8.5
Le cycle de l'acide citrique
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Q1: What is the citric acid cycle and where does it occur?
The citric acid cycle is a closed loop of eight enzymatic reactions occurring in the mitochondrial matrix that is critical for glucose catabolism. Also called the Krebs cycle or TCA cycle, it processes acetyl CoA through a series of redox, dehydration, hydration, and decarboxylation reactions. The cycle regenerates oxaloacetate, allowing continuous energy extraction from glucose.
Q2: How does acetyl CoA enter the citric acid cycle?
Acetyl CoA, a two-carbon molecule produced from pyruvate oxidation production of acetyl coa in mitochondria, donates its acetyl group to oxaloacetate, a four-carbon molecule. This combination forms citrate, a six-carbon intermediate that initiates the cycle. The CoA group is released and diffuses away to combine with another acetyl group.
Q3: What are the main products generated by one turn of the citric acid cycle?
Each cycle produces three NADH molecules, one FADH2 molecule, one ATP (or GTP), and releases two carbon dioxide molecules. These electron carriers, NADH and FADH2, are high-energy molecules used in the electron transport chain to generate additional ATP through oxidative phosphorylation.
Q4: Why does the citric acid cycle run twice per glucose molecule?
Pyruvate oxidation produces two acetyl CoA molecules from each glucose molecule. Since the citric acid cycle processes one acetyl CoA per turn, the cycle must run twice to completely oxidize all carbons from a single glucose molecule and extract all available energy.
Q5: What happens during the oxidation steps of the citric acid cycle?
Oxidation steps reduce NAD+ to NADH and FAD to FADH2, capturing high-energy electrons. These reductions occur when isocitrate is oxidized to alpha-ketoglutarate, when alpha-ketoglutarate forms succinyl CoA, when succinate is oxidized to fumarate, and when malate is oxidized back to oxaloacetate.
Q6: How is ATP generated directly in the citric acid cycle?
During step five, a phosphate group replaces the CoA group on succinyl CoA, forming succinate. This phosphate is then transferred to GDP or ADP, producing GTP or ATP through substrate-level phosphorylation. This direct mechanism generates one ATP per cycle turn.
Q7: How do NADH and FADH2 contribute to overall ATP production?
NADH and FADH2 produced in the citric acid cycle provide electrons to the electron transport chain, which uses these electrons to pump protons across the inner mitochondrial membrane. This creates a proton gradient that drives ATP synthesis, generating the majority of ATP from glucose catabolism.