Glycogenolysis regulation

The existence of two forms of phosphorylase with different catalytic activities, which were capable of being enzymatically interconverted, suggested that the two forms might be involved in regulation of glycogenolysis in muscle tissue. However, in the early studies, whenever phosphorylase was assayed in extracts of muscle it was always found to be in the a form. This was the case even in extracts prepared from... 

It is important to point ont that these four mechanisms are not mntuaUy exclnsive. Indeed, it is probable that, for some reactions, aU fonr mechanisms play a role in regnlation of flnx and this combination conld provide an enormons increase in sensitivity. An example is the regulation of the enzyme phosphorylase in mnscle and liver, and hence the process of glycogenolysis (Chapters 6 and 12). 

Major effector proteins for G-pro-tein-coupled receptors include adenylate cyclase (ATP intracellular messenger cAMP), phospholipase C (phos-phatidylinositol intracellular messengers inositol trisphosphate and di-acylglycerol), as well as ion channel proteins. Numerous cell functions are regulated by cellular cAMP concentration, because cAMP enhances activity of protein kinase A, which catalyzes the transfer of phosphate groups onto functional proteins. Elevation of cAMP levels inter alia leads to relaxation of smooth muscle tonus and enhanced contractility of cardiac muscle, as well as increased glycogenolysis and lipolysis (p. 

Glycogen stores are regulated by a balance between glycogen synthesis (glycogenesis) and breakdown (glycogenolysis). 

The final step in signal transduction is the action of cAMP on the regulatory subunit of the enzyme, protein kinase A. This ubiquitous enzyme then phosphorylates and activates enzymes with functions specific to different cells and organs. In fat cells, protein kinase A activates lipase, which mobilizes fatty acids in muscle and liver cells, it regulates glycogenolysis and glycogen synthesis. 

Pancreatic polypeptide Polypeptide (36 residues) Glycogenolysis, gastrointestinal regulation... 

Serebrennikova, T.P., Silkina, E.N., Shmelev, V.K., Morozova, A.L. and Nesterov, V.P. (1991). The regulation of glycogenolysis in skeletal muscle of annular bream during intensive performance as dependent on temperature (In Russian). Zhumal Evolutsionnoy Biokhimii i Physiologii 27,427-431. 

Since phosphorylase kinase not only activates phosphorylase, but also phospho-rylates glycogen synthase thereby decreasing its activity, the regulation of phosphorylase kinase by calcium may also provide a mechanism for co-ordinating the rates of glycogenolysis and glycogen synthesis during muscle contraction. 

Autonomic outputs. Hypoglycaemia and hypothermia both lead to snstained sympathetic responses. Subjects feel hungry and eat if possible, bnt they refine their other actions to suit the circumstances. Hypoglycaemia requires hepatic glycogenolysis and glnconeogenesis, while hypothermia requires increased heat prodnction and a redistribntion of blood flow. Sympathetic activity is controlled by the hypothalamns, which instrncts the adrenal medulla to secrete adrenalin. This is a rather blunt control, and so localised sympathetic responses (such as blood flow regulation) are mediated by individnal nerves. Parasympathetic activity can also respond to the hypothalamus, which controls the nnclens of the solitary tract. 

In the course of studies designed to elucidate the mechanism by which cyclic AMP regulates phosphorylase activation and glycogenolysis in... 

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