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Fatty Acid Synthesis and Regulation

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1,205prerequisites beneath it

Fatty Acid Oxidation (β-Oxidation)Pentose Phosphate Pathway→→Cholesterol Synthesis and Regulation Phospholipid Biosynthesis

Core Idea

Fatty acid synthesis (lipogenesis) is an anabolic pathway that builds fatty acids from acetyl-CoA units, primarily in the liver, adipose tissue, and mammary glands. The process requires acetyl-CoA carboxylase (catalyzes the first committed step, forming malonyl-CoA) and fatty acid synthase (a large multienzymatic complex catalyzing the iterative condensation, reduction, and dehydration of malonyl units). Unlike β-oxidation, synthesis uses NADPH (from the pentose phosphate pathway and malic enzyme) rather than NAD⁺/FAD. Fatty acid synthesis is allosterically activated by citrate and inhibited by AMP, palmitoyl-CoA, and glucagon.

Explainer

If β-oxidation is the process of chopping fatty acids into two-carbon acetyl-CoA units, fatty acid synthesis is essentially the reverse challenge: stitching two-carbon units back together into a long hydrocarbon chain. But cells do not simply run β-oxidation backwards. Instead, fatty acid synthesis uses a completely different set of enzymes, a different cellular compartment (the cytoplasm rather than the mitochondrial matrix), and a different electron carrier — NADPH instead of NADH and FADH₂. This separation is a recurring theme in metabolism: catabolic and anabolic pathways for the same molecule are kept distinct so the cell can regulate them independently.

The pathway begins with a critical problem: acetyl-CoA is produced inside mitochondria, but synthesis happens in the cytoplasm. Acetyl-CoA cannot cross the inner mitochondrial membrane directly, so it is shuttled out as citrate (via the citrate shuttle), then regenerated in the cytoplasm. Once there, acetyl-CoA carboxylase (ACC) catalyzes the first committed step: adding a CO₂ to acetyl-CoA to form malonyl-CoA, a three-carbon activated building block. This carboxylation is the key regulatory point of the entire pathway — ACC is activated by citrate (signaling energy abundance) and inhibited by palmitoyl-CoA (the end product, providing feedback inhibition) and by phosphorylation driven by AMP-activated protein kinase when cellular energy is low.

From malonyl-CoA, the heavy lifting is done by fatty acid synthase (FAS), a large homodimeric enzyme complex that carries the growing chain on an acyl carrier protein domain. Each cycle adds two carbons: malonyl-CoA condenses with the growing chain (releasing the CO₂ that was added by ACC), then the resulting β-keto group is reduced, dehydrated, and reduced again — four reactions per cycle, each consuming NADPH. After seven cycles, the 16-carbon saturated fatty acid palmitate is released. The NADPH consumed at each reduction step comes from two sources you already know: the pentose phosphate pathway (which produces NADPH in its oxidative phase) and the malic enzyme reaction that converts malate to pyruvate in the cytoplasm.

The regulation of fatty acid synthesis reflects the cell's overall energy state. When glucose is plentiful and energy stores are full, citrate accumulates in mitochondria and is exported to the cytoplasm, where it both provides the carbon source (via acetyl-CoA) and allosterically activates ACC. Insulin stimulates lipogenesis by activating ACC phosphatase, while glucagon and epinephrine suppress it by promoting ACC phosphorylation. This hormonal control ensures that the body synthesizes fat only when energy intake exceeds immediate needs — the biochemical basis of why chronic caloric surplus leads to fat accumulation.

Practice Questions 5 questions

Prerequisite Chain

Understanding Zero → The Number Zero → Counting to Five → Counting to 10 → Counting to 20 → Counting a Set of Objects Up to 20 → Cardinality: The Last Number Counted → Matching Numerals to Quantities → Subitizing Small Quantities → Addition Within 10 → Number Bonds to 10 → Addition Within 20 → Doubles and Near Doubles → Doubles Facts Within 10 → Near Doubles Facts Within 20 → Mental Math Strategies for Addition → Mental Math: Adding and Subtracting Tens → Addition Within 100 → Repeated Addition as Multiplication → Multiplication as Equal Groups → Multiplication: Arrays → Basic Multiplication Facts (0s, 1s, 2s, 5s, 10s) → Multiplication Facts Within 100 → Division as Equal Sharing → Division as Grouping (Measurement Division) → Division: Grouping (Repeated Subtraction) Model → Division: Fair Sharing Model → Division as Equal Sharing → Division as Grouping → Basic Division Facts → Division Facts Within 100 → Multiplication and Division Fact Families → Relationship Between Multiplication and Division → Division Facts as Inverse of Multiplication → Remainders and Quotients in Division → Division Word Problems → Multi-Step Word Problems → Solving Multi-Step Word Problems → Multiplication Word Problems → Division Word Problems → Introduction to Long Division → Factors and Multiples → Prime and Composite Numbers → Equivalent Fractions → Relating Fractions and Decimals → Decimal Place Value → Integers and the Number Line → Comparing and Ordering Integers → Absolute Value → Adding Integers → Subtracting Integers → Multiplying Integers → Dividing Integers → Unit Rates → Proportions → Percent Concept → Converting Between Fractions, Decimals, and Percents → Operations with Rational Numbers → Two-Step Equations → Solving Multi-Step Equations → Equations with Variables on Both Sides → Angle Pairs: Complementary, Supplementary, and Vertical → Parallel Lines and Transversals → Corresponding Angles → Alternate Interior Angles → Triangle Angle Sum Theorem → Exterior Angle Theorem → Triangle Inequality Theorem → Similar Triangles: AA Similarity → Similar Triangles: SSS and SAS Similarity → Proportions in Similar Triangles → Right Triangle Trigonometry Introduction → Sine, Cosine, and Tangent Ratios → Trigonometric Ratios Review → Radian Measure → Converting Between Degrees and Radians → The Unit Circle → Graphing Sine and Cosine → Graphing Tangent and Reciprocal Trigonometric Functions → Derivatives of Trigonometric Functions → Antiderivatives → Indefinite Integrals → Basic Integration Rules → Riemann Sums → Definite Integral Definition → Double Integrals: Definition and Setup → Iterated Integrals and Fubini's Theorem → Double Integrals over Rectangular Regions → Double Integrals over General Regions → Applications of Double Integrals: Area, Mass, and Moments → Triple Integrals in Cartesian Coordinates → Triple Integrals in Cylindrical and Spherical Coordinates → Change of Variables and the Jacobian Determinant → Applications of Triple Integrals: Volume and Mass → Vector Fields and Their Representations → Line Integrals of Vector Fields → Work and Circulation → Line Integrals of Scalar and Vector Functions → Fundamental Theorem for Line Integrals → Conservative Vector Fields → Conservative Vector Fields and Potential Functions → Curl and Divergence of Vector Fields → Curl and Divergence → Divergence Theorem → Electric Flux and Divergence Theorem → Gauss's Law: Integral Form and Meaning → Solving Problems with Gauss's Law → Conductors in Electrostatic Equilibrium → Capacitance and Capacitors → Dielectrics → Dielectric Constant and Relative Permittivity → Electric Field Inside Dielectric Materials → Dielectric Materials and Polarization → Dielectric Susceptibility and Permittivity → Energy Density in Electric Fields → Electric Current and Current Density → Electrical Resistance and Resistivity → Ohm's Law and Circuit Elements → Electromotive Force (EMF) and Batteries → Kirchhoff's Circuit Laws: Voltage and Current → DC Circuit Network Analysis Methods → Transient Response in RC Circuits → RC Circuits → LC and RLC Circuits → AC Circuits: Fundamentals → Impedance and Reactance → AC Power and Resonance → Electromagnetic Waves → Postulates of Special Relativity → Time Dilation → Length Contraction → Lorentz Transformation → Relativistic Velocity Addition → Relativistic Momentum and Energy → Mass-Energy Equivalence and E=mc² → Photons as Particles with Energy and Momentum → Planck-Einstein Relation: Energy and Frequency → Photoelectric Effect → The Photon: Light as Quanta → Compton Scattering → Wave-Particle Duality → de Broglie Wavelength → The Schrödinger Equation → State Vectors and Wavefunctions → Quantum Superposition → Quantum Entanglement → Bell Theorem and Bell Inequalities → Postulates of Quantum Mechanics → Observables and Quantum Operators → Commutators and Commutation Relations → Quantum Angular Momentum → Quantum Mechanical Treatment of Hydrogen → Solving the Schrödinger Equation for Hydrogen Atom → Quantum Numbers → Electron Configuration → Periodic Trends → Covalent Bonding → Electronegativity and Bond Polarity → Ionic Bonding → Lewis Structures → VSEPR Theory and Molecular Geometry → Molecular Geometry and Electron Pair Geometry → Molecular Polarity and Dipole Moments → Intermolecular Forces → States of Matter and Phase Changes: Melting, Boiling, and Sublimation → Gas Laws and the Ideal Gas Equation → Gas Stoichiometry and Volume-Volume Calculations → Thermochemistry and Enthalpy → Heat Capacity and Calorimetry → Entropy and Molecular Disorder → Spontaneity and ΔG → Entropy and Gibbs Free Energy → Chemical Equilibrium → Acid-Base Chemistry → Weak Acid Ionization → Weak Base Ionization → Acid and Base Strength: Ka, Kb, and Ionization → Leaving Groups and Nucleofugality → SN2 Substitution Reactions → SN1 Substitution Reactions → E1 Elimination Reactions → Alcohols and Ethers: Structure, Properties, and Nomenclature → Reactions of Alcohols → Aldehydes and Ketones: Structure and Reactivity → Oxidation Reactions in Organic Chemistry → Oxidation of Alcohols to Aldehydes and Ketones → Aldehyde and Ketone Structure and Nomenclature → Nucleophilic Addition to Aldehydes and Ketones → Carboxylic Acids and Their Derivatives → IUPAC Nomenclature of Carbonyls and Carboxylic Acids → IUPAC Nomenclature of Alkenes → Electrophilic Addition to Alkenes → Aromaticity and Benzene → Electrophilic Aromatic Substitution (EAS) → Nucleophilic Aromatic Substitution (SNAr) → Nucleophilic Acyl Substitution → Amines: Structure, Basicity, and Reactions → Amine Reactivity: Nucleophilicity and Basicity → Amino Acid Structure and Properties → Peptide Bonds and Polypeptide Formation → Protein Primary Structure → Protein Secondary Structure → Protein Tertiary Structure → Enzyme Structure and Function → Transcription: DNA to RNA → RNA Types and Structure → RNA Structure and Intramolecular Base Pairing → RNA Processing and Splicing → Translation: RNA to Protein → Ribosomes: Protein Synthesis Machines → Translation: Initiation and Elongation → Post-Translational Modifications → Proteasomal Degradation and Ubiquitin-Mediated Marking → Cell Cycle Regulation and Checkpoints → Cell Cycle Checkpoints: Ensuring Genome Integrity → Cell Cycle Checkpoints and Cancer Prevention → Mitotic Spindle Checkpoint and Chromosome Segregation → Kinetochore Structure and Function → Mitochondria: Structure and Function → Cellular Respiration Overview → Glycolysis → Glycolysis: Mechanism and Regulation → Pentose Phosphate Pathway → Fatty Acid Synthesis and Regulation

Longest path: 224 steps · 1205 total prerequisite topics

Prerequisites (2)

Fatty Acid Oxidation (β-Oxidation)hard Pentose Phosphate Pathwaysoft

Leads To (2)

Cholesterol Synthesis and Regulationsoft Phospholipid Biosynthesishard