Glycogen phosphorylase catalytic activity

The next key point is to realize that each enzyme in the pathway exists in both active and inactive forms. cAMP initiates a cascade of reactions by activating protein kinase A (PK-A)," the active form of which activates the next enzyme in the sequence, and so on. At the end of the day, glycogen phosphorylase is activated and glucose or ATP is produced. This signaling pathway is a marvelous amplification system. A few molecules of glucagon or adrenaline may induce formation of many molecules of cAMP, which may activate many of PK-A, and so on. The catalytic power of enzymes is magnified in cascades of this sort. 

The breakdown of glycogen in skeletal muscles and the liver is regulated by variations in the ratio of the two forms of glycogen phosphorylase. The a and b forms differ in their secondary, tertiary, and quaternary structures the active site undergoes changes in structure and, consequently, changes in catalytic activity as the two forms are interconverted. 

In the late 1930s, Carl and Gerty Cori (Box 15-1) discovered that the glycogen phosphorylase of skeletal muscle exists in two interconvertible forms glycogen phosphorylase a, which is catalytically active, and... 

The major substrate of phosphorylase b kinase is phosphorylase b which is phos-phorylated on a single serine residue at position 14, resulting in conversion to the more catalytically active form phosphorylase a [70], Phosphorylation of skeletal muscle phosphorylase also results in conversion of the Mr 200000 dimeric b form to the Mr 400000 tetrameric a form, whereas phosphorylation of the liver enzyme does not alter its dimeric structure [82]. Phosphorylase a is much less dependent than phosphorylase b upon the allosteric activator AMP [82], Since the activity of phosphorylase is rate-limiting for glycogen breakdown, its activation by phosphorylase b kinase results in enhanced glycogenolysis and glucose release from the liver. 

Unlike other pyridoxal phosphate-dependent enzymes, in which it is the carbonyl group that is essential for catalysis, the internal Schiff base between pyridoxal phosphate and lysine in glycogen phosphorylase can be reduced with sodium borohydride without affecting catalytic activity. Thus, while pyridoxal phosphate is essential for phosphorylase activity, it does not act by the same kind of mechanism as in amino acid metabolism. 

The catalytic activity of enzymes is controlled in several ways. Reversible allosteric control is especially important. For example, the first reaction in many biosynthetic pathways is allosterically inhibited by the ultimate product of the pathway. The inhibition of aspartate trans carbamoyl as e by cytidine triphosphate (Section 10.1) is a well-understood example offeedback inhibition. This type of control can be almost instantaneous. Another recurring mechanism is reversible covalent modification. For example, glycogen phosphorylase, the enzyme catalyzing the breakdovm of glycogen, a storage form of sugar, is activated by phosphorylation of a particular serine residue when glucose is scarce (Section 21.2.1). 

Glycogen phosphorylase b is an enzyme that catalyzes the phospho-lytic breakdown of glycogen to glucose-1-phosphate (Equation 18.11 and Figure 18.17). Crystals of the phosphorylase are catalytically active, but the reaction is too fast to study directly by diffraction methods. A gly-cosylic substrate analogue, heptenitol, is slowly converted to heptulose-2-phosphate, presumably by the same mechanism. 

Pyridoxal 5 -phosphate (PLP) was noticed to be a constituent of rabbit muscle phosphorylase in 1957, and since that time it has been shown that all a-glucan phosphorylases which give phosphorolysis products with retention of configuration contain PLP. The exact role of the PLP is still not known, though it has been shown that these a-glucan phosphorylases have an absolute requirement for PLP and that the Schiflfbase formed between PLP and glycogen phosphorylase can be reduced with borohydride without eliminating the catalytic activity of the enzyme. The P n.m.r. spectrum of PLP bound to phosphorylase b shows that deprotonation of the 5 -... 

Enzyme activity can be regulated by covalent modification or by noncovalent (allosteric) modification. A few enzymes can undergo both forms of modification (e.g., glycogen phosphorylase and glutamine synthetase). Some covalent chemical modifications are phosphorylation and dephosphorylation, acetylation and deacetylation, adeny-lylation and deadenylylation, uridylylation and deuridyly-lation, and methylation and demethylation. In mammalian systems, phosphorylation and dephosphorylation are most commonly used as means of metabolic control. Phosphorylation is catalyzed by protein kinases and occurs at specific seryl (or threonyl) residues and occasionally at tyrosyl residues these amino acid residues are not usually part of the catalytic site of the enzyme. Dephosphorylation is accomplished by phosphoprotein phosphatases ... 

Mobilization of glycogen stores also requires the participation of a debranching enzyme because phosphorylase ceases to cleave a-l,4-glycosidic linkages four glucosyl residues from an a-1,6-branch site. The debranching enzyme has two catalytic activities a transferase activity and a glucosidase... 

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