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Foundation Sciences · Biochemistry
Krebs Cycle
The Krebs Cycle, also known as the Citric Acid Cycle or TCA cycle, is a series of chemical reactions used by all aerobic organisms to generate energy. It takes place in the mitochondrial matrix and involves the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins into carbon dioxide and chemical energy in the form of ATP, NADH, and FADH2.
📌 Learning Objectives
- Describe the overall purpose and location of the Krebs Cycle within cellular respiration.
- Explain the key inputs and outputs of the Krebs Cycle.
- Identify the main regulatory points and rate-limiting steps within the cycle.
- Discuss the role of co-factors, particularly B vitamins, in Krebs Cycle enzyme function.
- Correlate dysfunctions of the Krebs Cycle with relevant clinical conditions.
- Outline the interconnections between the Krebs Cycle and other metabolic pathways.
📋 Overview
The Krebs cycle (Citric Acid Cycle/TCA) is the final common pathway for the oxidation of carbohydrates, fats, and proteins. It occurs in the mitochondrial matrix (except Succinate Dehydrogenase) and is critical for generating high-energy electron carriers (NADH, FADH2) for the Electron Transport Chain (ETC). In UK exams, focus is less on every enzyme and more on co-factors (B vitamins), rate-limiting steps, and clinical consequences of failure (lactic acidosis).
🔬 Basic Science
• Citrate Synthase: Condenses Acetyl-CoA (2C) and Oxaloacetate (4C) into Citrate (6C).
• Isocitrate Dehydrogenase: Rate-limiting enzyme; inhibited by high ATP/NADH.
• α-Ketoglutarate Dehydrogenase: Multi-enzyme complex requiring 5 co-factors: Thiamine (B1), Riboflavin (B2), Niacin (B3), Pantothenic acid (B5), and Lipoic acid.
• Succinate Dehydrogenase: Unique as it is also 'Complex II' of the ETC and the only cycle enzyme embedded in the inner mitochondrial membrane.
• Isocitrate Dehydrogenase: Rate-limiting enzyme; inhibited by high ATP/NADH.
• α-Ketoglutarate Dehydrogenase: Multi-enzyme complex requiring 5 co-factors: Thiamine (B1), Riboflavin (B2), Niacin (B3), Pantothenic acid (B5), and Lipoic acid.
• Succinate Dehydrogenase: Unique as it is also 'Complex II' of the ETC and the only cycle enzyme embedded in the inner mitochondrial membrane.
🏥 Clinical Relevance
• Wernicke-Korsakoff & Beriberi: B1 deficiency halts alpha-ketoglutarate dehydrogenase. Leads to ATP depletion in high-demand tissues (brain/heart). Common SBA scenario: Chronic alcoholic with ataxia and ophthalmoplegia.
• Arsenic Poisoning: Specifically inhibits Lipoic acid (cofactor for alpha-ketoglutarate dehydrogenase), mimicking B1 deficiency.
• Metabolic Acidosis: Failure of the TCA cycle (e.g., hypoxia, mitochondrial disease) leads to pyruvate shunting to lactate, causing a high-anion gap metabolic acidosis.
• Alcoholism: Ethanol metabolism increases the NADH:NAD+ ratio, which inhibits the cycle and leads to ketoacidosis and hepatic steatosis (fatty liver).
• Arsenic Poisoning: Specifically inhibits Lipoic acid (cofactor for alpha-ketoglutarate dehydrogenase), mimicking B1 deficiency.
• Metabolic Acidosis: Failure of the TCA cycle (e.g., hypoxia, mitochondrial disease) leads to pyruvate shunting to lactate, causing a high-anion gap metabolic acidosis.
• Alcoholism: Ethanol metabolism increases the NADH:NAD+ ratio, which inhibits the cycle and leads to ketoacidosis and hepatic steatosis (fatty liver).
🧪 Investigations
• Serum Lactate: Elevated when TCA cycle or ETC is impaired (anaerobic shift).
• Anion Gap: Calculated in the context of metabolic acidosis [ (Na+K) - (Cl+HCO3) ].
• B1 Status: Measured via erythrocyte transketolase activity or direct serum thiamine pyrophosphate.
• MRI Brain: In suspected Wernicke’s (TCA cycle failure), look for high signal in the mamillary bodies and periaqueductal grey.
• Anion Gap: Calculated in the context of metabolic acidosis [ (Na+K) - (Cl+HCO3) ].
• B1 Status: Measured via erythrocyte transketolase activity or direct serum thiamine pyrophosphate.
• MRI Brain: In suspected Wernicke’s (TCA cycle failure), look for high signal in the mamillary bodies and periaqueductal grey.
💊 Management
Management focuses on addressing co-factor deficiencies, such as intravenous Thiamine (Pabrinex) in Wernicke-Korsakoff syndrome. For genetic TCA cycle defects, management is largely supportive, involving high-calorie diets and avoidance of metabolic stressors. In cases of mitochondrial toxins, specific antidotes or supportive care may be required.
Revision Resources – expand the sections below for high-yield notes, exam pearls, key facts and further reading.
MLA High-Yield Notes & Quick Revision ⌄
SBA Trap: If a patient with suspected thiamine deficiency is given IV Glucose before Vitamin B1, you can worsen the neurological damage. This is because glucose metabolism spikes the demand for B1 (needed for PDC and Krebs cycle), causing a 'crash' in remaining enzyme activity. Always give thiamine (Pabrinex) BEFORE or WITH glucose.
Metabolic acidosis (e.g., lactic acidosis)
Nutritional deficiencies (e.g., B vitamin deficiencies)
Mitochondrial diseases
Toxicity (e.g., arsenic poisoning)
Energy metabolism disorders
- Also known as the Citric Acid Cycle or TCA cycle.
- Occurs in the mitochondrial matrix.
- Central to aerobic respiration, oxidising acetyl-CoA.
- Produces 3 NADH, 1 FADH2, 1 GTP (ATP), and 2 CO2 per cycle.
- Requires B vitamin-derived co-factors.
- Regulated by ATP/ADP ratio and NADH/NAD+ ratio.
Exam Pearls ⌄
⭐ High Yield
The Krebs Cycle occurs in the mitochondrial matrix, except for succinate dehydrogenase which is on the inner mitochondrial membrane.
It is the final common oxidative pathway for carbohydrates, fats, and proteins.
Key products are NADH and FADH2, which feed into the Electron Transport Chain for ATP synthesis.
Co-factors derived from B vitamins (e.g., thiamine, riboflavin, niacin, pantothenic acid) are essential for several enzyme activities.
Acetyl-CoA (from pyruvate, fatty acids, amino acids) is the primary entry molecule.
Regulation primarily occurs at isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase.
One molecule of GTP (converted to ATP) is produced per cycle via substrate-level phosphorylation.
The cycle produces CO2 as a waste product of carbon oxidation.
💡 Clinical Pearl
Thiamine (Vitamin B1) Deficiency: Can impair pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase, leading to lactic acidosis and neurological symptoms (e.g., Wernicke-Korsakoff syndrome).
Arsenic Poisoning: Inhibits pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase, disrupting the cycle and cellular energy production.
Mitochondrial Disorders: Genetic defects in Krebs Cycle enzymes can lead to a range of metabolic and neurological symptoms due to impaired energy generation.
Fumarase Deficiency: A rare autosomal recessive disorder causing severe neurological impairment and metabolic acidosis due to accumulation of fumarate and other cycle intermediates.
⚠️ Exam Tip — Common Mistakes
Confusing the location of the Krebs Cycle with glycolysis (cytosol vs. mitochondria).
Forgetting that the cycle is amphibolic (involved in both catabolism and anabolism).
Underestimating the importance of B vitamin co-factors for enzyme function.
Not recognising that the primary purpose is to generate electron carriers (NADH, FADH2), not large amounts of ATP directly.
Attributing all CO2 production in respiration solely to the Krebs Cycle, overlooking pyruvate decarboxylation.
Key Facts ⌄
Location: Mitochondrial matrix.
Products (per Acetyl-CoA): 3 NADH, 1 FADH2, 1 GTP/ATP, 2 CO2.
Rate-limiting step: Isocitrate Dehydrogenase.
Thiamine (B1) Dependency: Alpha-ketoglutarate dehydrogenase and Pyruvate dehydrogenase (PDC) require Thiamine Pyrophosphate (TPP).
Substrate Mnemonics: 'Citrate Is Krebs' Starting Substrate For Making Oxaloacetate' (Citrate, Isocitrate, α-Ketoglutarate, Succinyl-CoA, Succinate, Fumarate, Malate, Oxaloacetate).
Related Topics ⌄
References ⌄
- TeachMePhysiology - The Citric Acid Cycle
- BNF - Thiamine
- GMC MLA Content Map
Further Resources
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