Dephosphorylation, indirect

Both phosphorylase a and phosphorylase kinase a are dephosphorylated and inactivated by protein phos-phatase-1. Protein phosphatase-1 is inhibited by a protein, inhibitor-1, which is active only after it has been phosphorylated by cAMP-dependent protein kinase. Thus, cAMP controls both the activation and inactivation of phosphorylase (Figure 18-6). Insulin reinforces this effect by inhibiting the activation of phosphorylase b. It does this indirectly by increasing uptake of glucose, leading to increased formation of glucose 6-phosphate, which is an inhibitor of phosphorylase kinase. 

Renewed docking of the catalytic subunit requires the removal of the phosphate residue at the G subunit phosphorylated at the P2 site. This takes place via the protein phosphatases 2A and 2B (calcineurin). Thus, a cascade of protein phosphatases is involved in the regulation of dephosphorylation of key enzymes of glycogen degradation, whereby a phosphatase, namely protein phosphatase I, is indirectly activated by other protein phosphatases. With calcineurin, a Ca -dependent protein phosphatase is involved and thus it is possible to influence glycogen metabolism via Ca -mediated signals. 

Some saponins from plants are quite phytotoxic,36 but little is known of their mode(s) of action. Some of these compounds have effects on membrane properties, due to their detergent-like activity. However, all membrane effects are apparently not due to nonspecific effects on membrane lipids. Kauss and Jeblick23 provided indirect evidence that digitonin effects on plant cell Ca2+ uptake are due to effects on membrane protein phosphorylation/dephosphorylation processes. Betulin (Fig. 10.1), a phytotoxic saponin,22 is known for its pharmaceutical activity. It is an inhibitor of DNA topoisomerase.45 The natural product-based DNA topoisomerase inhibitor drug podophyllotoxin is also phytotoxic.37... 

Glucagon decreases cholesterol synthesis in isolated hepatocytes [131,132] apparently because it reduces the fraction of hydroxymethylglutaryl-CoA reductase in the active form [131,132], This is due to an increase in reductase kinase activity [133], However, there is no evidence that cAMP-dependent protein kinase phos-phorylates either the reductase, reductase kinase or reductase kinase kinase [134], It has been proposed that the phosphorylation state of these enzymes is indirectly controlled through changes in the activity of protein phosphatase I [132,134], This phosphatase can dephosphorylate and activate the reductase [134,135] and its activity can be controlled by a heat stable inhibitor (inhibitor 1), the activity of which is increased by cAMP-dependent phosphorylation [136,137], Since the phosphorylated forms of acetyl-CoA carboxylase, ATP-citrate lyase, pyruvate kinase, phos-phorylase, phosphorylase kinase and glycogen synthase are also substrates for protein phosphatase I [135], this mechanism could also contribute to their phosphorylation by glucagon. 

The phase difference between the saturation function of the receptor and extracellular cAMP depends on the value of parameter c, which measures the relative affinity of the ligand for the two receptor states. A negligible phase shift should be observed as long as the R state possesses a higher affinity for cAMP (c < 1) in the opposite case, the saturation function should reach its maximum after the peak in cAMP. A detailed experimental study, not yet available, would permit us to obtain information on that point and could indirectly )neld indications as to the value of coefficient c. The curves in fig. 5.30a and b also show that for the parameter values chosen (and, in particular, for c = 10), the maximum of the fraction of active, dephosphorylated receptor shortly precedes the maximum in cAMP, while the increase in the fraction of desensitized receptor follows the peak in cAMP. 

At present, little is known about the phosphatidate phosphohydrolase step in the biosynthetic pathway to triacylglycerols. The enzyme was specifically measured by Moore et al, (1973) in subcellular fractions of castor bean endosperm and found to be located mainly in the endoplasmic reticulum, with some activity in the soluble fraction. In leaves, the highest specific activity was found in an undefined particulate fraction, whereas the m or part of the activity was soluble (Heinz, 1977). Other evidence for a specific phosphatidate phosphohydrolase activity in chloroplast membranes was indirect (Joyard and Douce, 1977). Care must always be taken to ensure that the activity is not due to a nonspecific phosphatase in view of the report by Blank and Snyder (1970) that wheat germ contains an acid phosphatase capable of dephosphorylating phosphatidic acid. The enzyme described by Moore et al. 

Effects of insuiin on phosphoryiation and activity of various enzymes and proteins. The effects are indirect, i.e. Insuiin binds to its receptor on the cell membrane, initiating a chain of events, culminating in the phosphorylation (kinase reaction) or dephosphorylation (phosphatase action) of the enzyme. 

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