MCAT BioChem: Advanced Metabolic Integration

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Net ATP Yield (Palmitate Oxidation)

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Oxidizing palmitate (16C) yields 106 Net ATP. Calculation: 7 FADH2 (1.5 x 7 = 10.5) + 7 NADH (2.5 x 7 = 17.5) + 8 Acetyl-CoA (10 x 8 = 80) = 108 Gross ATP. Subtract 2 ATP for activation cost (Palmitate + ATP -> Palmitoyl-CoA). 108 - 2 = 106 ATP.

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Allosteric Regulation of Pyruvate Kinase (L)

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Pyruvate kinase is inhibited by ATP and Alanine (signaling high energy) and activated by Fructose-1,6-bisphosphate (Feed-forward activation). Covalent regulation involves Phosphorylation (by PKA) which Inhibits the enzyme during fasting to conserve PEP for gluconeogenesis.

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Thermodynamic Driving Force of ATP Synthase

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The driving force is the Proton-Motive Force (PMF), composed of the Chemical Gradient (delta pH) and Electrical Gradient (delta Psi). The equation is: delta G = -nF(delta E) + 2.3RT(delta pH). Typically, the electrical gradient (charge separation) provides the majority of the energy for ATP synthesis in the mitochondria.

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Mechanism of PEP Carboxykinase (PEPCK)

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This enzyme converts Oxaloacetate (OAA) to Phosphoenolpyruvate (PEP) during gluconeogenesis. It requires GTP as an energy source. Decarboxylation occurs (removing CO2) to drive the reaction forward, utilizing a metal ion cofactor (Mn2+ or Mg2+) to stabilize the enolate intermediate.

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Anaplerotic Role of Pyruvate Carboxylase

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Pyruvate Carboxylase converts Pyruvate to Oxaloacetate (OAA) using ATP and Biotin (CO2 carrier). This replenishes the TCA cycle when intermediates (especially alpha-ketoglutarate) are siphoned off for amino acid biosynthesis (e.g., glutamate synthesis). It is allosterically activated by Acetyl-CoA.