Glycogen is a highly branched polysaccharide of glucose units, serving as a storage form of glucose in animals. Liver and muscle provide familiar examples. The classical second messenger experiments measured regulation of an enzyme involved in glycogen breakdown, glycogen phosphorylase. [ref]
https://openstax.org/books/biology-2e/pages/3-2-carbohydrates. Clark, Douglas and Choi, Biology 2e, 3.2: glycogen
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