The glycogen cycle encompasses both the synthesis (glycogenesis) and breakdown (glycogenolysis) of glycogen. Since the question specifically asks for the cycle, I will outline the steps of glycogen synthesis and breakdown.
However, given the provided reference only details Glycogenesis, I will focus on expanding that process in this response.
Glycogenesis, the process of synthesizing glycogen from glucose, primarily occurs in the liver and muscle tissues. The steps are as follows:
Steps of Glycogenesis (Glycogen Synthesis)
Glycogenesis builds up glycogen stores when glucose is plentiful. Here's a breakdown of the steps:
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Glucose Phosphorylation: Glucose is phosphorylated to glucose-6-phosphate by glucokinase (in the liver) or hexokinase (in muscles). This traps glucose inside the cell.
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Glucose-1-Phosphate Formation: Glucose-6-phosphate is then converted to glucose-1-phosphate by phosphoglucomutase.
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Glucose Activation: Glucose-1-phosphate reacts with uridine triphosphate (UTP) to form UDP-glucose. This is catalyzed by UDP-glucose pyrophosphorylase. UDP-glucose is the activated form of glucose used in glycogen synthesis.
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Glucose Addition to the Polymer Backbone: Glycogen synthase adds UDP-glucose to the non-reducing end of a glycogen molecule, releasing UDP. This forms an α-1,4-glycosidic bond.
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Branch Formation: Branching enzyme (amylo-α(1,4 to 1,6)-transglucosidase) creates branches by transferring a chain of several glucose residues from one glycogen chain to another. These are attached via α-1,6-glycosidic bonds. Branching increases the solubility of glycogen and creates more non-reducing ends for glycogen synthase to act on.
| Step | Enzyme Involved | Description |
|---|---|---|
| Glucose Phosphorylation | Glucokinase/Hexokinase | Glucose is phosphorylated to glucose-6-phosphate. |
| Glucose-1-Phosphate Formation | Phosphoglucomutase | Glucose-6-phosphate is converted to glucose-1-phosphate. |
| Glucose Activation | UDP-glucose pyrophosphorylase | Glucose-1-phosphate reacts with UTP to form UDP-glucose. |
| Polymer Extension | Glycogen Synthase | UDP-glucose is added to the glycogen chain via α-1,4-glycosidic bonds. |
| Branching | Branching Enzyme (Amylo-(1,4 to 1,6)-Transglucosidase) | Branches are created via α-1,6-glycosidic bonds, increasing solubility and non-reducing ends. |
