# The Krebs Cycle (Tricarboxylic Acid Cycle / Citric Acid Cycle) ## A Comprehensive, Detailed Account --- # 1. INTRODUCTION AND HISTORICAL BACKGROUND ## 1.1 Discovery The cycle was elucidated in **1937 by Sir Hans Adolf Krebs** (a German-born British biochemist), for which he received the **Nobel Prize in Physiology or Medicine in 1953** (shared with Fritz Lipmann). Krebs worked primarily with pigeon breast muscle, which has a very high rate of oxidative metabolism, to piece together the cyclic nature of the pathway. The cycle is called by three names: - **Krebs cycle** — after its discoverer - **Citric acid cycle** — because citrate (citric acid) is the first product formed - **Tricarboxylic acid (TCA) cycle** — because the first two intermediates (citrate and isocitrate) are tricarboxylic acids (contain three carboxyl groups) ## 1.2 Location - **Mitochondrial matrix** in eukaryotes - **Cytoplasm** in prokaryotes (which lack mitochondria) - All the enzymes of the TCA cycle are located in the mitochondrial matrix **except succinate dehydrogenase (Complex II)**, which is embedded in the **inner mitochondrial membrane**. ## 1.3 Fundamental Role The TCA cycle is the **final common oxidative pathway** for carbohydrates, fats, and amino acids. All these fuels are ultimately converted to **acetyl-CoA** (the "universal metabolic fuel"), which enters the cycle. It is an **amphibolic pathway** — serving both: - **Catabolic functions** (oxidation of acetyl-CoA to CO₂ with energy capture) - **Anabolic functions** (providing intermediates for biosynthesis) --- # 2. OVERVIEW OF THE CYCLE ## 2.1 Net Reaction The overall reaction for one turn of the cycle: **Acetyl-CoA + 3 NAD⁺ + FAD + GDP + Pᵢ + 2 H₂O → CoA-SH + 3 NADH + 3 H⁺ + FADH₂ + GTP + 2 CO₂** Key points: - **2 carbon atoms** enter as acetyl-CoA - **2 carbon atoms** leave as **2 CO₂** (but these are NOT the same 2 carbons that entered — this is isotopically demonstrable and important) - **3 NADH**, **1 FADH₂**, and **1 GTP** are produced per turn - **2 molecules of water** are consumed - Oxaloacetate (OAA) is regenerated — it is the acceptor molecule and is not consumed ## 2.2 Preparatory Step: Pyruvate Dehydrogenase Complex (PDH Complex) Before acetyl-CoA enters the TCA cycle, pyruvate (from glycolysis) must be converted to acetyl-CoA. This is accomplished by the **pyruvate dehydrogenase complex (PDH complex)**, a massive multi-enzyme complex located in the mitochondrial matrix. ### Reaction: **Pyruvate + CoA-SH + NAD⁺ → Acetyl-CoA + CO₂ + NADH + H⁺** This is an **irreversible**, **oxidative decarboxylation** reaction. ### Components of PDH Complex: | Component | Enzyme | Prosthetic Group / Coenzyme | |-----------|--------|----------------------------| | E1 | Pyruvate dehydrogenase (pyruvate decarboxylase) | **TPP** (Thiamine pyrophosphate, from Vitamin B₁) | | E2 | Dihydrolipoyl transacetylase | **Lipoic acid** (lipoamide), **CoA-SH** (from pantothenic acid, Vitamin B₅) | | E3 | Dihydrolipoyl dehydrogenase | **FAD** (from Vitamin B₂), **NAD⁺** (from Vitamin B₃/Niacin) | **Five coenzymes required: TPP, Lipoic acid, CoA-SH, FAD, NAD⁺** Mnemonic: **"TLC For Nancy"** or **"Tender Loving Care For Nancy"** ### Mechanism (5 steps): 1. **Decarboxylation**: E1 (with TPP) decarboxylates pyruvate → hydroxyethyl-TPP + CO₂ 2. **Oxidation and Transfer**: The hydroxyethyl group is oxidized and transferred to lipoamide on E2 → acetyl-lipoamide 3. **Transacetylation**: E2 transfers the acetyl group to CoA-SH → **Acetyl-CoA** + dihydrolipoamide 4. **Regeneration of lipoamide**: E3 (with FAD) oxidizes dihydrolipoamide back to lipoamide → FADH₂ 5. **Regeneration of FAD**: FADH₂ transfers electrons to NAD⁺ → **NADH + H⁺** + FAD ### Regulation of PDH: **PDH is regulated by covalent modification (phosphorylation/dephosphorylation):** - **PDH kinase** → phosphorylates E1 → **inactivates** PDH - **PDH phosphatase** → dephosphorylates E1 → **activates** PDH **PDH kinase is activated by:** - High ATP/ADP ratio - High NADH/NAD⁺ ratio - High Acetyl-CoA/CoA ratio - (All signals of energy sufficiency) **PDH kinase is inhibited by:** - Pyruvate - ADP - Ca²⁺ **PDH phosphatase is activated by:** - Ca²⁺ (important in exercising muscle) - Mg²⁺ - Insulin > ### 🔴 CLINICAL: Pyruvate Dehydrogenase Deficiency > - **X-linked** (E1α subunit gene is on the X chromosome), though autosomal forms exist > - Most common cause of **congenital lactic acidosis** > - Pyruvate accumulates → converted to lactate (by LDH) → **lactic acidosis** > - Also converted to alanine (by ALT) → **hyperalaninemia** > - **Clinical features**: Intellectual disability, developmental delay, seizures, hypotonia, ataxia > - **CNS features** may resemble **Leigh syndrome** (subacute necrotizing encephalomyelopathy) > - **Treatment**: Ketogenic diet (bypasses the need for PDH by providing acetyl-CoA from fatty acid oxidation), thiamine supplementation (some mutations are thiamine-responsive), dichloroacetate (inhibits PDH kinase, keeping PDH active) > ### 🔴 CLINICAL: Arsenic Poisoning > - **Arsenic (arsenite, As³⁺)** binds to the sulfhydryl groups of **lipoic acid** (dihydrolipoamide) in E2 > - This inhibits both **PDH complex** and **α-ketoglutarate dehydrogenase complex** > - Results in accumulation of pyruvate and α-ketoglutarate > - Impairs TCA cycle function → decreased ATP production > - Symptoms: garlic-like breath odor, severe GI symptoms, rice-water stools, peripheral neuropathy, Mees' lines on nails > - Treatment: **Dimercaprol (BAL)** or **Succimer (DMSA)** > ### 🔴 CLINICAL: Thiamine (B₁) Deficiency > - TPP is required by PDH, α-KGDH, and also by transketolase (pentose phosphate pathway) and branched-chain α-keto acid dehydrogenase > - **Beriberi**: Wet (cardiac — high-output heart failure, edema) and Dry (neurological — peripheral neuropathy) > - **Wernicke's encephalopathy**: Confusion, ophthalmoplegia, ataxia (classic triad) > - **Korsakoff syndrome**: Anterograde and retrograde amnesia, confabulation > - Common in **chronic alcoholics** (alcohol impairs thiamine absorption and storage) > - **Always give thiamine BEFORE glucose** in suspected deficiency — glucose load increases thiamine demand and can precipitate acute Wernicke's --- # 3. THE EIGHT REACTIONS OF THE TCA CYCLE — IN DETAIL ## Mnemonic for intermediates: **"Can I Keep Selling Sex For Money, Officer?"** - **C**itrate - **I**socitrate - α-**K**etoglutarate (α-oxoglutarate) - **S**uccinyl-CoA - **S**uccinate - **F**umarate - **M**alate - **O**xaloacetate --- ## REACTION 1: Citrate Synthase ### Reaction: **Oxaloacetate (C₄) + Acetyl-CoA (C₂) + H₂O → Citrate (C₆) + CoA-SH** ### Details: - **Enzyme**: Citrate synthase - **Type**: Aldol condensation followed by hydrolysis (Claisen condensation) - **Irreversible** reaction (ΔG°' = −31.4 kJ/mol, highly exergonic) - This is a **REGULATORY step** (rate-limiting in some tissues) - The reaction proceeds in two steps: 1. Condensation of acetyl-CoA with OAA → citryl-CoA (enzyme-bound intermediate) 2. Hydrolysis of citryl-CoA → citrate + CoA-SH (this hydrolysis drives the reaction forward) ### Regulation of Citrate Synthase: - **Inhibited by**: ATP, NADH, succinyl-CoA, citrate (product inhibition), long-chain fatty acyl-CoA - **Activated by**: ADP, NAD⁺ - Availability of substrates (OAA and acetyl-CoA) is crucial — OAA concentration in the mitochondrial matrix is very low (~micromolar) and is often rate-limiting > ### 🔴 CLINICAL: Citrate and Fatty Acid Synthesis > - Citrate is transported out of the mitochondria via the **citrate shuttle (tricarboxylate transport system/citrate-malate antiporter)** > - In the cytoplasm, **ATP-citrate lyase** cleaves citrate back to OAA + acetyl-CoA > - Cytoplasmic acetyl-CoA is then used for **de novo fatty acid synthesis** and **cholesterol synthesis** > - This links TCA cycle to **lipogenesis** > - Citrate in the cytoplasm also **activates acetyl-CoA carboxylase (ACC)**, the rate-limiting enzyme of fatty acid synthesis > - Citrate also **inhibits PFK-1** (phosphofructokinase-1 in glycolysis) — acting as a signal that the cell has sufficient energy --- ## REACTION 2: Aconitase ### Reaction: **Citrate (C₆) ⇌ Isocitrate (C₆)** (via cis-aconitate as an enzyme-bound intermediate) ### Details: - **Enzyme**: Aconitase (aconitate hydratase) - **Type**: Isomerization (dehydration followed by rehydration) - **Reversible** reaction - Contains an **iron-sulfur cluster [4Fe-4S]** — Fe²⁺ in this cluster participates in substrate binding (not electron transfer) - Two-step process: 1. Dehydration of citrate → cis-aconitate + H₂O 2. Rehydration of cis-aconitate → isocitrate (the –OH group moves from C-3 to C-2) - At equilibrium: citrate (90%), isocitrate (4%), cis-aconitate (6%) — but the reaction is pulled forward because isocitrate is rapidly consumed by the next step - Aconitase is **stereospecific** — it acts only on the part of citrate derived from OAA, not the acetyl-CoA part. This is important for understanding why the two CO₂ molecules released are initially derived from OAA carbons (see below on isotope labeling) > ### 🔴 CLINICAL: Fluoroacetate Poisoning (Compound 1080) > - **Fluoroacetate** (a rodenticide, also found in certain toxic plants like *Dichapetalum cymosum* in South Africa) enters the cell > - Converted to **fluoroacetyl-CoA** → condenses with OAA (by citrate synthase) → **fluorocitrate** > - Fluorocitrate is a potent **competitive inhibitor of aconitase** > - Results in accumulation of citrate ("**lethal synthesis**" — a term coined by Sir Rudolph Peters) > - TCA cycle is blocked → energy depletion → death (primarily cardiac and CNS effects) > - No specific antidote; treatment is supportive. Glycerol monoacetate has been used experimentally > ### 🔴 CLINICAL: Aconitase and Oxidative Stress / Iron-Responsive Element Binding Protein (IRP) > - Cytoplasmic aconitase (c-aconitase) has a dual function: > - When iron is sufficient → [4Fe-4S] cluster intact → functions as aconitase > - When iron is low → loses Fe-S cluster → becomes **Iron Regulatory Protein 1 (IRP1)** → binds to **Iron Responsive Elements (IREs)** on mRNAs > - Binds 5' IRE of ferritin mRNA → **blocks translation** (reduces iron storage) > - Binds 3' IRE of transferrin receptor mRNA → **stabilizes mRNA** (increases iron uptake) > - Reactive oxygen species (ROS) can damage the Fe-S cluster of aconitase, contributing to mitochondrial dysfunction in conditions of oxidative stress > - **Friedreich's ataxia**: Deficiency of **frataxin** (a mitochondrial iron chaperone) → iron accumulation in mitochondria → oxidative damage to Fe-S clusters (including aconitase) → impaired TCA cycle and electron transport chain --- ## REACTION 3: Isocitrate Dehydrogenase (IDH) ### Reaction: **Isocitrate (C₆) + NAD⁺ → α-Ketoglutarate (C₅) + CO₂ + NADH + H⁺** ### Details: - **Enzyme**: Isocitrate dehydrogenase (IDH3 = NAD⁺-dependent, mitochondrial) - **Type**: Oxidative decarboxylation - **Irreversible** reaction (ΔG°' = −20.9 kJ/mol) - This is the **RATE-LIMITING STEP** of the TCA cycle (the major regulatory point) - Mn²⁺ or Mg²⁺ is required as cofactor - First CO₂ is released here (the **first decarboxylation**) - First NADH is generated here - Two-step mechanism: 1. Oxidation of isocitrate → oxalosuccinate (enzyme-bound intermediate) + NADH 2. Decarboxylation of oxalosuccinate → α-ketoglutarate + CO₂ ### Important Note — IDH Isoforms: | Isoform | Location | Coenzyme | Regulatory Role | |---------|----------|----------|-----------------| | **IDH1** | Cytoplasm | NADP⁺ | Generates NADPH for biosynthesis and antioxidant defense | | **IDH2** | Mitochondrial matrix | NADP⁺ | Generates NADPH in mitochondria | | **IDH3** | Mitochondrial matrix | NAD⁺ | **TCA cycle enzyme** — rate-limiting, allosterically regulated | ### Regulation of IDH3: - **Activated by**: ADP, Ca²⁺, NAD⁺, Mg²⁺/Mn²⁺ - **Inhibited by**: ATP, NADH (product inhibition) - **Ca²⁺** activation is particularly important during muscle contraction — increased Ca²⁺ from sarcoplasmic reticulum stimulates the TCA cycle to meet energy demands > ### 🔴 CLINICAL: IDH1 and IDH2 Mutations in Cancer > This is an extremely important clinical correlation: > - **Gain-of-function mutations** in IDH1 (R132H most common) and IDH2 (R140Q, R172K) are found in: > - **~80% of WHO grade II-III gliomas and secondary glioblastomas** > - **~20% of acute myeloid leukemia (AML)** > - Cholangiocarcinoma, chondrosarcoma > - Mutant IDH acquires a **neomorphic activity**: instead of converting isocitrate → α-KG, it converts **α-KG → 2-hydroxyglutarate (2-HG)** (an "oncometabolite") > - 2-HG is a competitive inhibitor of **α-KG-dependent dioxygenases**, including: > - **TET2** (DNA demethylase) → **DNA hypermethylation** → altered gene expression → blocks differentiation > - **Jumonji-domain histone demethylases** → **histone hypermethylation** > - **Prolyl hydroxylases (PHDs)** → stabilization of **HIF-1α** → pseudohypoxic state → angiogenesis > - **Collagen prolyl hydroxylases** → altered collagen maturation (relevant in chondrosarcoma) > - This creates a **CpG island methylator phenotype (CIMP)** and blocks normal cell differentiation > - **Targeted therapies**: > - **Ivosidenib** (AG-120) — IDH1 inhibitor (FDA-approved for AML and cholangiocarcinoma) > - **Enasidenib** (AG-221) — IDH2 inhibitor (FDA-approved for relapsed/refractory AML) > - **Vorasidenib** — brain-penetrant dual IDH1/2 inhibitor (approved for low-grade gliomas) > - IDH mutation is generally associated with **better prognosis** in gliomas compared to IDH-wildtype > - **D-2-Hydroxyglutaric aciduria**: Inherited deficiency of D-2-HG dehydrogenase → accumulation of 2-HG → encephalopathy, seizures, cardiomyopathy --- ## REACTION 4: α-Ketoglutarate Dehydrogenase Complex (α-KGDH) ### Reaction: **α-Ketoglutarate (C₅) + CoA-SH + NAD⁺ → Succinyl-CoA (C₄) + CO₂ + NADH + H⁺** ### Details: - **Enzyme**: α-Ketoglutarate dehydrogenase complex - **Type**: Oxidative decarboxylation - **Irreversible** reaction (ΔG°' = −33.5 kJ/mol — the most exergonic reaction of the TCA cycle) - Second CO₂ is released here (the **second decarboxylation**) - Second NADH is produced - This complex is **structurally and mechanistically analogous to the PDH complex** ### Components: | Component | Enzyme | Coenzyme | |-----------|--------|----------| | E1 | α-Ketoglutarate dehydrogenase | **TPP** | | E2 | Dihydrolipoyl transsuccinylase | **Lipoic acid, CoA-SH** | | E3 | Dihydrolipoyl dehydrogenase | **FAD, NAD⁺** | - Same **5 coenzymes** (TPP, lipoic acid, CoA, FAD, NAD⁺) as PDH - E3 component is **identical** to E3 of PDH complex (encoded by the same gene) ### Regulation: - **Inhibited by**: Succinyl-CoA (product inhibition), NADH, ATP/GTP - **Activated by**: Ca²⁺, ADP, NAD⁺ - Also inhibited by **arsenic** (attacks lipoic acid, same as PDH) > ### 🔴 CLINICAL: α-Ketoglutarate and Amino Acid Metabolism > - α-Ketoglutarate is a key link between the TCA cycle and amino acid metabolism > - It can be transaminated to form **glutamate** (by various aminotransferases) > - Glutamate can be further aminated to form **glutamine** (by glutamine synthetase) > - Glutamate is the major substrate for **glutamate dehydrogenase (GDH)**, which can regenerate α-KG + NH₄⁺ > - In the brain, α-KG → glutamate → GABA (via glutamic acid decarboxylase, which requires **pyridoxal phosphate/B₆**) > - Deficiency of B₆ → decreased GABA → **seizures** (this is why isoniazid toxicity causes seizures — INH depletes B₆) > ### 🔴 CLINICAL: α-KGDH Deficiency > - Very rare > - Leads to α-ketoglutaric aciduria > - Presents with developmental delay, hypotonia, metabolic acidosis, and extrapyramidal symptoms > - Decreased activity of α-KGDH has been observed in neurodegenerative diseases: **Alzheimer's disease**, **Parkinson's disease**, **Wernicke-Korsakoff syndrome** — suggesting vulnerability of this enzyme to oxidative stress and thiamine deficiency --- ## REACTION 5: Succinyl-CoA Synthetase (Succinate Thiokinase) ### Reaction: **Succinyl-CoA (C₄) + GDP + Pᵢ → Succinate (C₄) + CoA-SH + GTP** ### Details: - **Enzyme**: Succinyl-CoA synthetase (also called succinate thiokinase) - **Type**: Substrate-level phosphorylation - **Reversible** reaction - This is the **only substrate-level phosphorylation** in the TCA cycle - The energy from the thioester bond of succinyl-CoA is used to phosphorylate GDP → GTP - GTP can be converted to ATP by **nucleoside diphosphate kinase**: GTP + ADP → GDP + ATP - Two isoforms exist: - **GTP-specific** (predominant in **liver** and other tissues) - **ATP-specific** (uses ADP → ATP directly; found in some tissues) ### Mechanism (3 steps): 1. The succinyl group is transferred from CoA to an enzyme histidine residue → succinyl-enzyme + CoA-SH 2. The succinyl group is displaced by inorganic phosphate → succinate + phospho-histidine-enzyme 3. The phosphoryl group is transferred to GDP → GTP > ### 🔴 CLINICAL: Succinyl-CoA at the Crossroads > Succinyl-CoA is a critical metabolite with several metabolic connections: > > **1. Heme Synthesis**: Succinyl-CoA + Glycine → δ-aminolevulinic acid (δ-ALA), catalyzed by **ALA synthase** (rate-limiting enzyme of heme synthesis, requires PLP/B₆) > - This reaction occurs in the mitochondrial matrix > - Defects in heme synthesis → **Porphyrias** > - **Lead poisoning** inhibits ALA dehydratase and ferrochelatase → accumulation of ALA and protoporphyrin IX → abdominal pain, neuropathy, basophilic stippling > - **Sideroblastic anemia**: Can result from defective ALA synthase (X-linked form) or B₆ deficiency → ringed sideroblasts on bone marrow biopsy > > **2. Odd-chain fatty acid oxidation**: Propionyl-CoA → methylmalonyl-CoA → succinyl-CoA (requires **vitamin B₁₂**) > - **Methylmalonic acidemia**: Deficiency of methylmalonyl-CoA mutase or vitamin B₁₂ → accumulation of methylmalonic acid → metabolic acidosis, developmental delay > - **Vitamin B₁₂ deficiency**: Causes both methylmalonic aciduria (distinguishes B₁₂ from folate deficiency) and megaloblastic anemia (methyl trap hypothesis) > > **3. Ketone body metabolism**: Succinyl-CoA donates its CoA to acetoacetate → acetoacetyl-CoA (by succinyl-CoA:acetoacetate CoA transferase, also called thiophorase) > - This enzyme is present in extrahepatic tissues but **absent in liver** — that's why the liver produces but cannot utilize ketone bodies > > **4. Amino acid catabolism**: Valine, isoleucine, methionine, and threonine are catabolized to succinyl-CoA > ### 🔴 CLINICAL: Succinyl-CoA Synthetase Deficiency (mtDNA Depletion Syndrome) > - Rare autosomal recessive disorder > - Deficiency of succinyl-CoA synthetase (SUCLG1 or SUCLA2 mutations) > - Leads to **mitochondrial DNA depletion syndrome** — because succinyl-CoA synthetase interacts with nucleoside diphosphate kinase (NDPK), which is needed for maintaining the mitochondrial deoxyribonucleotide pool > - Presents with **Leigh-like syndrome**, methylmalonic aciduria, lactic acidosis, hypotonia, dystonia --- ## REACTION 6: Succinate Dehydrogenase (Complex II) ### Reaction: **Succinate (C₄) + FAD (enzyme-bound) → Fumarate (C₄) + FADH₂** ### Details: - **Enzyme**: Succinate dehydrogenase (SDH) - **Type**: Oxidation (dehydrogenation — removal of 2 H atoms in a trans configuration) - **Reversible** reaction - This is the **ONLY enzyme** that: 1. Is **embedded in the inner mitochondrial membrane** (all other TCA cycle enzymes are in the matrix) 2. Is a component of both the **TCA cycle** and the **electron transport chain (Complex II)** 3. Uses **FAD** rather than NAD⁺ as the electron acceptor (because the free energy change of this oxidation is insufficient to reduce NAD⁺) 4. Does **NOT produce NADH** 5. Is the only TCA cycle enzyme **not encoded by mtDNA** (all subunits are nuclear-encoded, but this applies to the other TCA enzymes too — actually ALL TCA enzymes are nuclear-encoded) ### Prosthetic Groups and Cofactors: - **FAD** (covalently bound to a histidine residue of SDHA subunit — this is unique; most flavoproteins have non-covalently bound FAD) - **Iron-sulfur clusters** [2Fe-2S], [4Fe-4S], [3Fe-4S] - **Ubiquinone (CoQ)** binding site - **Heme b** (does not participate in electron transfer; may protect against ROS) ### SDH Subunits (4 subunits): | Subunit | Function | |---------|----------| | SDHA | Catalytic — contains covalently-bound FAD, substrate binding | | SDHB | Contains Fe-S clusters, electron transfer | | SDHC | Membrane anchor, ubiquinone binding | | SDHD | Membrane anchor, ubiquinone binding | ### Electron flow in Complex II: Succinate → FAD → [Fe-S clusters] → Ubiquinone (CoQ) → enters ETC at Complex III ### Regulation: - **Competitive inhibition by malonate** (a structural analog of succinate — this was actually used historically by Krebs to help elucidate the cycle) - Also inhibited by **oxaloacetate** (powerful competitive inhibitor) - Activated by intermediates that increase substrate availability > ### 🔴 CLINICAL: SDH Mutations — Tumor Suppressor Gene / Paraganglioma-Pheochromocytoma Syndrome > - SDH subunit genes (SDHA, SDHB, SDHC, SDHD, SDHAF2) are **tumor suppressor genes** > - Loss-of-function mutations lead to: > - **Hereditary paraganglioma-pheochromocytoma syndrome** > - **Gastrointestinal stromal tumors (GISTs)** — especially **Carney-Stratakis syndrome** (paraganglioma + GIST) > - **Renal cell carcinoma** (SDH-deficient RCC — distinctive morphology) > - **Pituitary adenomas** (rare) > > **Mechanism**: SDH loss → accumulation of **succinate** → succinate acts as an **oncometabolite**: > - Inhibits **prolyl hydroxylases (PHDs)** → stabilization of **HIF-1α** → activation of pseudohypoxic pathway → increased VEGF, glycolysis (Warburg effect), angiogenesis > - Inhibits **TET enzymes** and **Jumonji-domain demethylases** → DNA and histone hypermethylation → epigenetic dysregulation > - This mechanism is analogous to 2-HG in IDH mutations > > **SDHB mutations** are associated with **malignant/metastatic paraganglioma** (worst prognosis among SDH mutations) > **SDHD mutations** show **parent-of-origin effect** (disease manifests only when inherited from father — maternal imprinting) > > **Succinate:Fumarate ratio** in tumor tissue can be used as a biomarker for SDH deficiency > ### 🔴 CLINICAL: Malonate as Competitive Inhibitor > - Malonate is a dicarboxylic acid (HOOC-CH₂-COOH) structurally similar to succinate (HOOC-CH₂-CH₂-COOH) > - Classic example of **competitive inhibition** taught in enzymology > - Malonate blocks the TCA cycle at succinate dehydrogenase > - This was used by Krebs experimentally and is a common exam question --- ## REACTION 7: Fumarase (Fumarate Hydratase) ### Reaction: **Fumarate (C₄) + H₂O → L-Malate (C₄)** ### Details: - **Enzyme**: Fumarase (fumarate hydratase, FH) - **Type**: Hydration (addition of water across the double bond in a trans manner) - **Reversible** (near equilibrium) - **Stereospecific**: Produces only **L-malate** (not D-malate); acts only on the **trans** double bond of fumarate (not on maleate, the cis isomer) - Exists as both mitochondrial and cytoplasmic isoforms (encoded by the same gene, different targeting) > ### 🔴 CLINICAL: Fumarase (FH) Deficiency — Fumaric Aciduria > - **Autosomal recessive** — extremely rare > - Most severe: brain malformations, severe intellectual disability, seizures, FTT (failure to thrive) > - Fumarate accumulates → acts as an oncometabolite (similar to succinate) > > **FH and Cancer (Hereditary Leiomyomatosis and Renal Cell Cancer — HLRCC)**: > - **Heterozygous germline FH mutations** → **HLRCC syndrome** (Reed's syndrome): > - Cutaneous and uterine **leiomyomas** (fibroids — often numerous and painful) > - Aggressive **type 2 papillary renal cell carcinoma** (very aggressive, can metastasize even when small) > - Mechanism: Loss of FH → fumarate accumulates → inhibits PHDs → HIF-1α stabilization → pseudohypoxia > - Fumarate also causes **succination of proteins** — fumarate reacts with cysteine residues (e.g., succination of KEAP1 → activates NRF2 antioxidant pathway → altered redox biology) > - These tumors rely heavily on **glycolysis** (Warburg effect) because TCA cycle is disrupted > ### 🔴 CLINICAL: Fumarate in Urea Cycle > - In the urea cycle, **argininosuccinate lyase** cleaves argininosuccinate → arginine + **fumarate** > - This fumarate can enter the TCA cycle, providing a link between the urea cycle and TCA cycle (**Krebs bicycle**) > - **Argininosuccinate lyase deficiency** → argininosuccinic aciduria (urea cycle disorder) --- ## REACTION 8: Malate Dehydrogenase (MDH) ### Reaction: **L-Malate (C₄) + NAD⁺ → Oxaloacetate (C₄) + NADH + H⁺** ### Details: - **Enzyme**: Malate dehydrogenase (MDH) - **Type**: Oxidation - **Reversible** reaction (but thermodynamically unfavorable in the forward direction: ΔG°' = +29.7 kJ/mol) - Despite the positive ΔG°', the reaction is **pulled forward** because: 1. OAA is rapidly consumed by citrate synthase (very exergonic next step) 2. NADH is rapidly reoxidized by the ETC 3. OAA concentration is maintained very low - Third and final NADH of the cycle is produced here ### Two isoforms: - **Mitochondrial MDH (MDH2)**: TCA cycle enzyme - **Cytoplasmic MDH (MDH1)**: Participates in the **malate-aspartate shuttle** (transfers reducing equivalents from cytoplasmic NADH into mitochondria) > ### 🔴 CLINICAL: Malate-Aspartate Shuttle > - NADH produced in the cytoplasm (e.g., from glycolysis) cannot directly cross the inner mitochondrial membrane > - The **malate-aspartate shuttle** (operates in liver, kidney, heart): > 1. OAA + NADH → Malate + NAD⁺ (cytoplasmic MDH) > 2. Malate enters mitochondria via malate-α-KG antiporter > 3. Malate + NAD⁺ → OAA + NADH (mitochondrial MDH) > 4. OAA is transaminated to aspartate, which exits to cytoplasm > - Transfers electrons as NADH (yields 2.5 ATP per NADH) > - Compare with **glycerol-3-phosphate shuttle** (muscle, brain): uses cytoplasmic NADH to produce mitochondrial FADH₂ (yields only 1.5 ATP) > ### 🔴 CLINICAL: OAA Availability and Ketogenesis ("Fats burn in the flame of carbohydrates") > - When OAA is depleted (e.g., starvation, uncontrolled diabetes, low carbohydrate intake): > - OAA is diverted to **gluconeogenesis** (via PEPCK) > - Less OAA available to condense with acetyl-CoA > - Acetyl-CoA accumulates > - Excess acetyl-CoA is diverted to **ketone body synthesis** (ketogenesis) in the liver > - This explains the classic saying: **"Fats burn in the flame of carbohydrates"** — you need OAA (derived from carbohydrate metabolism) to keep the TCA cycle running to oxidize acetyl-CoA (from fat) > - In **diabetic ketoacidosis (DKA)**: > - Insulin deficiency → unrestrained lipolysis → massive fatty acid oxidation → excess acetyl-CoA > - OAA is consumed for gluconeogenesis (glucagon-driven) > - → Ketone body production exceeds utilization → ketoacidosis > - Ketone bodies: acetoacetate, β-hydroxybutyrate (measured in blood), acetone (gives fruity breath odor) --- # 4. SUMMARY TABLE OF TCA CYCLE REACTIONS | Step | Substrate | Product | Enzyme | Type | Coenzyme | Energy Product | Reversibility | Notes | |------|-----------|---------|--------|------|----------|----------------|---------------|-------| | 1 | OAA + Acetyl-CoA | Citrate | Citrate synthase | Condensation | — | — | **Irreversible** | Rate-limiting in some tissues | | 2 | Citrate | Isocitrate | Aconitase | Isomerization | Fe-S cluster | — | Reversible | Via cis-aconitate | | 3 | Isocitrate | α-Ketoglutarate | Isocitrate dehydrogenase (IDH3) | Oxidative decarboxylation | NAD⁺, Mn²⁺ | **NADH + CO₂** | **Irreversible** | **Rate-limiting step** | | 4 | α-Ketoglutarate | Succinyl-CoA | α-KGDH complex | Oxidative decarboxylation | TPP, lipoate, CoA, FAD, NAD⁺ | **NADH + CO₂** | **Irreversible** | Analogous to PDH | | 5 | Succinyl-CoA | Succinate | Succinyl-CoA synthetase | Substrate-level phosphorylation | — | **GTP** | Reversible | Only substrate-level phosphorylation | | 6 | Succinate | Fumarate | Succinate dehydrogenase | Oxidation | FAD (covalent) | **FADH₂** | Reversible | Complex II of ETC | | 7 | Fumarate | Malate | Fumarase | Hydration | — | — | Reversible | Stereospecific for L-malate | | 8 | Malate | OAA | Malate dehydrogenase | Oxidation | NAD⁺ | **NADH** | Reversible | Pulled forward by citrate synthase | --- # 5. ENERGY YIELD OF ONE TURN OF THE TCA CYCLE | Reduced Coenzyme / High-Energy Compound | Number | ATP Equivalent (per molecule) | Total ATP | |----------------------------------------|--------|-------------------------------|-----------| | NADH (from IDH) | 1 | 2.5 | 2.5 | | NADH (from α-KGDH) | 1 | 2.5 | 2.5 | | NADH (from MDH) | 1 | 2.5 | 2.5 | | FADH₂ (from SDH) | 1 | 1.5 | 1.5 | | GTP (substrate-level) | 1 | 1.0 | 1.0 | | **TOTAL per acetyl-CoA** | | | **10 ATP** | If we include the PDH step (pyruvate → acetyl-CoA produces 1 NADH = 2.5 ATP): **Total per pyruvate = 10 + 2.5 = 12.5 ATP** **Total per glucose (2 pyruvates) through complete oxidation:** - Glycolysis: 2 ATP + 2 NADH (= 5 or 3 ATP depending on shuttle) = 7 or 5 ATP - 2 × PDH: 2 NADH = 5 ATP - 2 × TCA cycle: 20 ATP - **Grand total: ~30-32 ATP per glucose** (using modern P/O ratios of 2.5 for NADH and 1.5 for FADH₂) --- # 6. REGULATION OF THE TCA CYCLE ## 6.1 Three Irreversible Steps — Major Regulatory Points The three irreversible reactions are catalyzed by: 1. **Citrate synthase** (Reaction 1) 2. **Isocitrate dehydrogenase** (Reaction 3) — **principal rate-limiting enzyme** 3. **α-Ketoglutarate dehydrogenase** (Reaction 4) ## 6.2 Regulatory Mechanisms Summary | Condition | Effect on TCA Cycle | Mechanism | |-----------|-------------------|-----------| | High NADH/NAD⁺ | ↓ (inhibited) | Inhibits IDH3, α-KGDH, citrate synthase, PDH | | High ATP | ↓ (inhibited) | Inhibits IDH3, citrate synthase | | High ADP/AMP | ↑ (stimulated) | Activates IDH3 | | High Ca²⁺ | ↑ (stimulated) | Activates PDH phosphatase, IDH3, α-KGDH | | High succinyl-CoA | ↓ (inhibited) | Inhibits α-KGDH, citrate synthase | | High citrate | ↓ (inhibited) | Inhibits citrate synthase (product inhibition) | | Substrate availability (OAA, acetyl-CoA) | ↑ (stimulated) | More substrate for citrate synthase | ## 6.3 Ca²⁺ as a Key Activator Calcium ions activate **three** key mitochondrial dehydrogenases: 1. **PDH phosphatase** (activates PDH indirectly) 2. **IDH3** (direct allosteric activation) 3. **α-KGDH** (direct allosteric activation) This is physiologically important: during **muscle contraction**, Ca²⁺ is released from the sarcoplasmic reticulum → enters mitochondria → stimulates TCA cycle → increases ATP production to meet the energy demands of contraction. Similarly, in the **heart**, adrenergic stimulation and hormones increase intracellular Ca²⁺ → increased TCA cycle flux. --- # 7. ANAPLEROTIC REACTIONS "Anaplerosis" means "filling up." Since TCA cycle intermediates are constantly drained for biosynthetic purposes (cataplerosis), they must be replenished. ## 7.1 Major Anaplerotic Reactions: | Reaction | Enzyme | Product | Tissue | |----------|--------|---------|--------| | Pyruvate + CO₂ + ATP → OAA + ADP + Pᵢ | **Pyruvate carboxylase** | **OAA** | Liver, kidney (mitochondria) | | Pyruvate + CO₂ + NADPH → Malate + NADP⁺ | Malic enzyme (reverse) | Malate | Various | | Phosphoenolpyruvate + CO₂ + GDP → OAA + GTP | PEP carboxykinase (reverse) | OAA | Liver | | Glutamate + NAD(P)⁺ → α-KG + NH₄⁺ + NAD(P)H | Glutamate dehydrogenase | **α-KG** | Liver | | Amino acid catabolism | Various transaminases | Various TCA intermediates | Various | | Propionyl-CoA → succinyl-CoA | Propionyl-CoA carboxylase + methylmalonyl-CoA mutase | **Succinyl-CoA** | Liver | ## 7.2 Pyruvate Carboxylase — The Most Important Anaplerotic Enzyme **Pyruvate + CO₂ + ATP → OAA + ADP + Pᵢ** - Located in the **mitochondrial matrix** - Requires **biotin** (vitamin B₇/H) as prosthetic group - **Allosterically activated by acetyl-CoA** — this is a crucial regulatory feature: - When acetyl-CoA accumulates (e.g., from fatty acid oxidation), it signals that more OAA is needed to condense with acetyl-CoA - This simultaneously provides substrate for gluconeogenesis (OAA → PEP) and keeps the TCA cycle running - Acetyl-CoA is an **obligate activator** — without it, pyruvate carboxylase is virtually inactive > ### 🔴 CLINICAL: Pyruvate Carboxylase Deficiency > - **Autosomal recessive**, very rare > - Three types (A, B, C) of varying severity > - Results in: > - **Lactic acidosis** (pyruvate shunted to lactate) > - **Hypoglycemia** (impaired gluconeogenesis — OAA is the starting point) > - **Hyperammonemia** (impaired urea cycle — OAA needed for aspartate production) > - **Elevated lysine and citrulline in blood** > - **Decreased ketogenesis** in Type B > - Type B (French form): severe, usually fatal in infancy > - Type A (North American form): moderate, associated with Leigh-like syndrome > ### 🔴 CLINICAL: Biotin Deficiency > - Biotin is required by four carboxylases: pyruvate carboxylase, acetyl-CoA carboxylase, propionyl-CoA carboxylase, and β-methylcrotonyl-CoA carboxylase > - Causes: Raw egg whites (contain **avidin**, which binds biotin tightly), prolonged parenteral nutrition, biotinidase deficiency > - Symptoms: Dermatitis (periorificial), alopecia, metabolic acidosis > - **Multiple carboxylase deficiency** (holocarboxylase synthetase deficiency or biotinidase deficiency): All four carboxylases affected → organic aciduria, metabolic acidosis, seizures, skin rash --- # 8. CATAPLEROTIC REACTIONS (Removal of Intermediates for Biosynthesis) This is the **anabolic function** of the TCA cycle: | TCA Intermediate | Biosynthetic Product | |-----------------|---------------------| | **Citrate** | Fatty acid synthesis, cholesterol synthesis (via ATP-citrate lyase → cytoplasmic acetyl-CoA) | | **α-Ketoglutarate** | Glutamate, glutamine, proline, arginine (amino acid synthesis); neurotransmitter synthesis (glutamate, GABA) | | **Succinyl-CoA** | Heme synthesis (ALA synthase); porphyrin metabolism | | **OAA** | Gluconeogenesis (OAA → PEP via PEPCK); Aspartate (by transamination) → pyrimidine synthesis, urea cycle, asparagine | | **Fumarate** | Connection to urea cycle (produced by argininosuccinate lyase) | | **Malate** | Gluconeogenesis (via OAA in cytoplasm); shuttle systems | --- # 9. AMPHIBOLIC NATURE OF THE TCA CYCLE The TCA cycle is **amphibolic** because it serves both catabolic and anabolic roles: ### Catabolic: - Oxidizes acetyl-CoA → CO₂ - Generates reducing equivalents (NADH, FADH₂) for ETC/oxidative phosphorylation - Generates GTP ### Anabolic: - Provides intermediates for biosynthesis of: - Glucose (gluconeogenesis from OAA) - Fatty acids (from citrate) - Amino acids (from α-KG, OAA) - Heme (from succinyl-CoA) - Pyrimidines (from OAA → aspartate) - Purines (from α-KG → glutamate → glutamine) --- # 10. ENTRY POINTS OF AMINO ACIDS INTO THE TCA CYCLE This is clinically and biochemically important for understanding amino acid catabolism: | TCA Intermediate | Amino Acids | |-----------------|-------------| | **Acetyl-CoA** | Leucine*, isoleucine, lysine*, tryptophan, phenylalanine, tyrosine, threonine | | **Pyruvate (→ Acetyl-CoA)** | Alanine, serine, glycine, cysteine, tryptophan, threonine | | **α-Ketoglutarate** | Glutamate, glutamine, proline, arginine, histidine | | **Succinyl-CoA** | Valine, isoleucine, methionine, threonine | | **Fumarate** | Phenylalanine, tyrosine, aspartate | | **OAA** | Aspartate, asparagine | *Leucine and lysine are **purely ketogenic** (produce only acetyl-CoA/acetoacetyl-CoA) *Isoleucine, phenylalanine, tyrosine, tryptophan, threonine are **both glucogenic and ketogenic** --- # 11. INHIBITORS AND POISONS OF THE TCA CYCLE | Inhibitor/Poison | Target | Mechanism | |-----------------|--------|-----------| | **Fluoroacetate** | Aconitase | Converted to fluorocitrate (lethal synthesis) — competitive inhibitor | | **Malonate** | Succinate dehydrogenase | Competitive inhibitor (structural analog of succinate) | | **Arsenite (As³⁺)** | PDH, α-KGDH | Binds lipoic acid sulfhydryl groups | | **Mercury, lead** | Multiple enzymes | React with sulfhydryl groups | | **NADH accumulation** | IDH3, α-KGDH, citrate synthase, PDH | Product inhibition / allosteric inhibition | | **Beriberi (thiamine deficiency)** | PDH, α-KGDH | Lack of TPP coenzyme | --- # 12. ISOTOPE LABELING STUDIES AND SYMMETRY OF CITRATE ## 12.1 The "Ogston Effect" (Prochirality of Citrate) - Citrate is a **symmetric molecule** (has a plane of symmetry) but is treated **asymmetrically** by aconitase - In 1948, Alexander Ogston explained this: although citrate appears symmetric, when bound to the enzyme (a three-point attachment), the two –CH₂COO⁻ groups become distinguishable - Citrate is a **prochiral molecule** — it is not chiral itself, but becomes chiral when one of two identical groups is modified - Aconitase always acts on the –CH₂COO⁻ group derived from **OAA** (not from acetyl-CoA) - This means: - The two CO₂ molecules released in the first turn of the cycle (at IDH and α-KGDH) come from the carbon atoms originally in **OAA**, NOT from acetyl-CoA - The acetyl-CoA carbons are retained in the cycle intermediates and are released as CO₂ only in **subsequent turns** - This was proven using ¹⁴C-labeled acetyl-CoA in isotope tracing experiments --- # 13. RELATIONSHIP OF TCA CYCLE TO ELECTRON TRANSPORT CHAIN (ETC) - The TCA cycle produces **NADH and FADH₂** — these are "electron carriers" - These must be reoxidized by the **electron transport chain** (ETC) on the inner mitochondrial membrane: - NADH → Complex I → CoQ → Complex III → Cytochrome c → Complex IV → O₂ - FADH₂ (from SDH/Complex II) → CoQ → Complex III → Cytochrome c → Complex IV → O₂ - The TCA cycle is therefore **strictly aerobic** — it requires oxygen indirectly (to reoxidize NADH and FADH₂) - If oxygen is absent → ETC stops → NADH cannot be reoxidized → NAD⁺ is depleted → TCA cycle stops - This is why the TCA cycle only operates under **aerobic conditions** > ### 🔴 CLINICAL: Hypoxia and TCA Cycle > - In tissue hypoxia (e.g., ischemia, carbon monoxide poisoning, cyanide poisoning): > - ETC is inhibited → NADH accumulates → NAD⁺ depleted > - TCA cycle stops > - Cells switch to anaerobic glycolysis → lactate production → **lactic acidosis** > - **Cyanide** inhibits Complex IV → blocks entire ETC → accumulation of NADH → TCA cycle arrest > - Treatment: hydroxocobalamin, sodium thiosulfate, amyl nitrite > - **Carbon monoxide** binds hemoglobin (forming carboxyhemoglobin) AND inhibits Complex IV --- # 14. TCA CYCLE IN DIFFERENT METABOLIC STATES ## 14.1 Fed State (Postprandial) - High insulin - Glucose → pyruvate → acetyl-CoA → TCA cycle runs fully - Excess citrate exits to cytoplasm for fatty acid synthesis - TCA cycle is active for energy production ## 14.2 Fasting State - Low insulin, high glucagon - OAA diverted to gluconeogenesis - Acetyl-CoA (from fatty acid β-oxidation) exceeds OAA availability - Excess acetyl-CoA → ketone body synthesis - TCA cycle activity decreases in liver (OAA depleted) - TCA cycle activity increases in muscle/heart (using acetyl-CoA from fatty acids and ketone bodies) ## 14.3 Exercise - Increased Ca²⁺ → activates PDH, IDH3, α-KGDH - Increased ADP → activates IDH3 - TCA cycle flux increases dramatically - Muscle can increase TCA cycle activity 100-fold during intense exercise ## 14.4 Diabetes Mellitus - In uncontrolled type 1 DM: - Insulin deficiency → excessive lipolysis → massive β-oxidation → excess acetyl-CoA - Gluconeogenesis drains OAA - → Ketoacidosis (DKA) - In type 2 DM: - Chronic substrate overload → mitochondrial dysfunction → increased ROS from ETC - Impaired TCA cycle efficiency may contribute to metabolic complications --- # 15. ADDITIONAL CLINICAL CORRELATIONS ## 15.1 Leigh Syndrome (Subacute Necrotizing Encephalomyelopathy) - Caused by mutations affecting: - PDH complex - ETC complexes (especially Complex I, IV) - TCA cycle enzymes (rare) - mtDNA mutations - Bilateral symmetric lesions in brainstem and basal ganglia - Presents in infancy: developmental regression, hypotonia, lactic acidosis, respiratory failure - Elevated lactate in blood and CSF ## 15.2 Reye's Syndrome - Acute encephalopathy + fatty liver in children - Associated with aspirin use during viral illness (influenza, varicella) - Proposed mechanism: impaired mitochondrial function → disrupted β-oxidation and TCA cycle - Decreased OAA and impaired urea cycle → hyperammonemia ## 15.3 Cancer Metabolism and the Warburg Effect - Many cancer cells preferentially use **aerobic glycolysis** (glycolysis even in the presence of oxygen) — the **Warburg effect** - TCA cycle intermediates are diverted for biosynthesis (anabolic demands of rapidly dividing cells): - Citrate → lipid synthesis (for new membranes) - OAA → aspartate → nucleotide synthesis - α-KG → glutamate → non-essential amino acid synthesis - **Glutamine addiction**: Many cancers rely heavily on glutamine as a carbon and nitrogen source → glutamine → glutamate → α-KG (enters TCA cycle) — this is called **"glutaminolysis"** - Mutations in TCA cycle enzymes (IDH, SDH, FH) act as driver mutations in specific cancers (discussed above) ## 15.4 Organic Acidurias Related to TCA Cycle | Disorder | Enzyme Deficiency | Accumulated Metabolite | |----------|-------------------|----------------------| | D-2-Hydroxyglutaric aciduria | D-2-HG dehydrogenase | D-2-hydroxyglutarate | | L-2-Hydroxyglutaric aciduria | L-2-HG dehydrogenase | L-2-hydroxyglutarate | | Fumaric aciduria | Fumarase | Fumarate | | Succinic semialdehyde dehydrogenase deficiency | SSADH | γ-hydroxybutyric acid (GHB) | | α-Ketoglutaric aciduria | α-KGDH | α-Ketoglutarate | ## 15.5 Aluminum Toxicity - Aluminum inhibits aconitase (competes with iron for the Fe-S cluster) - Also inhibits other TCA cycle enzymes - Relevant in chronic renal failure patients on aluminum-containing antacids - Contributes to **dialysis dementia** (aluminum encephalopathy) ## 15.6 Mitochondrial Diseases — General Principles - Any defect in TCA cycle enzymes, ETC, or mitochondrial membrane integrity → impaired oxidative metabolism - Tissues with highest oxidative demand are most affected: **brain, heart, skeletal muscle, liver, kidney** - Features: lactic acidosis, myopathy, encephalopathy, seizures - **Ragged red fibers** on muscle biopsy (Gomori trichrome stain) — indicate mitochondrial proliferation/dysfunction --- # 16. VITAMINS AND COENZYMES IN THE TCA CYCLE | Vitamin | Active Form | Enzyme(s) Requiring It | |---------|-------------|----------------------| | **Thiamine (B₁)** | TPP | PDH (E1), α-KGDH (E1) | | **Riboflavin (B₂)** | FAD | PDH (E3), α-KGDH (E3), SDH | | **Niacin (B₃)** | NAD⁺ | IDH3, α-KGDH (E3), MDH, PDH (E3) | | **Pantothenic acid (B₅)** | CoA-SH | PDH, α-KGDH, citrate synthase | | **Biotin (B₇)** | Biotin (carboxylated) | Pyruvate carboxylase (anaplerosis) | | **Lipoic acid** | Lipoamide | PDH (E2), α-KGDH (E2) | --- # 17. STEREOCHEMICAL AND THERMODYNAMIC CONSIDERATIONS ## 17.1 Thermodynamics: | Reaction | ΔG°' (kJ/mol) | ΔG (kJ/mol, in vivo) | Notes | |----------|----------------|----------------------|-------| | Citrate synthase | −31.4 | −53.9 | Highly favorable | | Aconitase | +6.3 | ~0 | Near equilibrium | | IDH3 | −20.9 | −7.1 | Rate-limiting | | α-KGDH | −33.5 | −43.9 | Most exergonic | | Succinyl-CoA synthetase | −2.9 | −0.8 | Near equilibrium | | SDH | 0 | ~0 | Near equilibrium | | Fumarase | −3.8 | −0.9 | Near equilibrium | | MDH | +29.7 | −3.3 | Pulled forward in vivo | Note: The in vivo ΔG values are very different from ΔG°' values because actual concentrations of reactants and products differ greatly from standard conditions. ## 17.2 Stereochemistry: - Aconitase: Acts stereospecifically on citrate (prochiral) → produces only 2R,3S-isocitrate - IDH3: Produces 2-oxoglutarate with retention of configuration - Fumarase: Trans-addition of H₂O to fumarate → produces only L-malate - SDH: Removes 2H from succinate in a trans elimination → produces only trans-fumarate (not cis-maleate) --- # 18. GLYOXYLATE CYCLE (Not in humans — for completeness) - Found in plants, bacteria, and some fungi — NOT in mammals - Modified TCA cycle that allows net synthesis of glucose from acetyl-CoA (from fat) - Key enzymes unique to the glyoxylate cycle: 1. **Isocitrate lyase**: Isocitrate → succinate + glyoxylate 2. **Malate synthase**: Glyoxylate + acetyl-CoA → malate - Net result: 2 Acetyl-CoA → 1 OAA (which can be used for gluconeogenesis) - **Animals cannot perform net gluconeogenesis from acetyl-CoA** because they lack these enzymes - This is why fatty acids (which produce acetyl-CoA) cannot be converted to glucose in humans (except for odd-chain fatty acids which produce some propionyl-CoA → succinyl-CoA) --- # 19. EVOLUTION AND COMPARATIVE BIOCHEMISTRY - The TCA cycle is **universally conserved** across aerobic organisms — testament to its ancient origin - Some anaerobic organisms run a **reductive (reverse) TCA cycle** — used for CO₂ fixation (autotrophic carbon fixation in some bacteria and archaea) - The individual reactions of the TCA cycle may have existed separately before being assembled into a cycle during evolution --- # 20. SUMMARY OF KEY HIGH-YIELD POINTS 1. **Location**: Mitochondrial matrix (except SDH in inner membrane) 2. **Entry molecule**: Acetyl-CoA (C₂) 3. **Regenerated molecule**: OAA (C₄) 4. **CO₂ released**: 2 per turn (at IDH and α-KGDH) 5. **NADH produced**: 3 per turn 6. **FADH₂ produced**: 1 per turn 7. **GTP produced**: 1 per turn (substrate-level phosphorylation) 8. **Total ATP equivalent**: 10 per acetyl-CoA 9. **Rate-limiting enzyme**: Isocitrate dehydrogenase (IDH3) 10. **Three irreversible steps**: Citrate synthase, IDH3, α-KGDH 11. **Only substrate-level phosphorylation**: Succinyl-CoA synthetase 12. **Only FAD-linked enzyme / Complex II**: Succinate dehydrogenase 13. **Key activator**: Ca²⁺ (activates PDH phosphatase, IDH3, α-KGDH) 14. **Key inhibitors**: NADH, ATP, succinyl-CoA, citrate 15. **Amphibolic**: Both catabolic and anabolic 16. **Most important anaplerotic enzyme**: Pyruvate carboxylase (biotin-dependent, activated by acetyl-CoA) 17. **Strictly aerobic**: Requires O₂ indirectly (for NADH/FADH₂ reoxidation via ETC) 18. **Five coenzymes for PDH/α-KGDH**: TPP, lipoic acid, CoA, FAD, NAD⁺ --- This comprehensive account covers the biochemistry, enzymology, regulation, thermodynamics, metabolic connections, and extensive clinical correlations of the Krebs cycle, from the preparatory PDH reaction through all eight steps, anaplerosis, cataplerosis, and their relevance to human disease. 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