GLYCOGEN SYNTHASE
This is a tetrameric enzyme comprising four identical subunits. The enzyme may exist in either the phosphorylated or the de-phosphorylated form.
The phosphorylated form (known as glycogen synthase D) contains a (reversibly) bound phosphate at one of the serine residues on each subunit. This version of the enzyme is less active. It predominates during muscle contraction. Glycogen synthase is phosphorylated by protein kinases. The activity of this version of the synthase enzyme is dependent on the [glucose-6-phosphate], hence the "D" in the name. When [glucose-6-phosphate] > 1mM, the D-form is active. This is because glucose-6-phosphate is an allosteric effector -by binding to a particular site on the synthase enzyme it increases the affinity of the enzyme for its substrate - UDP glucose.
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