Glycogen glucose-6-phosphate effect

Figure 22.17 Summary of mechanisms to maintain the ATP/ADP concentration ratio in hypoxic myocardium. A decrease in the ATP/ADP concentration ratio increases the concentrations of AMP and phosphate, which stimulate conversion of glycogen/ glucose to lactic acid and hence ATP generation from glycolysis. The changes also increase the activity of AMP deaminase, which increases the formation and hence the concentration of adenosine. The latter has two major effects, (i) It relaxes smooth muscle in the arterioles, which results in vasodilation that provides more oxygen for aerobic ATP generation (oxidative phosphorylation). (ii) It results in decreased work by the heart (i.e. decrease in contractile activity), (mechanisms given in the text) which decreases ATP utilisation.

Figure 6.19 Regulation of the synthesis of glycogen from glucose in liver and muscle. Insulin is the major factor stimulating glycogen synthesis in muscle it increases glucose transport into the muscle and the activity of glycogen synthase, activity which is also activated by glucose 6-phosphate but inhibited by glycogen. The latter represents a feedback mechanism and the former a feedforward. The mechanism by which glycogen inhibits the activity is not known. The mechanism for the insulin effect is discussed in Chapter 12.

The dephosphorylated form of the carboxylase does not require citrate for activity, but the phosphorylated form of the enzyme can be activated by citrate in vitro. This reaction is reminiscent of the effect of glucose-6-phosphate on glycogen synthase. The active, dephosphorylated form of glycogen synthase has only a small requirement for glucose-6-phosphate, whereas high concentrations of this activator are required to activate the phosphorylated form of glycogen synthase. 

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. 

Anthelmintic drugs can act in a variety of ways, by influencing the metabolism of the parasite (e.g. dismption of glucose and glycogen metabolism) or neuromuscular effects (e.g. inhibition of acetyl cholinesterase - organo-phosphates). The compounds are usually administered to animals orally, by injections, as feed additives or in the form of pour-on preparations. 

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