Phosphorylase regulation

Increasing sophistication of glycogen phosphorylase regulation Section 21,3.3 The a-amylase family Section 21,4.3... 

Structural Insights, Glycogen Phosphorylase, looks closely at the structural mechanisms of phosphorylase regulation, examining the efects of allosteric effectors and serine phosphorylation. 

Figure 21.10. Phosphorylase Regulation. Both phosphorylase h and phosphorylase a exist as equilibria between an...

Figure 21.10 Phosphorylase regulation. Both phosphorylase b and phosphorylase a exist as equilibria between an active R state and a less-active T state. Phosphorylase b is usually inactive because the equilibrium favors the T state. Phosphorylase a is usually active because the equilibrium favors the R state. Regulatory structures are shown in blue and green.

How does the regulation of phosphorylase in the liver differ from the scheme for phosphorylase regulation in muscle shovm in Figure 21.2 ... 

Glycogen Phosphorylase Allosteric Regulation and Covalent Modification 473... 

The principal enzymes controlling glycogen metabolism—glycogen phosphorylase and glycogen synthase— are regulated by allosteric mechanisms and covalent modifications due to reversible phosphorylation and... 

Glycogen Synthase Phosphorylase Activity Are Reciprocally Regulated (Figure 18-7)... 

REGULATION OF GLYCOGEN METABOLISM IS EFFECTED BY A BALANCE IN ACTIVITIES BETWEEN GLYCOGEN SYNTHASE PHOSPHORYLASE (Figure 18-8)... 

The biosynthesis of purines and pyrimidines is stringently regulated and coordinated by feedback mechanisms that ensure their production in quantities and at times appropriate to varying physiologic demand. Genetic diseases of purine metabolism include gout, Lesch-Nyhan syndrome, adenosine deaminase deficiency, and purine nucleoside phosphorylase deficiency. By contrast, apart from the orotic acidurias, there are few clinically significant disorders of pyrimidine catabolism. 

Phosphorylase was studied in depth. The enzyme from muscle was different from that catalyzing the same reaction in liver. Muscle phosphorylase but not that from liver, was activated by AMP, an early example of enzyme regulation by a ligand which was not a substrate. [Allosteric regulation was not postulated until the work of Jacob and... 

A well-known example, indeed the first enzyme that was shown to be regulated by the phosphorylation/ dephosphorylation mechanism, is glycogen phosphorylase, which catalyses the breakdown of glycogen (Box 3.7). 

The existence of two forms of phosphorylase with different catalytic activities, which were capable of being enzymatically interconverted, suggested that the two forms might be involved in regulation of glycogenolysis in muscle tissue. However, in the early studies, whenever phosphorylase was assayed in extracts of muscle it was always found to be in the a form. This was the case even in extracts prepared from... 

Figure 3.12 The regulation of phosphorylase activity by reversible phosphoiylation. A reversible phosphorylation process is also known as an interconversion cycle the latter term is preferred in this text, since the individual reactions must be irreversible, which can be confusing if the term reversible is used to describe the overall process. In resting muscle, almost all phosphorylase is in the b form.

Figure 3.29 Control of an enzyme activity by multiple allosteric regulators. The enzyme glycogen phosphorylase b in muscle is regulated by changes in the concentrations of AMP and inosine monophosphate (IMP) (which are activators) and ATP and glucose 6-phosphate (G6P), which are inhibitors.

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