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Magazine Chapter 12: Q18CP (page 391)
Describe how phosphorylation and dephosphorylation control the activity of glycogen phosphorylase
Short Answer
The phosphorylation of amino acid residue - Serine 14 promotes the conformational change of glycogen phosphorylase鈥檚 inactive (T form) to its active form (R form).
Step by step solution
Introduction
Glycogen Phosphorylase exists in two forms.A phosphoryl group is esterified to Serine 14 in the phosphorylated state of the enzyme, phosphorylase a. Phosphorylase b is the dephosphorylated form of the enzyme.
The enzymatically inactive T state and the enzymatically active R state are the two conformational states of glycogen phosphorylase.
Activation of phosphorylase b
The enzyme can take on either the enzymatically inactive T conformation (shown above) or the catalytically active R conformation (shown below). The effectors ATP, AMP, and G6P allosterically regulate the conformation of phosphorylase b, which is usually in the T state under physiological conditions.
Activation of phosphorylase a
The phosphorylated form of this enzyme, phosphorylase a, on the other hand, is insensitive to these effectors and is generally in the R state unless there is a lot of glucose available. Thus, glycogen phosphorylase鈥檚 enzymatic activity is substantially governed by its phosphorylation and dephosphorylation rates under normal physiological conditions.
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