Graduate
  1. Physical Chemistry  1.1. Thermodynamics  1.1.1. Laws of Thermodynamics  1.1.2. Enthalpy and heat capacity  1.1.3. Entropy and free energy  1.1.4. Phase Equilibrium  1.1.5. Statistical thermodynamics  1.1.6. Thermodynamic cycle  1.1.7. Fugacity and activity  1.2. Quantum Chemistry  1.2.1. Principles of quantum mechanics  1.2.2. Schrödinger Equation  1.2.3. Particle in a box  1.2.4. Operators and eigenvalues  1.2.5. Atomic orbitals  1.2.6. Molecular orbital theory  1.2.7. Perturbation theory  1.2.8. Valence bond theory  1.2.9. Hybridization and chemical bonding  1.3. Chemical kinetics  1.3.1. Rate laws and reaction mechanisms  1.3.2. Collision theory  1.3.3. Transition state theory  1.3.4. Enzyme Kinetics  1.3.5. Chain Reactions and Polymerization  1.3.6. Photochemical reactions  1.4. Statistical mechanics  1.4.1. Partitioning functions  1.4.2. Molecular distribution functions  1.4.3. Boltzmann Distribution  1.4.4. Bose–Einstein and Fermi–Dirac statistics  1.5. Spectroscopy  1.5.1. Rotational Spectroscopy  1.5.2. Vibrational Spectroscopy  1.5.3. Electronic Spectroscopy  1.5.4. Nuclear magnetic resonance spectroscopy  1.5.5. Mass Spectrometry  1.5.6. Raman Spectroscopy  1.5.7. Electron paramagnetic resonance spectroscopy  1.6. Surface and colloidal chemistry  1.6.1. Absorption isotherm  1.6.2. Surface tension and wetting  1.6.3. Colloidal Stability  1.6.4. Catalysis  1.6.5. Emulsions and micelles  1.7. Electrochemistry  1.7.1. Nernst equation  1.7.2. Electrochemical cells  1.7.3. Conductivity and mobility  1.7.4. War  1.7.5. Fuel cells  1.7.6. Electrolysis
  2. Organic chemistry  2.1. Reaction mechanism  2.1.1. Nucleophilic substitution reactions  2.1.2. Electrophilic addition reactions  2.1.3. Elimination reactions  2.1.4. Rearrangement reactions  2.1.5. Radical reactions  2.1.6. Pericyclic reactions  2.2. Spectroscopy and structural determination  2.2.1. UV-Vis Spectroscopy  2.2.2. IR Spectroscopy  2.2.3. NMR Spectroscopy  2.2.4. Mass Spectrometry  2.2.5. X-ray crystallography  2.3. Stereoscopic  2.3.1. Chirality and optical activity  2.3.2. Structural analysis  2.3.3. Geometrical isomerism  2.3.4. Dynamic Stereochemistry  2.4. Organometallic Chemistry  2.4.1. Organolithium and organomagnesium reagents  2.4.2. Palladium-catalyzed cross-coupling reactions  2.4.3. Transition Metal Complexes  2.4.4. Metal–carbon bond  2.5. Polymer chemistry  2.5.1. Polymerization mechanism  2.5.2. Polymer Characteristics  2.5.3. Biodegradable Polymer  2.5.4. Conducting polymer  2.5.5. Supramolecular polymers  2.6. Medicinal chemistry  2.6.1. Drug design and development  2.6.2. Pharmacokinetics and pharmacodynamics  2.6.3. Structure-activity relationships  2.6.4. Molecular docking and drug screening
  3. Inorganic chemistry  3.1. Coordination chemistry  3.1.1. Crystal field theory  3.1.2. Ligand field theory  3.1.3. Spectrochemical Series  3.1.4. Chelation and stability  3.2. Organometallic Chemistry  3.2.1. Metal Carbonyls  3.2.2. Catalysis by organometallic complexes  3.2.3. Metallocenes  3.3. Bio-inorganic chemistry  3.3.1. Metalloproteins and enzymes  3.3.2. The role of metals in biological systems  3.3.3. Metal ion transport and storage  3.4. Solid state chemistry  3.4.1. Crystal Structures  3.4.2. Band theory of solids  3.4.3. Superconductors  3.4.4. Defects in the crystal  3.5. Lanthanides and Actinides  3.5.1. Electronic configuration in lanthanides and actinides  3.5.2. Coordination chemistry of the lanthanides  3.5.3. Magnetic Properties of Lanthanides
  4. Analytical chemistry  4.1. Chromatography  4.1.1. Gas Chromatography  4.1.2. High Performance Liquid Chromatography  4.1.3. Thin Layer Chromatography  4.2. Spectroscopic Techniques  4.2.1. Atomic absorption spectroscopy  4.2.2. X-ray diffraction  4.2.3. Inductively coupled plasma spectroscopy  4.3. Electroanalytical methods  4.3.1. Potentiometry  4.3.2. Voltammetry  4.3.3. Colometry  4.4. Mass Spectrometry  4.4.1. Ionization Technique  4.4.2. Fragmentation Pattern  4.5. Chemometrics  4.5.1. Multidisciplinary analysis  4.5.2. Machine Learning in Chemistry
  5. Theoretical and Computational Chemistry  5.1. Molecular dynamics simulation  5.1.1. Force fields and energy minimization  5.1.2. Monte Carlo simulation in molecular dynamics simulations  5.2. Quantum chemical methods  5.2.1. Hartree–Fock theory  5.2.2. Density functional theory  5.2.3. Semi-empirical methods  5.3. Computational drug design  5.3.1. Molecular Docking  5.3.2. QSAR Modeling  5.3.3. Virtual Screening
  6. Biochemistry  6.1. Enzyme Kinetics  6.1.1. Michaelis-Menten kinetics  6.1.2. Inhibition mechanism  6.2. Metabolism and Bioenergetics  6.2.1. Glycolysis  6.2.2. Citric acid cycle  6.2.3. Electron transport chain  6.3. molecular Biology  6.3.1. DNA replication and repair  6.3.2. Protein synthesis  6.3.3. Gene Regulation  6.4. Structural Biochemistry  6.4.1. Protein Folding  6.4.2. Membrane Biophysics
  7. Environmental Chemistry  7.1. Atmospheric Chemistry  7.1.1. Greenhouse gases  7.1.2. Ozone depletion  7.1.3. Air pollutants and smoke  7.2. Water Chemistry  7.2.1. pH and water hardness  7.2.2. Wastewater treatment  7.2.3. Aquatic Chemistry  7.3. Soil Chemistry  7.3.1. Heavy metal contamination  7.3.2. Soil pH and Buffering  7.3.3. Nutrient cycling  7.4. Toxicology and Chemical Safety  7.4.1. Toxicokinetics  7.4.2. Risk Assessment in Toxicology and Chemical Safety in Environmental Chemistry  7.4.3. Effects of pollutants on the environment

Graduate → Biochemistry → Metabolism and Bioenergetics ↓


Citric acid cycle


The citric acid cycle, also known as the Krebs cycle or tricarboxylic acid (TCA) cycle, is a central component of cellular respiration. It plays a key role in the metabolic pathway where cells generate energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins into carbon dioxide and a chemical called ATP (adenosine triphosphate).

Citrate Isocitrate α-ketoglutarate succinyl-coa Succinate fumarate Malate oxaloacetate

Overview of the cycle

The cycle begins with the condensation of acetyl-CoA with oxaloacetate to form citrate, catalyzed by an enzyme called citrate synthase. This reaction is important because it controls the flow of carbon through the cycle.

oxaloacetate + acetyl-CoA → citrate + CoA

The citrate molecule undergoes isomerization process to form isocitrate, which is catalyzed by aconitase. It involves a hydration step followed by a dehydration step.

Citrate ⇌ Isocitrate

Decarboxylation and energy production

It then undergoes oxidative decarboxylation by isocitrate dehydrogenase to form α-ketoglutarate, producing NADH and releasing CO2.

Isocitrate + NAD + → α-ketoglutarate + NADH + H + + CO 2

α-Ketoglutarate undergoes another oxidative decarboxylation to form succinyl-CoA, catalyzed by the α-ketoglutarate dehydrogenase complex. This step also forms NADH and CO2.

α-Ketoglutarate + NAD + + CoA → Succinyl-CoA + NADH + H + + CO 2

Regeneration of oxaloacetate

The cycle continues with the conversion of succinyl-CoA to succinate, catalyzed by succinyl-CoA synthetase, and is coupled to the phosphorylation of GDP to GTP (which can subsequently form ATP).

Succinyl-CoA + GDP + PI → Succinate + CoA + GTP

Succinate then undergoes oxidation to form fumarate through the action of succinate dehydrogenase, which is also involved in the electron transport chain. This step generates FADH2.

Succinate + FAD → fumarate + FADH2

The conversion of fumarate to malate is catalyzed by fumarase, which adds water to the double bond.

fumarate + H 2 O → malate

Finally, malate is oxidized by malate dehydrogenase, regenerating oxaloacetate, producing NADH.

Malate + NAD + → Oxaloacetate + NADH + H +

Energetic yield

In the oxidation of one acetyl-CoA molecule by the citric acid cycle, the major reduced cofactors are produced: three NADH, one FADH2, and one GTP (or ATP). Each NADH can be converted by the electron transport chain into about 2.5 molecules of ATP, and each FADH2 into about 1.5 molecules of ATP.

Regulation of the cycle

The citric acid cycle is strictly regulated through allosteric regulation of key enzymes. These include citrate synthase, isocitrate dehydrogenase, and the α-ketoglutarate dehydrogenase complex. High levels of ATP and NADH indicate a high-energy state of the cell and inhibit these enzymes, while high levels of ADP and NAD + indicate a low-energy state and activate the enzymes.

Importance in metabolism

Beyond energy production, the citric acid cycle provides several metabolic intermediates needed for various biosynthetic pathways. For example, α-ketoglutarate and oxaloacetate are important for amino acid synthesis. In addition, succinyl-CoA is important for heme synthesis.

Scene summary

The integration of the cycle with other metabolic pathways highlights its central role.

Glucose → pyruvate → acetyl-CoA → citric acid cycle Fat → Fatty Acids → Acetyl-CoA → Citric Acid Cycle Protein → Amino Acids → Citric Acid Cycle Intermediates

Conclusion

The citric acid cycle is an important aerobic pathway for energy production in cells. It not only generates ATP and other energy molecules to fuel cellular activities, but also contributes to various anabolic processes by providing building blocks for macromolecules. Its efficiency and regulation are essential for the proper functioning of living organisms.


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Citric acid cycle

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