Glycogen phosphorylase catalytic site

FIGURE 15.15 (a) The structure of a glycogen phosphorylase monomer, showing the locations of the catalytic site, the PLP cofactor site, the allosteric effector site, the glycogen storage site, the tower helix (residnes 262 throngh 278), and the snbnnit interface. 

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. 

Figure 21.6. Structure of Glycogen Phosphorylase. This enzyme forms a homodimer one subunit is shown in white and the other in yellow. Each catalytic site includes a pyridoxal-phosphate (PLP) group, linked to lysine 680 of the enzyme. The binding site for the phosphate (Pj) substrate is shown.

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 ... 

Protein phosphatase-1 (Mg +/ATP-dependent phosphatase multisubstrate protein phosphatase M.W. of catalytic subunit is 35,000 major enzyme in regulation of glycogen metabolism in skeletal muscle dephosphorylates glycogen phosphorylase, jS-subunit of phosphorylase kinase, and at least three sites of glycogen synthase regulated by inhibitor-1, inhibitor-2, and GSK-3 -t- Mg +- ATP). 

Figure 21.6 Structure of glycogen phosphorylase. This enzyme forms a homodimer one subunit is shown in white and the other in yellow. Each catalytic site includes a pyridoxal phosphate PLP group, linked to lysine 680 of the enzyme. The binding site for the phosphate (Pj) substrate is shown. Not icp that the catalytic site lies between the C-termina domain and the glycogen-binding site, A narrow crevice, which binds four or five glucose units of glycogen, connects the two sites. The separation of the sites allows the catalytic site to phosphorolyze several glucose units before the enzyme must rebind the glycogen substrate. [Drawn from INOl.pdb.]...

Crystallographic studies of the structures of rabbit muscle glycogen phosphorylase b, complexed with D-g/wco-heptenitol or a maltosaccharide plus inorganic phosphate (Pi), confirm that, the a-anomeric configuration of the product of these phosphorolytic reactions is dictated topologically. As illustrated in Fig. 3, the cnitol binds at the catalytic site of phosphorylase in a... 

Fig. 9.9. Activation of muscle glycogen phosphorylase by AMP and by phosphorylation. Muscle glycogen phosphorylase is composed of two identical subunits. The substrate binding sites in the active catalytic site are denoted by S. AMP binds to the allosteric site, a site separate from the active catalytic site. Glycogen phosphorylase kinase can transfer a phosphate from ATP to one serine residue in each subunit. Either phosphorylation or binding of AMP causes a change in the active site that increases the activity of the enzyme. The first event at one subunit facilitates the subsequent events that convert the enzyme to the fully active form.

Conformational changes have lost some of their mystical associations in recent years. From X-ray crystallography there is now an atomic-level resolution model for both the active, phosphorylated and inactive, dephosphorylated forms of the enzyme glycogen phosphorylase (11). From comparison of these structures, it is seen that the effect of phosphorylation of serine-14 of each subunit is to create an ordered helical conformation at each amino-terminus which in consequence binds more closely to the surface of the glycogen phosphorylase dimer. This produces rotation of each subunit about an axis perpendicular to the axis of symmetry of the dimer. This structural change clearly alters substrate binding at the catalytic site, even though the catalytic pyridoxal phosphates are located more than 30 A from the phosphoserine (12). 

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