A polysaccharide called glycogen plays a key function in energy storage in animals. It is a homoglycan of glucose and is highly branched for efficient storage and release of glucose. Glycogen is mainly stored in the liver and muscle cells, the liver being the regulator of blood glucose levels and the muscle requiring the breakdown of glucose for energy for muscle contractions. Glycogen is synthesized through three main enzymes. Glycogenin forms glucose primers of at least four monomers, catalyzing the addition of glucose chains to its aspartic acid R-groups. Glycogenin is required to start all new glycogen molecules. Then, glycogen synthase catalyzes the addition of glucose to the primer one at a time through alpha (1,4) bonds. Next, the branching enzyme forms the branches of glycogen. It transfers a minimum of seven-unit chains of glucose, breaking an alpha (1,4) bond, and connects them to another chain through an alpha (1,6) bond. On the other hand, glycogen degradation occurs using two main enzymes. Glycogen phosphorylase removes monomers from non-reducing ends up until four units away from a branch point. Then, the debranching enzyme removes the three-unit chunk from the branch and attaches it to the main chain. It can then break the alpha (1,6) bond to remove the branch point glucose. This allows for glycogen phosphorylase to continue to degrade the chains of glucose.
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