L11 - The Citric Acid Cycle

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Last updated 10:00 PM on 8/25/26
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12 Terms

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Goal of TCA

oxidizing every single carbon of glucose for energy

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TCA found in

matrix of mitochondria

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Total ATP from oxidation =

28 ATP

+2 ATP from glycolysis and 2 GTP = 32 ATP total

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Reactions of The Citric Acid Cycle (TCA cycle)

  1. Acetyl-CoA + Oxaloacetate/OAA (Gluconeogenesis) → Citrate via Citrate synthase (Creates CoA-SH byproduct); Large -ΔG; Irreversible

  2. Citrate → Isocitrate via aconitase; Dehydrating and rehydrating to move an OH group (Tert to Sec alc), creating a more accessible area for reduction

  3. Isocitrate → α-Ketoglutarate (α-KG) via Isocitrate dehydrogenase (Needs NAD(P)+), creates NAD(P)H + H+ + CO2 as byproduct; Irreversible

  4. α-KG → Succinyl-CoA via α-Ketoglutarate dehydrogenase complex (Needs CoA-SH + NAD+), creates NADH + CO2 byproduct; This complex is similar to PDH (Has all 3 Ex subunits); Irreversible

  5. Succinyl-CoA → Succinate via succinyl-CoA synthetase (substrate-level phosphorylation), removes CoA group (CoA-SH), and turns GDP/ADP to GTP/ATP

  6. Succinate → Fumarate via succinate dehydrogenase (Needs FAD), makes FADH2 byproduct

  7. Fumarate → L-Malate via fumarase

  8. L-Malate → Oxaloacetate via L-malate dehydrogenase (Needs NAD+) NADH + H+ byproduct

If not specified, reversible

Notes: Step 8: Oxaloacetate sink (Low concentration) drives the reaction forward

There are several ways to enter the TCA cycle

<ol><li><p>Acetyl-CoA + Oxaloacetate/OAA (Gluconeogenesis) → Citrate via <strong>Citrate synthase</strong> (Creates CoA-SH byproduct); Large -ΔG; Irreversible</p></li><li><p>Citrate → Isocitrate via <strong>aconitase</strong>; Dehydrating and rehydrating to move an OH group (Tert to Sec alc), creating a more accessible area for reduction</p></li><li><p>Isocitrate → α-Ketoglutarate (α-KG) via<strong> Isocitrate dehydrogenase</strong> (Needs NAD(P)<sup>+</sup>), creates NAD(P)H + H<sup>+</sup> + CO<sub>2</sub> as byproduct; Irreversible</p></li><li><p>α-KG → Succinyl-CoA via <strong>α-Ketoglutarate dehydrogenase complex</strong> (Needs CoA-SH + NAD<sup>+</sup>), creates NADH + CO<sub>2</sub> byproduct; This complex is similar to PDH (Has all 3 E<sub>x</sub> subunits); Irreversible</p></li><li><p>Succinyl-CoA → Succinate via <strong>succinyl-CoA synthetase</strong> (substrate-level phosphorylation), removes CoA group (CoA-SH), and turns GDP/ADP to GTP/ATP</p></li><li><p>Succinate → Fumarate via <strong>succinate dehydrogenase </strong>(Needs FAD), makes FADH<sub>2</sub> byproduct</p></li><li><p>Fumarate → <sub>L</sub>-Malate via <strong>fumarase</strong></p></li><li><p><sub>L</sub>-Malate → Oxaloacetate via<strong> <sub>L</sub>-malate dehydrogenase </strong>(Needs NAD<sup>+</sup>) NADH + H<sup>+</sup> byproduct</p></li></ol><p>If not specified, reversible</p><p>Notes: Step 8: Oxaloacetate sink (Low concentration) drives the reaction forward</p><p>There are several ways to enter the TCA cycle</p>
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TCA Mnemonic (Enzymes)

Can (Citrate synthase)

Anyone (Aconitase)

Interested (Isocitrate dehydrogenase)

Kindly (a-Ketoglutarate dehydrogenase)

Send (Succinyl-CoA synthetase)

Some (Succinate dehydrogenase)

Freaking (Fumarase)

Money (Malate dehydrogenase)

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TCA Mnemonic (Products)

Can → Citrate

I → Isocitrate

Keep → α-Ketoglutarate

Selling → Succinyl-CoA

Sex → Succinate

For → Fumarate

Money → Malate

Officer → Oxaloacetate

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Citrate synthase inhibitors and activators

Inhibitors: NADH, succinyl-CoA, citrate, ATP

Activators: ADP

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Isocitrate dehydrogenase inhibitors and activators

Inhibitors: ATP

Activators: Ca2+, ADP

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α-Ketoglutarate dehydrogenase complex inhibitors and activators

Inhibitors: succinyl-CoA, NADH

Activators: Ca2+

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Amphibolic

Contributes precursors for anabolic processes

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Cataplorotic reaction

Reaction that contributes to emptying a certain cycle or pathway

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Anaplerotic reactions

Reactions that contributes to replenishing metabolites within a cycle