), see figure 7.11). The map showed electron density at the binding site for bound maltoheptose in phosphorylase b crystals. The phosphorylase b difference Fourier maps calculated for the Laue case used fractional intensity changes (difference ratio method) as discussed earlier in section 7.4. 

Glycogen phosphorylase b is the less active form of glycogen phosphorylase. It differs from glycogen phosphorylase a in that it is not phosphorylated and that it requires AMP for activity. Glycogen phosporylase b is a substrate for the enzyme glycogen phosphorylase b kinase, which converts the b form to the a form by adding two phosphates. The reaction is stimulated in the presence of calcium via interaction of calmodulin with glycogen phosphorylase b kinase... 

The a form is derived from the b form by phosphorylation of the b form by the enzyme glycogen phosphorylase b kinase (Figure 13.18). 

The breakdown of glycogen (glycogenolysis) requires two enzymes, glycogen phosphorylase and (... 

The fact that two target enzymes, glycogen phosphorylase and glycogen synthase, are modified in the same way by the same enzymes links the opposing processes of synthesis and breakdown of glycogen even more intimately. 

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