Glycogen Synthesis

Cheatsheet Content

### Overview of Glycogen Synthesis - **Definition:** The process of synthesizing glycogen from glucose. - **Purpose:** Store glucose for future energy needs, primarily in the liver and skeletal muscles. - **Location:** Cytosol of cells. - **Key Precursor:** Glucose-6-phosphate. ### Key Steps in Glycogen Synthesis #### 1. Glucose Phosphorylation - **Enzyme:** Hexokinase (muscle), Glucokinase (liver) - **Reaction:** Glucose + ATP $\rightarrow$ Glucose-6-phosphate + ADP - **Significance:** Traps glucose inside the cell and activates it for further reactions. #### 2. Isomerization - **Enzyme:** Phosphoglucomutase - **Reaction:** Glucose-6-phosphate $\rightleftharpoons$ Glucose-1-phosphate - **Significance:** Converts G6P to G1P, which is the direct precursor for UDP-glucose. #### 3. UDP-Glucose Formation - **Enzyme:** UDP-glucose pyrophosphorylase - **Reaction:** Glucose-1-phosphate + UTP $\rightarrow$ UDP-glucose + PPi - **Significance:** UDP-glucose is the activated form of glucose, necessary for glycogen chain elongation. PPi hydrolysis drives the reaction forward. #### 4. Glycogenin Initiation - **Enzyme:** Glycogenin - **Function:** Acts as a primer by attaching the first few glucose residues (typically 4-8) to itself via tyrosine residues, forming a short oligosaccharide chain. - **Significance:** Glycogen synthase cannot initiate a new glycogen chain de novo; it requires a pre-existing primer. #### 5. Glycogen Chain Elongation - **Enzyme:** Glycogen synthase - **Function:** Adds glucose residues from UDP-glucose to the non-reducing end of the growing glycogen chain via $\alpha(1 \rightarrow 4)$ glycosidic bonds. - **Regulation:** This is the rate-limiting step and a major point of regulation. #### 6. Branching - **Enzyme:** Glycogen branching enzyme (amylo-$\alpha(1 \rightarrow 4)$ to $\alpha(1 \rightarrow 6)$ transglycosylase) - **Function:** Transfers a block of 6-8 glucose residues from the non-reducing end of a growing $\alpha(1 \rightarrow 4)$ chain to an internal glucose residue via an $\alpha(1 \rightarrow 6)$ glycosidic bond. - **Significance:** - Increases glycogen solubility. - Creates more non-reducing ends, allowing for faster synthesis and degradation. - Increases the storage capacity of glycogen. ### Regulation of Glycogen Synthesis #### Hormonal Control - **Insulin:** - **Effect:** Stimulates glycogen synthesis. - **Mechanism:** Activates protein phosphatases (e.g., PP1), which dephosphorylate and activate glycogen synthase, and dephosphorylate and inactivate glycogen phosphorylase (inhibiting breakdown). - **Glucagon (liver only) & Epinephrine (muscle & liver):** - **Effect:** Inhibit glycogen synthesis. - **Mechanism:** Activate protein kinases (e.g., PKA), which phosphorylate and inactivate glycogen synthase, and phosphorylate and activate glycogen phosphorylase (promoting breakdown). #### Allosteric Regulation - **Glucose-6-phosphate:** - **Effect:** Allosterically activates glycogen synthase. - **Significance:** High levels of G6P (indicating abundant glucose) promote glycogen storage. - **ATP:** - **Effect:** High ATP levels can indirectly promote glycogen synthesis by signaling high energy status. #### Reciprocal Regulation with Glycogenolysis - Glycogen synthesis and degradation are reciprocally regulated to prevent a futile cycle. When one is active, the other is inhibited. ### Glycogenolysis (Glycogen Breakdown) #### Overview - **Definition:** The process of breaking down glycogen into glucose or glucose-6-phosphate. - **Purpose:** Release glucose for energy when needed. - **Location:** Cytosol of cells. #### Key Steps 1. **Phosphorolysis:** - **Enzyme:** Glycogen phosphorylase - **Reaction:** Glycogen $(n \text{ residues}) + \text{Pi} \rightarrow \text{Glycogen} (n-1 \text{ residues}) + \text{Glucose-1-phosphate}$ - **Significance:** Releases glucose units from the non-reducing ends of glycogen chains as glucose-1-phosphate. It cannot break $\alpha(1 \rightarrow 6)$ bonds or cleave past 4 glucose units from a branch point. 2. **Debranching:** - **Enzyme:** Debranching enzyme (4-$\alpha$-glucanotransferase and amylo-$\alpha(1 \rightarrow 6)$-glucosidase activities) - **Function:** - **Transferase activity:** Moves 3 of the 4 glucose residues from a branch to a non-reducing end of another chain. - **Glucosidase activity:** Hydrolyzes the remaining single glucose residue at the $\alpha(1 \rightarrow 6)$ branch point, releasing it as free glucose. - **Significance:** Allows glycogen phosphorylase to continue breaking down the linear chain. 3. **Isomerization:** - **Enzyme:** Phosphoglucomutase - **Reaction:** Glucose-1-phosphate $\rightleftharpoons$ Glucose-6-phosphate - **Significance:** Converts G1P to G6P, which can then enter glycolysis or, in the liver, be dephosphorylated to free glucose. 4. **Glucose Release (Liver Only):** - **Enzyme:** Glucose-6-phosphatase - **Reaction:** Glucose-6-phosphate $\rightarrow$ Glucose + Pi - **Significance:** The liver is able to release free glucose into the bloodstream to maintain blood glucose homeostasis. Muscle cells lack this enzyme, so G6P enters glycolysis directly for muscle's own energy needs. #### Regulation of Glycogenolysis - **Hormonal Control:** - **Glucagon (liver) & Epinephrine (muscle & liver):** Stimulate glycogen breakdown by activating glycogen phosphorylase through phosphorylation (via PKA). - **Insulin:** Inhibits glycogen breakdown by activating protein phosphatases (e.g., PP1), which dephosphorylate and inactivate glycogen phosphorylase. - **Allosteric Regulation:** - **AMP (in muscle):** Activates glycogen phosphorylase, signaling low energy status. - **ATP & Glucose-6-phosphate (in muscle):** Inhibit glycogen phosphorylase, signaling high energy status. - **Glucose (in liver):** Inhibits liver glycogen phosphorylase, signaling high blood glucose.