Glycogen phosphorylase activity

The cAMP formed by adenylyl cyclase (Figure 15.20) does not persist because 5 -phosphodiesterase activity prevalent in cells hydrolyzes cAMP to give 5 -AMP. Caffeine inhibits 5 -phosphodi-esterase activity. Describe the effects on glycogen phosphorylase activity that arise as a consequence of drinking lots of caffeinated coffee. 

Enzymes have evolved such that their values (or A o.s values) for substrate(s) are roughly equal to the in vivo concentration(s) of the substrate (s). Assume that glycogen phosphorylase is assayed at [P[] = A o.s in the absence and presence of AMP or ATP. Estimate from Figure 15.15 the relative glycogen phosphorylase activity when (a) neither AMP or ATP is present, (b) AMP is present, and (c) ATP is present. 

FIGURE 6-31 Regulation of glycogen phosphorylase activity by covalent modification. In the more active form of the enzyme, phosphorylase a, specific Ser residues, one on each subunit, are phosphorylated. Phosphorylase a is converted to the less active phosphorylase b by enzymatic loss of these phosphoryl groups, promoted by phosphorylase phosphatase. Phosphorylase b can be reconverted (reactivated) to phosphorylase a by the action of phosphorylase kinase. 

Correct answer = B. Epinephrine and glucagon both cause increased glycogen degradation in the liver. Therefore, glycogen phosphorylase activity is increased, whereas glycogen synthase activity is decreased. Both cAMP-dependent protein kinase and its substrate, phosphorylase kinase, are also activated. 

Fig. 5.37. Modulation of glycogen phosphorylase activity by covalent modulation...

DiMarco JP, Miles W, Akhtar M, Milstein S, Sharma AD, Platia E, McGovern B, Scheinmman MM, Govier WC (1990) Adenosine for paroxysmal supraventricular tachycardia dose ranging and comparison with verapamil. Assessment in placebo-controlled, multicenter trials. The Adenosine for PSVT Study Group. Ann Intern Med 113(2) 104-110 Dobson JG (1978) Reduction by adenosine of the isoproterenol-induced increase in cyclic 3 , 5 -monophosphate formation and glycogen phosphorylase activity in rat heart muscle. Circ Res 43(5) 785-792... 

Fig. 2. Regulation of (a) glycogen phosphorylase activity and (b) glycogen synthase activity by phosphorylation (covalent modification).

In this laboratory exercise, you will study the effects of phosphorylation and allosteric regulation on the activity of glycogen phosphorylase. In the first experiment, you will phosphorylate glycogen phosphorylase b in vitro using y-[32P]ATP and glycogen phosphorylase kinase. In the second experiment, you will study the effect of phosphorylation on glycogen phosphorylase activity, as well as the effect of AMP on glycogen phosphorylase b activity with the use of a coupled enzymatic (kinetic) assay (Fig. 15-3). 

Figure 15-3 Coupled enzyme assay to determine glycogen phosphorylase activity. As long as the activity of phosphoglucomutase and glucose-6-phosphate dehydrogenase are not rate-limiting, the AA340/At is directly proportional to the A[glucose-l -phosphate]/Atime.

Day 2 In Vitro Assay for Phosphorylase Kinase and Glycogen Phosphorylase Activity... 

Determine the glycogen phosphorylase activity present in tubes D to J using the following equation ... 

NOTE No dilution factor (20) is required for the calculation of glycogen phosphorylase activity in tubes H to J, since the enzyme stock solution was added directly to the assay. Otherwise, the same formula can be used for tubes H to J. 

Compare the glycogen phosphorylase activity between the reactions in tubes I and J. What effect does AMP have on the activity of glycogen phosphorylase b Can you estimate how much the activity of the enzyme is increased or decreased in the presence of this allosteric effector ... 

Muscle pyridoxal phosphate is released into the circulation (as pyridoxal) in starvation as muscle glycogen reserves are exhausted and there is less requirement for glycogen phosphorylase activity. Under these conditions, it is potentially available for redistribution to other tissues, especially the liver and kidneys, to meet the increased requirement for gluconeogenesis from amino acids (Black et al., 1978). However, during both starvation and prolonged bed rest, there is a considerable increase in urinary excretion of 4-pyridoxic acid, suggesting that much of the vitamin Be released as a result of depletion of muscle glycogen and atrophy of muscle is not redistributed, but rather is ca-tabolized and excreted (Cobum et al., 1995). 

The molecular basis of HFI involves the deficiency of normal D-fructose 1-phosphate aldolase activity, and low activity of D-fructose 1,6-bisphosphate aldolase in liver, intestine, and renal cortex. Accumulated D-fructose 1-phosphate leads to a diminution of liver-glycogen phosphorylase activity, causing severe hypoglycemia. An accumulation of D-fructose 1-phosphate produces renal acidification and subcellular pathology of the jejunum and liver. 

Maltodextrin phosphorylase and glycogen phosphorylase (Reaction (6)) occur in many bacteria and catalyzes the synthesis as well as the phosphorolysis of a-4-glucosidic linkages present in glycogen or starch. Maltodextrin phosphorylase however is only induced in the presence of maltodextrins, and for the microorganisms studied, glycogen phosphorylase activity is insufficient to account for their rate of glycogen accumulation. Moreover,... 

ACTIVE FIGURE 7.9 Glycogen phosphorylase activity is subject to allosteric control and covalent modification via phosphorylation. The phosphorylated form is more active. The enzyme that puts a phosphate group on phosphorylase is called phosphorylase kinase. Sign in at www.thomsonedu. com/login to explore an interactive version of this figure. 

Figure 18.5 summarizes some of the salient control features that affect glycogen phosphorylase activity. The enzyme is a dimer that exists in two forms, the inactive T (taut) form and the active R (relaxed) form. In the T form (and only in the T form), it can be modified by phosphorylation of a specific serine residue on each of the two subunits. The esterification of the serines to phosphoric acid is catalyzed by the enzyme phosphorylase kinase the dephosphorylation is catalyzed by phosphoprotein phosphatase. The phosphorylated form of glycogen phosphorylase is called phosphorylase a, and the dephosphorylated form is called phosphorylase h. The switch from phosphorylase b to phosphorylase a is the major form of control over the actiHty of phosphorylase The response time of the changes is on the order of seconds to nunutes. Phosphoiylase is... 

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