Glycogen hormonal control

Glycogen degradation and glycogen synthesis are controlled both by allosteric regulation and by hormonal control. 

Hormonal control Glycogen metabolism is tightly controlled by hormones. When blood glucose by epinephrine levels fall, glucagon is secreted by the a cells of the pancreas and acts on the and glucagon liver to stimulate glycogen breakdown to glucose which is then released into... 

The glycogenesis and glycogenolysis enzymes subject to hormonal control are glycogen synthase and phosphorylase, respectively. Briefly, glycogen synthase is inhibited by high cellular cAMP and Ca2+ levels, whereas phorphorylase is stimulated. The mechanisms for accomplishing this are quite complex. 

How is the enzyme dephosphorylated and activated Insulin stimulates the carboxylase by causing its dephospkorylation. It is not clear which of the phosphatases activates the carboxylase in response to insulin. The hormonal control of acetyl CoA carboxylase is reminiscent of that of glycogen synthase (Section 21.5.2). 

In addition to the activator/inhibitor controls of citrate and fatty ac.yl-CoA, the enzyme is also under hormonal control (note the similarities to glycogen control) ... 

Figure 21.14 Pathway Integration Hormonal control of glycogen breakdown. Glucagon stimulates liver glycogen breakdown when blood glucose is low. Epinephrine enhances glycogen breakdown in muscle and the liver to provide fuel for muscle contraction.

Skeletal muscle cells lack glucagon receptors. Hormonal control of glycogen degradation is achieved by epinephrine via P-adrenergic activation of adenylate cyclase, resulting in enhanced cytoplasmic cyclic AMP levels. Neural activation of skeletal muscle cells considerably increases the cytoplasmic Ca level. Cyclic AMP and Ca " act in a synergistic fashion to fully express the activity of glycogen phosphorylase in the process described above (Devlin, 1992). 

Short-term hormonal control of protein phosphatases involved in hepatic glycogen metabolism, W. Stalmans, M. Bollen, B. Toth, and P. Gergely, 1990, Adv. Enzyme Reg., 305327. 

H) In vivo studies The studies in vitro discussed above clearly showed that for mammalian glycogen synthase (or at least for the rabbit muscle enzyme) the important phosphorylation sites for inactivation were site 2 and (2a SerlO), sites 3a and 3b. However, it is of interest to know which are the important phosphorylation sites under hormonal control in vivo. In rabbit muscle enzyme, intravenous insulin administration doubles the Glc-6-P activity ratio, with a decrease in phosphorylation of all the sites. " " Epinephrine, which increases the phosphate content of the glycogen synthase, increases the phosphorylation at practically all the phosphorylation sites." " " ... 

As was the case with glycolysis and gluconeogenesis, it would be futile for the cell to carry out glycogen synthesis and degradation simultaneously. The results achieved by the action of one pathway would be imdone by the other. This problem is avoided by a series of hormonal controls that activate the enzymes of one pathway while inactivating the enzymes of the other pathway. 

This elegant system of hormonal control ensures that the reactions involved in glycogen degradation and synthesis do not compete with one another. In this way they provide glucose when the blood level is too low, and they cause the storage of glucose in times of excess. 

ATP + glycogen phosphorylase <1> (conversion to an AMP-independent form, key enzyme of neural and hormonal control of glycogen metabolism) [29]... 

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