Glycogen intracellular concentration

Glycogen synthesis is important in two tissues, muscle and liver. In muscle the major factors regulating the rate of synthesis are insulin and the amount of glycogen already present in the muscle. In liver, the major factor is the intracellular concentration of glucose (see below) (Figure 6.19). 

Figure 6.31 (a) An increase in the intracellular concentration of glucose in the liver results in an increased activity of glycogen synthase and a decreased activity of glycogen phosphorylase. The mechanisms for these effects are shown in (b). 

Mechanisms. cAMP plays a central role as a second messenger in the regulation of cell metabolism (14). Binding of cAMP with the regulatory subunit of a protein kinase initiates a cascade of enzymatic reactions that ultimately lead to the breakdown of glycogen and release of glucose to the blood stream. The intracellular concentration of cAMP represents a balance between the action of adenylate cyclase (which produces cAMP... 

Protein kinase A provides a means for hormones to control metabolic pathways. Adrenaline and many other hormones increase the intracellular concentration of the allosteric regulator 3, 5 -cychc AMP (cAMP), which is referred to as a hormonal second messenger (Fig. 9.10). cAMP binds to regulatory subunits of protein kinase A, which dissociate and release the activated catalytic subunits (Fig. 9.11). Dissociation of inhibitory regulatory subunits is a common theme in enzyme regulation. The active catalytic subunits phosphorylate glycogen phosphorylase and other enzymes at serine residues. 

Figure 23 Intracellular concentration of glycogen and lipids in a lager yeast strain during fermentation of a 15 Plato wort.

Aspartate is thought to reduce ammonia or increase the TCA cycle flux, and then deliver the potassium and magnesium to the subcellular locations to normalize intracellular concentrations." These conditions could increase fatty acid oxidation, spare glycogen, and reduce ammonia-induced fatigue — and thus increase the time to exhaustion. However, there are no proven action mechanisms. 

Other nucleoside 5 -triphosphates demonstrate a standard free energy of hydrolysis equivalent to that of ATP. Their intracellular concentrations are low which restricts their function to selected biosynthetic pathways, e.g. uridine triphosphate (UTP) in glycogen biosynthesis (Section 11.5). Nucleoside-diphosphate kinase permits the phosphorylation of (d)NDPs to (d)NTPs at the expense of ATP and vice versa. 

Potassium is the second most abundant cation in the body and is found primarily in the intracellular fluid. Potassium has many important physiologic functions, including regulation of cell membrane electrical action potential (especially in the myocardium), muscular function, cellular metabolism, and glycogen and protein synthesis. Potassium in PN can be provided as chloride, acetate, and phosphate salts. One millimole of potassium phosphate provides 1.47 mEq of elemental potassium. Generally, the concentration of potassium in peripheral PN (PPN) admixtures should not exceed 80 mEq/L (80 mmol/L). While it is safer to also stick to the 80 mEq/L (80 mmol/L) limit for administration through a central vein, the maximum recommended potassium concentration for infusion via a central vein is 150 mEq/L (150 mmol/L).14 Patients with abnormal potassium losses (e.g., loop or thiazide diuretic therapy) may have higher requirements, and patients with renal failure may require potassium restriction. 

Glycogen and its enzymes are compartmentalized. Glycogen granules are only present in astrocytes of adult animals but are found in both astrocytes and neurons of immature animals. Of the enzymes involved in glycogen metabolism, glycogen phosphorylase is found in astrocytes only. Under steady-state conditions, it is probable that less than 10% of phosphorylase in brain is in the unphosphorylated b form (requiring AMP). This form is probably not very active at the low AMP concentrations present when intracellular glucose is sufficient to maintain ATP synthesis. Brain phosphorylase b kinase is activated indirectly by cAMP and by the molar concentrations... 

Phosphorylation also can modify an enzyme s sensi-tivity to allosteric effectors. Phosphorylation of glycogen phosphorylase reduces its sensitivity to the allosteric activator adenosine monophosphate (AMP). Thus, a covalent modification triggered by an extracellular signal can override the influence of intracellular allosteric regulators. In other cases, variations in the concentrations of intracellular effectors can modify the response to the covalent modification, depending on the metabolic state of affairs in the cell. 

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