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Biochemistry I - Carbohydrate Metabolism II

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    Study Guide Biochemistry I – Carbohydrate Metabolism II 1. The Gluconeogenic Pathway Gluconeogenesis is the pathway by which cells synthesize glucose from non - sugar sources , mainly amino acids and TCA cycle intermediates . This pathway is especially important during fasting, starvation, or intense exercise, when glucose must be supplied to tissues such as the brain and red blood cells. Although gluconeogenesis uses many of the same reactions as glycolysis, it cannot simply run glycolysis in reverse . This is because glycolysis contains three steps with large drops in free energy , making them essentially irreversible. These steps are catalyzed by: • Hexokinase • Phosphofructokinase • Pyruvate kinase To synthesize glucose efficiently, gluconeogenesis uses alternative reactions to bypass these steps. 1.1 Why Special Bypass Reactions Are Needed Reactions with very small free - energy changes operate near equilibrium . For these steps, simply increasing the concentration of products can drive the reaction in reverse. For example: • Adding glyceraldehyde - 3 - phosphate and dihydroxyacetone phosphate pushes the aldolase reaction toward fructose - 1,6 - bisphosphate. However, reactions with large negative free - energy changes (like pyruvate kinase) cannot be reversed effectively. Therefore, gluconeogenesis replaces these steps with new enzyme - catalyzed reactions . 1.2 Bypassing the Pyruvate Kinase Step To bypass pyruvate kinase, cells first convert pyruvate into oxaloacetate .

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    Study Guide Sources of Oxaloacetate Oxaloacetate can be formed in two main ways: 1. From Amino Acids Aspartate has the same carbon skeleton as oxaloacetate. Removal of the amino group converts aspartate into oxaloacetate. 2. From Pyruvate (Anapleurotic Reaction) Oxaloacetate can also be synthesized directly from pyruvate by adding CO ₂ . • This reaction occurs in the mitochondrial matrix • It is catalyzed by pyruvate carboxylase • It consumes one ATP • CO ₂ (as bicarbonate) is required This type of reaction, which replenishes TCA cycle intermediates, is called an anapleurotic reaction . In eukaryotic cells, oxaloacetate formed in the mitochondria is then transported to the cytosol by shuttle systems.

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    Study Guide Figure 1 1.3 Conversion of Oxaloacetate to Phosphoenolpyruvate Once in the cytosol, oxaloacetate is converted into phosphoenolpyruvate (PEP) . Figure 2 This step is catalyzed by pyruvate carboxykinase . Important points to remember: • Pyruvate carboxylase adds CO ₂ • Pyruvate carboxykinase removes CO ₂ and adds phosphate • GTP is used as the phosphate donor

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    Study Guide Even though CO ₂ is added and then removed, the net result is: • Conversion of pyruvate into phosphoenolpyruvate • Consumption of two high - energy phosphate bonds (ATP + GTP) This illustrates a general rule: Biosynthetic pathways often use several favorable steps to bypass one highly unfavorable step. 1.4 Bypassing the Phosphofructokinase Step The next irreversible step of glycolysis is catalyzed by phosphofructokinase . Gluconeogenesis bypasses this step using a phosphatase . • The enzyme fructose - 1,6 - bisphosphatase removes the phosphate at carbon 1 • The product is fructose - 6 - phosphate 1.5 Avoiding a Futile Cycle If phosphofructokinase and fructose - 1,6 - bisphosphatase were both active at high rates, ATP would be wasted. Together, these opposing reactions sum to: Fructose - 6 - phosphate + ATP → Fructose - 1,6 - bisphosphate + ADP Fructose - 1,6 - bisphosphate →

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    Study Guide Fructose - 6 - phosphate + phosphate This is called a futile cycle . To prevent excessive ATP loss: • The two enzymes are coordinately regulated • They respond to the same signals but in opposite ways ATP → ADP + phosphate For example: • Fructose - 2,6 - bisphosphate : o Activates phosphofructokinase o Inhibits fructose - 1,6 - bisphosphatase This regulation does not completely shut off either pathway. Instead, it ensures that carbon flow favors either glycolysis or gluconeogenesis , depending on the cell’s needs. The small ATP cost of the futile cycle is the price of precise metabolic control . 1.6 Bypassing the Hexokinase Step The final irreversible glycolytic step is catalyzed by hexokinase . Gluconeogenesis bypasses this step using another phosphatase . • This enzyme removes the phosphate from glucose - 6 - phosphate • It is activated by high levels of glucose - 6 - phosphate • This is an example of substrate - level regulation Notably, this effect is the opposite of glucose - 6 - phosphate’s effect on hexokinase during glycolysis. Key Takeaway • Gluconeogenesis synthesizes glucose from non - carbohydrate sources • It bypasses the three irreversible steps of glycolysis • Pyruvate kinase is bypassed using pyruvate carboxylase and pyruvate carboxykinase • Phosphofructokinase and hexokinase are bypassed using phosphatases • Coordinated regulation prevents wasteful ATP consumption • Biosynthesis requires energy investment for control and efficiency

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    Study Guide 2. Storage of Glucose as Glycogen Glucose is the main fuel used by our body for energy. However, glucose cannot always stay freely floating in the blood. To manage this, the body stores extra glucose in a special form called glycogen . This storage system helps keep blood glucose levels stable and ensures energy is available when needed. 2.1 Why Glycogen Is Important The liver plays a major role in maintaining blood glucose levels. It releases glucose into the bloodstream when blood sugar drops. At the same time, liver cells, muscle cells, and other tissues store glucose as glycogen , which is a large, branched polymer made of glucose units . Storing glucose as glycogen is efficient and allows the body to respond quickly to energy demands. Step 1: Making Glucose - 1 - Phosphate Glycogen synthesis begins with glucose - 1 - phosphate . • Glucose - 1 - phosphate is formed from glucose - 6 - phosphate • This conversion is carried out by the enzyme phosphoglucomutase • This enzyme is an isomerase , meaning it rearranges atoms within the same molecule Interestingly, glucose - 1 - phosphate is also produced when glycogen is broken down.

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    Study Guide Glycogen Breakdown vs Glycogen Synthesis The enzyme glycogen phosphorylase breaks glycogen into glucose - 1 - phosphate. • This reaction naturally favors glycogen breakdown • Because of this, the same pathway cannot efficiently work in reverse to make glycogen This means glycogen synthesis requires extra energy and must follow a different route. Step 2: Using Energy to Form UDP - Glucose To drive glycogen synthesis forward, the cell uses extra energy . • Glucose - 1 - phosphate reacts with UTP • This reaction forms UDP - glucose and inorganic pyrophosphate (PPi) The pyrophosphate is then broken down into two phosphate ions . This step releases energy and pulls the reaction forward , making UDP - glucose formation favorable.

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    Study Guide Figure 1 Step 3: Building the Glycogen Chain The enzyme glycogen synthase adds glucose units to glycogen. • It transfers glucose from UDP - glucose • The glucose is added to the non - reducing end of an existing glycogen molecule • An α - 1,4 glycosidic bond is formed • UDP is released Glycogen synthase cannot start a new glycogen chain . Another enzyme is required to begin the initial glycogen molecule.

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