Citric Acid Cycle Notes

Citric Acid Cycle / TCA Cycle / Kreb's Cycle

• Presented by: Dr. B.K. Manjunatha Goud, MBBS MD, Professor and Chairperson

• Date: 18/03/2025

• Time: 12.00 pm-1.00 pm

Learning Objectives

• Explain the role of the Pyruvate Dehydrogenase (PDH) complex in linking glycolysis to the TCA cycle.

• Analyze the steps of the TCA cycle, highlighting key regulatory mechanisms and the energetic yield of the cycle.

Citric Acid Cycle Overview

• The common pathway leading to complete oxidation of carbohydrates, fatty acids, and amino acids to $CO_2$.

• Some ATP is produced directly during the cycle.

• NADH produced will enter the Electron Transport Chain (ETC) to generate more ATP.

• A pathway providing many precursors for the biosynthesis of amino acids and nucleotides.

• Acts as both catabolic and anabolic (amphibolic role).

TCA Cycle and Related Pathways

• Inputs to Acetyl-CoA:

  • Glucose

  • Fatty Acids

  • Pyruvate

  • Alanine

  • Amino Acids

• TCA Cycle Intermediates & Connections:

  • Citrate (via Citrate Synthase)

  • Isocitrate (via Aconitase)

  • α-Ketoglutarate (links to Glutamate/Glutamine via GS/GOGAT)

  • Succinyl-CoA

  • Succinate

  • Fumarate

  • Malate

  • Oxaloacetate (links to Aspartate via AAT/AST)

Fate of Pyruvate

• For energy synthesis, pyruvate enters the mitochondria by active transport with the help of pyruvate translocase, a transport protein.

Pyruvate Dehydrogenase Complex

• Reaction Catalyzed: Pyruvate + CoA + $NAD^+$ → Acetyl CoA + $CO_2$ + NADH + $H^+$

• Pyruvate Oxidation: Pyruvate is converted to Acetyl-CoA.

Pyruvate Dehydrogenase Complex Coenzymes

• Requires 5 coenzymes:

  • TPP (Thiamine Pyrophosphate)

  • Lipoic Acid

  • Coenzyme A

  • FAD (Flavin Adenine Dinucleotide)

  • $NAD^+$ (Nicotinamide Adenine Dinucleotide)

Sources and Fates of Acetyl CoA

• Precursors:

  • Glucose

  • Fatty acids

  • Pyruvate

  • Amino Acids

• Acetyl CoA is used to synthesize:

  • Triglycerides

  • Phospholipids

  • Eicosanoids

  • Cholesterol

  • Ketone bodies

  • Steroid hormones

  • Bile salts

• Acetyl CoA is oxidized to $CO<em>2$ + $H</em>2O$ + energy (ATP).

• Acetyl CoA is a central intermediate in lipid metabolism.

Steps of the Citric Acid Cycle

• Condensation:

  • Acetyl-CoA + Oxaloacetate → Citrate (enzyme: citrate synthase, releases CoA-SH)

• Isomerization:

  • Citrate → cis-Aconitate → Isocitrate (enzyme: aconitase, involves dehydration and hydration)

• Oxidative Decarboxylation 1:

  • Isocitrate → Oxalosuccinate → α-Ketoglutarate (enzyme: isocitrate dehydrogenase, produces NADH and releases $CO_2$)

• Oxidative Decarboxylation 2:

  • α-Ketoglutarate → Succinyl-CoA (enzyme: α-ketoglutarate dehydrogenase complex, produces NADH and releases $CO_2$)

• Substrate-Level Phosphorylation:

  • Succinyl-CoA → Succinate (enzyme: succinyl-CoA synthetase, produces GTP which can be converted to ATP)

• Dehydrogenation:

  • Succinate → Fumarate (enzyme: succinate dehydrogenase, produces $FADH_2$)

• Hydration:

  • Fumarate → Malate (enzyme: fumarase, adds water)

• Dehydrogenation:

  • Malate → Oxaloacetate (enzyme: malate dehydrogenase, produces NADH)

Key Reactions and Enzymes

• Citrate Synthase: Acetyl CoA + Oxaloacetate → Citrate

• Isocitrate Dehydrogenase: Isocitrate → Oxalosuccinate

• Alpha-ketoglutarate Dehydrogenase: Alpha-ketoglutarate → Succinyl CoA

• Succinate Thiokinase: Succinyl CoA → Succinate

• Succinate Dehydrogenase: Succinate → Fumarate

• Malate Dehydrogenase: Malate → Oxaloacetate

Regulation of the Citric Acid Cycle

• ATP acts as an allosteric inhibitor of citrate synthase.

• Citrate allosterically inhibits phosphofructokinase (PFK), a key enzyme of glycolysis.

• Increased levels of $NAD^+$ and FAD stimulate the cycle.

• Isocitrate dehydrogenase is stimulated by ADP and inhibited by NADH.

• Alpha-ketoglutarate dehydrogenase is inhibited by succinyl CoA and NADH.

Non-Physiological Inhibitors

• Aconitase is inhibited by Fluoroacetate.

• Alpha-ketoglutarate dehydrogenase is inhibited by Arsenite.

• Succinate dehydrogenase is inhibited by Malonate.

Energetics

• Pyruvate Oxidation: 2.5 ATPs

• Isocitrate dehydrogenase: NADH yields 2.5 ATPs

• α-ketoglutarate dehydrogenase: NADH yields 2.5 ATPs

• Succinate thiokinase: Substrate-level phosphorylation yields 1 ATP/GTP

• Succinate dehydrogenase: $FADH_2$ yields 1.5 ATPs

• Malate dehydrogenase: NADH yields 2.5 ATPs

• Number of ATPs produced per Acetyl-CoA: 10

• Energetics of Krebs cycle: One cycle = 12.5 ATPs, so total TWO cycles: 25 ATPs

Overall Energetics of Glucose Oxidation

• Complete oxidation of ONE molecule of glucose:

• Aerobic glycolysis:

• Anaerobic glycolysis:

• Kreb’s cycle:

Malate-Aspartate Shuttle

• Transports reducing equivalents from NADH in the cytosol to NADH in the mitochondria

Glycerol Phosphate Shuttle

• Transports reducing equivalents from NADH in the cytosol to $FADH_2$ in the mitochondria

Significance and Conclusion

• Complete oxidation of acetyl CoA to $CO_2$.

• ATP generation.

• Final oxidative pathway.

• Integrates major metabolic pathways.