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.Preview Mode
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