Chapter16 - Glycolysis and Gluconeogenesis Topics you're not ...

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II. Individual reactions:GlucosekinasephosphataseGlucose 6-phosphateFructose 6-phosphatekinasephosphataseFructose 1,6-bisphosphateComment: In both cases, a (different) phosphatase cleaves a low-energy phosphate ester because tosynthesize an ATP via the exact reversal of the corresponding glycolytic reactions is energeticallyunfeasible.PEPOC OC OPO 3CH 2GDPPEP carboxykinaseGTPADPKinaseO OCC OOAAATPO OCCH 2CO OPyruvateCCH 3OCarboxylaseADPATP(biotin)Comment: To synthesize PEP from pyruvate requires the energy equivalent of 2 ATP and requires 2distinct steps, each of which requires an ATP (GTP in the second case). The carboxylase-catalyzedreaction is the anaplerotic reaction of the CAC. Thus, OAA is at an intersection involvinggluconeogenesis, the CAC, and also the glyoxalate cycle in plants. Since gluconeogenesis occurs in thecytoplasm whereas the CAC occurs in the mitochondria, and since the inner mitochondrial membrane isimpermeable to OAA, export of OAA intended for gluconeogenesis occurs via the malate shuttle:
PyruvatePyruvateATPMatrixADPOxaloacetateNADHCytoplasmNAD +MalateMalateNAD +NADHOxaloacetateNote that during this process mitochondrial levels of NADH decrease wherease cytoplasmic levelsincrease. In other words, reducing power is being exported from the mitochondria which is desirableunder conditions which favor biosynthesis (which gluconeogenesis is an example of).Note also that the last enzyme in gluconeogenesis, glucose-6-phoaphatase, is present only in those cellswhose metabolic duty is to export glucose, i.e., the liver and to a lesser extent the kidney.. Otherwise,glucose-6-phosphate cannot leave the cell.III. Energy yield of gluconeogenesis:ATPPyruvateGTPOxaloacetateATP3-phosphoglycerateADPOxaloacetateGDPPEPADP1,3 bisphosphoglycerateTotal energy cost per glucose = (2 ATP + 1 GTP)/pyruvate X 2 pyruvate/glucose = 6IV. Regulation - There are two aspects of regulation. Entry of pyruvate into gluconeogenesis isregulated because pyruvate carboxylase requires acetyl CoA. Note that this is the anaplerotic reactionof the CAC so production of OAA can either refill depleted CAC levels or be converted into glucose.The choice here is determined by ATP levels. As an indicator of an energy-rich condition high levels ofATP will inhibit the CAC and direct OAA into gluconeogenesis and vice versa. Thus, an energy-richcondition diverts OAA into biosynthesis, and energy-poor condition increases CAC activity and the
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