Biochemistry I - Carbohydrate Metabolism II - Quick Guides | Biochemistry

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Study GuideBiochemistry I–Carbohydrate Metabolism II1.The Gluconeogenic PathwayGluconeogenesisis the pathway by which cells synthesizeglucose from non-sugar sources,mainlyamino acidsandTCA cycle intermediates. This pathway is especially important duringfasting, starvation, or intense exercise, when glucose must be supplied to tissues such as the brainand red blood cells.Although gluconeogenesis uses many of the same reactions as glycolysis, itcannot simply runglycolysis in reverse. This is because glycolysis containsthree steps with large drops in freeenergy, making them essentially irreversible.These steps are catalyzed by:•Hexokinase•Phosphofructokinase•Pyruvate kinaseTo synthesize glucose efficiently, gluconeogenesis usesalternative reactionsto bypass these steps.1.1Why Special Bypass Reactions Are NeededReactions with very small free-energy changes operatenear equilibrium. For these steps, simplyincreasing the concentration of products can drive the reaction in reverse. For example:•Adding glyceraldehyde-3-phosphate and dihydroxyacetone phosphate pushes the aldolasereaction toward fructose-1,6-bisphosphate.However, reactions withlarge negative free-energy changes(like pyruvate kinase) cannot bereversed effectively. Therefore, gluconeogenesis replaces these steps withnew enzyme-catalyzedreactions.1.2Bypassing the Pyruvate Kinase StepTo bypass pyruvate kinase, cells first convertpyruvate into oxaloacetate.

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Study GuideSources of OxaloacetateOxaloacetate can be formed in two main ways:1. From Amino AcidsAspartate has thesame carbon skeletonas oxaloacetate. Removal of the amino group convertsaspartate into oxaloacetate.2. From Pyruvate (Anapleurotic Reaction)Oxaloacetate can also be synthesized directly from pyruvate by adding CO₂.•This reaction occurs in themitochondrial matrix•It is catalyzed bypyruvate carboxylase•Itconsumes one ATP•CO₂(as bicarbonate) is requiredThis type of reaction, which replenishes TCA cycle intermediates, is called ananapleurotic reaction.In eukaryotic cells, oxaloacetate formed in the mitochondria is thentransported to the cytosolbyshuttle systems.

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Study GuideFigure 11.3Conversion of Oxaloacetate to PhosphoenolpyruvateOnce in the cytosol, oxaloacetate is converted intophosphoenolpyruvate (PEP).Figure 2This step is catalyzed bypyruvate carboxykinase.Important points to remember:•Pyruvate carboxylaseadds CO₂•Pyruvate carboxykinaseremoves CO₂and adds phosphate•GTP is used as the phosphate donor

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Study GuideEven though CO₂is added and then removed, thenet resultis:•Conversion of pyruvate into phosphoenolpyruvate•Consumption oftwo high-energy phosphate bonds(ATP + GTP)This illustrates a general rule:Biosynthetic pathways often use several favorable steps to bypass one highly unfavorablestep.1.4Bypassing the Phosphofructokinase StepThe next irreversible step of glycolysis is catalyzed byphosphofructokinase. Gluconeogenesisbypasses this step using aphosphatase.•The enzymefructose-1,6-bisphosphataseremoves the phosphate at carbon 1•The product isfructose-6-phosphate1.5Avoiding a Futile CycleIf phosphofructokinase and fructose-1,6-bisphosphatase were both active at high rates, ATP would bewasted.Together, these opposing reactions sum to:Fructose-6-phosphate + ATP →Fructose-1,6-bisphosphate + ADPFructose-1,6-bisphosphate →

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Study GuideFructose-6-phosphate + phosphateThis is called afutile cycle.To prevent excessive ATP loss:•The two enzymes arecoordinately regulated•They respond to the same signals but inopposite waysATP → ADP + phosphateFor example:•Fructose-2,6-bisphosphate:oActivates phosphofructokinaseoInhibits fructose-1,6-bisphosphataseThis regulation does not completely shut off either pathway. Instead, it ensures thatcarbon flowfavors either glycolysis or gluconeogenesis, depending on the cell’s needs. The small ATP cost ofthe futile cycle is theprice of precise metabolic control.1.6Bypassing the Hexokinase StepThe final irreversible glycolytic step is catalyzed byhexokinase. Gluconeogenesis bypasses this stepusing anotherphosphatase.•This enzyme removes the phosphate fromglucose-6-phosphate•It is activated byhigh levels of glucose-6-phosphate•This is an example ofsubstrate-level regulationNotably, this effect is theoppositeof 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 usingpyruvate carboxylase and pyruvate carboxykinase•Phosphofructokinase and hexokinase are bypassed usingphosphatases•Coordinated regulation prevents wasteful ATP consumption•Biosynthesis requiresenergy investment for control and efficiency

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Study Guide2.Storage of Glucose as GlycogenGlucose is the main fuel used by our body for energy. However, glucose cannot always stay freelyfloating in the blood. To manage this, the body stores extra glucose in a special form calledglycogen.This storage system helps keep blood glucose levels stable and ensures energy is available whenneeded.2.1Why Glycogen Is ImportantTheliverplays a major role in maintaining blood glucose levels. It releases glucose into thebloodstream when blood sugar drops.At the same time,liver cells, muscle cells, and other tissuesstore glucose asglycogen, which isalarge, 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-PhosphateGlycogen synthesis begins withglucose-1-phosphate.•Glucose-1-phosphate is formed fromglucose-6-phosphate•This conversion is carried out by the enzymephosphoglucomutase•This enzyme is anisomerase, meaning it rearranges atoms within the same moleculeInterestingly, glucose-1-phosphate is also produced when glycogen is broken down.

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Study GuideGlycogen Breakdown vs Glycogen SynthesisThe enzymeglycogen phosphorylasebreaks glycogen into glucose-1-phosphate.•This reaction naturally favorsglycogen breakdown•Because of this, the same pathway cannot efficiently work in reverse to make glycogenThis meansglycogen synthesis requires extra energyand must follow a different route.Step 2: Using Energy to Form UDP-GlucoseTo drive glycogen synthesis forward, the cell usesextra energy.•Glucose-1-phosphate reacts withUTP•This reaction formsUDP-glucoseandinorganic pyrophosphate (PPi)The pyrophosphate is then broken down intotwo phosphate ions.This step releases energy andpulls the reaction forward, making UDP-glucose formation favorable.

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Study GuideFigure 1Step 3: Building the Glycogen ChainThe enzymeglycogen synthaseadds glucose units to glycogen.•It transfers glucose fromUDP-glucose•The glucose is added to thenon-reducing endof an existing glycogen molecule•Anα-1,4glycosidic bondis formed•UDPis releasedGlycogen synthasecannot start a new glycogen chain.Another enzyme is required to begin the initial glycogen molecule.

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