Fatty Acids and Triacylglycerols
Fatty acids and the triacylglycerols built from them are the fundamental molecules of lipid energy storage.
Fatty acids and the triacylglycerols built from them are the fundamental molecules of lipid energy storage. This subtopic covers how fatty acids are named and structured, why triacylglycerols make such efficient energy reserves, and the two opposing pathways — synthesis and beta-oxidation — that build and break down fatty acids.
Key Takeaways
Fatty acids are named by carbon count and double-bond count (e.g., 18:0); saturated fats pack tightly and are solid at room temperature, unsaturated fats are liquid.
Linoleic acid (omega-6) and alpha-linolenic acid (omega-3) are essential fatty acids humans must get from diet.
Triacylglycerols store energy efficiently because fatty acid carbons are highly reduced and the molecule is hydrophobic (no water weight).
Fatty acid synthesis: citrate shuttle moves acetyl-CoA to the cytoplasm → acetyl-CoA carboxylase forms malonyl-CoA (rate-limiting step) → fatty acid synthase builds palmitate via repeating condensation-reduction-dehydration-reduction cycles, using NADPH.
Beta-oxidation: fatty acyl-CoA synthetase activates the fatty acid → carnitine shuttle moves it into mitochondria → repeating oxidation-hydration-oxidation-cleavage cycles release acetyl-CoA, NADH, and FADH₂.
Malonyl-CoA links the two pathways by inhibiting carnitine acyltransferase I, so synthesis and oxidation don't run at the same time.
Odd-chain fatty acids end beta-oxidation with propionyl-CoA, converted via methylmalonyl-CoA (vitamin B12-dependent mutase) into succinyl-CoA.
Unsaturated fatty acids need extra enzymes (enoyl-CoA isomerase, and for polyunsaturated fats, 2,4-dienoyl-CoA reductase) to fix cis double bonds before beta-oxidation can proceed.
Fatty Acid Structure and Naming
Fatty acids are long-chain carboxylic acids: a carboxyl group at one end, with a long hydrocarbon chain extending from it. The carboxyl carbon is designated carbon 1, and the carbon next to it is the alpha carbon — a reference point used throughout fatty acid nomenclature and reactivity.
Fatty acids are named with two numbers: the total number of carbons in the chain, and the number of double bonds. A fatty acid written as 18:0, for example, has 18 carbons and no double bonds.
Saturated vs. Unsaturated Fatty Acids
Saturated fatty acids contain no double bonds — every carbon is connected by single bonds only. These straight chains pack tightly together, so saturated fatty acids are typically solid at room temperature.
Unsaturated fatty acids contain one or more double bonds, which introduce bends into the chain and prevent tight packing. As a result, unsaturated fatty acids are usually liquid at room temperature.
Essential Fatty Acids
Some unsaturated fatty acids are essential fatty acids — humans need them but cannot synthesize them, so they must come from the diet. The two key examples are linoleic acid, an omega-6 fatty acid, and alpha-linolenic acid (ALA), an omega-3 fatty acid. Both are required for normal cell function and for synthesizing other biologically important lipids.
Delta Notation vs. Omega Notation
Two notation systems describe the position of double bonds in unsaturated fatty acids:
Delta notation counts carbons starting from the carboxyl end, specifying the carbon number where the first double bond begins.
Omega notation counts carbons starting from the methyl end, indicating how far the first double bond sits from that end.
Both describe the same structure — they just use different reference points.
Triacylglycerols: The Body's Energy-Storage Molecule
The primary molecules used for lipid energy storage are triacylglycerols (triglycerides): three fatty acids attached to a glycerol backbone by ester linkages. Forming a triacylglycerol is a condensation reaction — one glycerol molecule reacts with three fatty acids, releasing three water molecules as the ester bonds form.
Triacylglycerols are ideal for energy storage for two reasons:
Fatty acid carbons are more reduced than carbohydrate carbons, meaning they carry more high-energy electrons — oxidizing fat releases more energy per carbon than sugar does.
Triacylglycerols are hydrophobic, so they can be stored compactly without the extra water weight that carbohydrates carry.
Unlike proteins and nucleic acids, which are directly encoded by DNA, lipid synthesis depends on metabolic pathways and substrate availability (acetyl-CoA, glycerol, fatty acids) — so lipid composition shifts with diet and metabolic state.
Fatty Acid Synthesis: Building Fat When Energy Is Abundant
Fatty acid metabolism has two opposing processes: synthesis, which stores energy, and beta-oxidation, which releases it. These don't run simultaneously in the same cell — the body builds fatty acids when energy is abundant and breaks them down when energy is needed.
Fatty acid synthesis occurs primarily in the cytoplasm of liver and adipose cells when insulin is high and energy is abundant — typically after a carbohydrate-rich meal, when excess glucose needs to be converted into stored fat.
The Citrate Shuttle: Getting Acetyl-CoA to the Cytoplasm
The building block for fatty acids is acetyl-CoA, but acetyl-CoA is produced inside the mitochondria while fatty acid synthesis happens in the cytoplasm — and acetyl-CoA can't cross the mitochondrial membrane directly. The citrate shuttle solves this: acetyl-CoA combines with oxaloacetate inside the mitochondria (via citrate synthase) to form citrate, which is transported into the cytosol and broken back down into acetyl-CoA and oxaloacetate. High citrate levels signal that the cell has plenty of energy and carbon available — an ideal time to synthesize fatty acids.
Acetyl-CoA Carboxylase and Malonyl-CoA: The Committed Step
The next step — and the committed, rate-limiting step of the whole pathway — is forming malonyl-CoA. The enzyme acetyl-CoA carboxylase converts acetyl-CoA into malonyl-CoA, requiring ATP, carbon dioxide, and biotin as a cofactor (biotin carries the CO₂ during the reaction).
This step is highly regulated: citrate activates acetyl-CoA carboxylase (energy levels are high), while long-chain fatty acyl-CoA molecules inhibit it (plenty of fatty acids already exist, so there's no need to make more).
Fatty Acid Synthase: Building the Chain Two Carbons at a Time
Malonyl-CoA is delivered into fatty acid synthase, a large multi-enzyme complex that holds the growing carbon chain on a carrier protein called ACP (acyl carrier protein).
Synthesis starts with two units — an acetyl group (the two-carbon primer) and a malonyl group — loaded onto the complex. Each cycle runs through four reactions:
Step | Reaction | What Happens |
|---|---|---|
1 | Condensation | Acetyl and malonyl groups join to form a four-carbon molecule; CO₂ is released |
2 | Reduction | NADPH donates electrons, converting to NADP⁺; lowers the molecule's energy |
3 | Dehydration | A water molecule is removed, creating a double bond in the chain |
4 | Reduction | A second NADPH removes the double bond, fully saturating the segment |
Each full cycle extends the chain by two carbons. The cycle repeats, using a new malonyl group each time, until the chain reaches 16 carbons, at which point fatty acid synthase releases palmitate — a saturated 16:0 fatty acid and the primary product of fatty acid synthesis in humans. Because fatty acid synthesis is an anabolic process building a large, reduced molecule, it requires a steady supply of NADPH throughout.
Elongation and Desaturation
Palmitate isn't necessarily the final fatty acid the body uses — it can be modified, primarily in the smooth endoplasmic reticulum (though some elongation also occurs in mitochondria):
Elongation adds more two-carbon units using malonyl-CoA, but through fatty acyl-CoA intermediates rather than ACP.
Desaturation introduces double bonds via fatty acyl-CoA desaturases, requiring oxygen and reducing equivalents.
Humans can't introduce double bonds beyond a certain position in the chain — which is exactly why some unsaturated fatty acids (the essential ones) must come from the diet.
Once synthesized and modified, fatty acids are typically stored as triacylglycerols. In the liver, they're packaged into VLDL and exported to adipose tissue for long-term storage.
Beta-Oxidation: Breaking Fat Down for Energy
Beta-oxidation occurs in the mitochondria during fasting or low-energy conditions, when insulin is low and glucagon/epinephrine are elevated — signaling the body to rely on stored fat rather than incoming glucose. Its purpose is to break fatty acids down to produce ATP.
Activation and the Carnitine Shuttle
Before oxidation can begin, a fatty acid must first be activated in the cytoplasm: the enzyme fatty acyl-CoA synthetase attaches the fatty acid to coenzyme A, using ATP (converted to AMP and pyrophosphate) — an energetically favorable, essentially irreversible step.
Long-chain fatty acyl-CoA can't cross the inner mitochondrial membrane on its own, so the cell uses the carnitine shuttle: carnitine acyltransferase I (on the outer mitochondrial membrane) transfers the fatty acid from CoA onto carnitine, forming fatty acyl-carnitine. A carnitine transporter carries this across the inner membrane, where another enzyme transfers the fatty acid back onto CoA — regenerating fatty acyl-CoA inside the matrix and releasing free carnitine to be reused.
This shuttle is tightly regulated: when fatty acid synthesis is active, malonyl-CoA inhibits carnitine acyltransferase I, preventing fatty acids from being synthesized and oxidized at the same time.
The Four-Reaction Beta-Oxidation Cycle
Once inside the mitochondrial matrix, fatty acids undergo beta-oxidation — a repeating cycle that removes two carbons at a time:
Step | Reaction | What Happens |
|---|---|---|
1 | Oxidation | Forms a double bond in the fatty acid |
2 | Hydration | Adds water across the double bond, forming a hydroxyl group |
3 | Oxidation | Oxidizes the hydroxyl group to a carbonyl (a beta-ketoacid) |
4 | Cleavage | Splits the beta-ketoacid into a shorter acyl-CoA plus acetyl-CoA |
Each cycle shortens the fatty acid by two carbons and produces three things: acetyl-CoA (enters the citric acid cycle for ATP), NADH, and FADH₂ (both feed the electron transport chain) — making fatty acid breakdown a three-way energy source at once. For long fatty acids like palmitate, this generates a very large amount of ATP, which is why fat is such an efficient energy source.
Odd-Chain Fatty Acids: The Propionyl-CoA Exception
Odd-chain fatty acids undergo beta-oxidation the same way as even-chain fatty acids until the final cycle. At that point, instead of yielding two acetyl-CoA molecules, the last three-carbon fragment produces one acetyl-CoA and one propionyl-CoA.
Propionyl-CoA can't directly enter the citric acid cycle. It's first carboxylated by propionyl-CoA carboxylase (using biotin and CO₂) into methylmalonyl-CoA, which is then rearranged by methylmalonyl-CoA mutase — an enzyme that requires vitamin B12 (as its adenosylcobalamin cofactor) — into succinyl-CoA, a citric acid cycle intermediate. This is how the carbons from odd-chain fatty acids enter central metabolism.
Unsaturated Fatty Acids Need Extra Enzymes
Unsaturated fatty acids also undergo beta-oxidation, but their double bonds interfere with the stereochemistry the standard beta-oxidation enzymes require. Those enzymes recognize a trans double bond between the alpha and beta carbons, but naturally occurring unsaturated fatty acids have cis double bonds at various positions — which must be rearranged before oxidation can continue.
Monounsaturated fatty acids need one extra enzyme: enoyl-CoA isomerase, which shifts the double bond's position and geometry into the correct trans configuration so the fatty acid can rejoin the normal cycle.
Polyunsaturated fatty acids need a second extra enzyme, 2,4-dienoyl-CoA reductase, which uses NADPH to reduce a conjugated double bond system that isomerase alone can't fix — producing an intermediate that isomerase can then convert into the correct trans configuration.
Common MCAT Mistakes
Thinking fatty acid synthesis and beta-oxidation both occur in the mitochondria. Synthesis happens in the cytoplasm (via fatty acid synthase), while beta-oxidation happens in the mitochondrial matrix — that's exactly why the citrate shuttle and carnitine shuttle exist, to move acetyl-CoA and fatty acyl-CoA across membranes in opposite directions.
Confusing which shuttle goes with which pathway. The citrate shuttle moves acetyl-CoA out of the mitochondria for synthesis; the carnitine shuttle moves fatty acyl-CoA into the mitochondria for beta-oxidation. Mixing them up reverses the direction and the pathway.
Forgetting that malonyl-CoA regulates both pathways. It's not just the substrate for chain elongation in synthesis — high malonyl-CoA also inhibits carnitine acyltransferase I, blocking beta-oxidation. This is the mechanism that keeps synthesis and breakdown from running at the same time.
Assuming all fatty acids finish beta-oxidation the same way. Even-chain, saturated fatty acids yield only acetyl-CoA. Odd-chain fatty acids leave a propionyl-CoA remainder that needs biotin and vitamin B12 to become succinyl-CoA, and unsaturated fatty acids need isomerase (and, for polyunsaturated fats, reductase) to fix cis double bonds before the standard cycle can finish them.
MCAT-Style Concept Check
Question: A fatty acid synthesis inhibitor blocks acetyl-CoA carboxylase. Which of the following is the most direct downstream consequence in a liver cell?
A) Beta-oxidation increases because malonyl-CoA no longer inhibits carnitine acyltransferase I
B) The citrate shuttle stops transporting acetyl-CoA out of the mitochondria entirely
C) Fatty acid synthase produces palmitate at a faster rate
D) NADPH accumulates because it can no longer be used in the citric acid cycle
Answer: A
Explanation: Acetyl-CoA carboxylase converts acetyl-CoA into malonyl-CoA, the committed step of fatty acid synthesis. Malonyl-CoA also inhibits carnitine acyltransferase I, so blocking its formation removes that inhibition and allows beta-oxidation to proceed (A). The citrate shuttle (B) is a separate transport step upstream of acetyl-CoA carboxylase and isn't shut down by inhibiting this one enzyme. Fatty acid synthase (C) needs malonyl-CoA as its substrate, so blocking malonyl-CoA formation would slow it, not speed it up. NADPH (D) is used in fatty acid synthesis, not the citric acid cycle, so this option misattributes its role.
FAQ
What's the difference between fatty acid synthesis and beta-oxidation?
Synthesis builds fatty acids from acetyl-CoA in the cytoplasm when energy is abundant, using the citrate shuttle, acetyl-CoA carboxylase, and fatty acid synthase. Beta-oxidation breaks fatty acids down in the mitochondria when energy is needed, using the carnitine shuttle and a repeating four-reaction cycle that releases acetyl-CoA, NADH, and FADH₂.
Why can't fatty acid synthesis and beta-oxidation happen in the same cell at the same time?
Malonyl-CoA, the intermediate formed during synthesis, inhibits carnitine acyltransferase I — the enzyme that shuttles fatty acids into the mitochondria for beta-oxidation. This keeps the two opposing pathways from running simultaneously and wasting energy.
What makes odd-chain fatty acids different during beta-oxidation?
Even-chain fatty acids break down entirely into acetyl-CoA. Odd-chain fatty acids leave a final three-carbon propionyl-CoA fragment, which requires biotin-dependent carboxylation and a vitamin B12-dependent mutase to convert into succinyl-CoA before it can enter the citric acid cycle.
Why do unsaturated fatty acids need extra enzymes for beta-oxidation?
Standard beta-oxidation enzymes require a trans double bond, but naturally occurring unsaturated fatty acids have cis double bonds. Enoyl-CoA isomerase fixes this for monounsaturated fatty acids; polyunsaturated fatty acids additionally need 2,4-dienoyl-CoA reductase to resolve a conjugated double bond system.
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