Lipid Metabolism
Fatty Acid Synthesis: Building the Chain Two Carbons at a Time
Fatty acid synthesis turns a fed cell's spare acetyl CoA into long-chain fat, and it is deliberately not beta-oxidation run backward. Three differences make that possible: the work happens in the cytoplasm on a dedicated carrier (ACP), it spends NADPH instead of banking NADH, and a single committed enzyme (acetyl CoA carboxylase) decides whether any fat gets made at all. Trace the carbon from citrate to palmitate and the whole pathway resolves into one stoichiometric ledger: 8 acetyl CoA, 7 ATP, and 14 NADPH per C16 chain.
One cookie. One slice of pizza. If you don’t burn it, your body turns it into fat using exactly this pathway. Not a vague metaphor for “fat” but the literal chemistry, running in your liver and fat cells whenever you eat more than you spend. The carbon in that cookie becomes acetyl CoA, and acetyl CoA, in the fed state, gets stitched into long fatty acid chains two carbons at a time.
Here is the trap to avoid from the first sentence. You have already learned beta-oxidation, the pathway that takes fat apart, and it is tempting to assume synthesis just runs it backward. It does not. Synthesis is a separate machine in a separate place, and the differences are the whole point.
Why it is not beta-oxidation in reverse
Three differences set the two pathways apart, and each one earns its keep.
Location. Degradation happens in the mitochondrion. Synthesis happens in the cytoplasm. The cell physically separates the factory from the demolition crew.
Carrier. Beta-oxidation carries its intermediates on coenzyme A. Synthesis carries them on a dedicated acyl carrier protein, ACP, using a long phosphopantetheine arm that swings the growing chain from one active site to the next without ever letting it float free.
Reducing power. Degradation produces the high-energy carriers NADH and FADH2. Synthesis consumes NADPH. That single letter, the P on NADPH, is the cell’s accounting trick: it keeps a separate currency for building and for burning so the two never get confused.
Why bother with all this separation? Because it lets the cell regulate the two directions independently and avoid a futile cycle, the wasteful spin of building and tearing down the same molecule at the same time. Keep that idea close. It is the spine of the whole chapter.
Stage 1: getting the carbon out, with reducing power for free
Acetyl CoA is made in the mitochondrion, but synthesis happens in the cytoplasm, and acetyl CoA cannot cross the mitochondrial membrane. So the cell sends it across in disguise.
When fuel is plentiful, citrate from the TCA cycle is exported to the cytoplasm. There, ATP-citrate lyase cleaves it back into acetyl CoA and oxaloacetate. Think of citrate as an Uber for acetyl groups: it carries the passenger across a membrane the passenger could not cross alone.
The leftover oxaloacetate then does something clever. It is reduced to malate, and malic enzyme decarboxylates malate to pyruvate, generating NADPH on the way out. So the citrate shuttle is a two-for-one: it delivers the carbon and it makes reducing power. Eight of the fourteen NADPH a palmitate needs come from this shuttle; the pentose phosphate pathway supplies the other six.
Stage 2: the committed step
Once the acetyl CoA is in the cytoplasm, acetyl CoA carboxylase (ACC) makes the decision that matters. This is the committed step, the one Chris flags as the exam goldmine, and it does one thing: it attaches CO2 to acetyl CoA to make malonyl CoA, spending one ATP.
ACC carries a biotin cofactor, a vitamin tethered to a lysine that acts as a CO2 taxi, picking up carbon dioxide at one active site and dropping it at another. It is the same mechanism pyruvate carboxylase uses, so you have seen it before.
Biotin’s grip on this story has a vivid history. In 1927, researchers in Wisconsin fed rats raw egg white and watched them sicken. The culprit was avidin, an egg-white protein that binds biotin so tightly (a dissociation constant near 10 to the minus 15 molar) that it sequesters the vitamin and starves the enzymes that need it. That accident is how biotin was discovered.
Once you make malonyl CoA, you are committed to making fat. That single sentence is worth memorizing.
Carbon delivery (a preamble to the tally). Citrate made in the mitochondrion is exported and cleaved in the cytoplasm, spending one ATP to release the acetyl group onto CoA — citrate is the shuttle that carries acetyl groups across a membrane acetyl-CoA itself cannot cross.
Clinical. Targeted by the cholesterol drug bempedoic acid, activated only in liver, which lowers LDL with less muscle effect than statins.
The leftover oxaloacetate is reduced to malate, then decarboxylated by malic enzyme, generating NADPH and returning pyruvate to the mitochondrion. Routed this way, about eight of the fourteen NADPH per palmitate come from the shuttle; the other six come from the pentose phosphate pathway.
The committed step. A biotin arm acts as a CO₂ taxi (the same trick pyruvate carboxylase uses), attaching CO₂ to acetyl-CoA at the cost of one ATP. Once malonyl-CoA exists, the carbon is committed to fat. The CO₂ added here is released again at condensation, so it is a thermodynamic lever, not a building block.
Regulation. Activated by citrate, inhibited by palmitoyl-CoA and by AMPK phosphorylation. Malonyl-CoA also blocks CPT-I, so the cell will not burn and build fat at once.
Clinical. AMPK, the cell's low-energy sensor, switches ACC off during exercise — the metabolic logic behind metformin and exercise both restraining fat synthesis.
Both substrates are loaded onto the acyl carrier protein arm of fatty acid synthase, a multidomain homodimer. ACP carries intermediates on a long phosphopantetheine swinging arm — the same chemistry as CoA, but tethered so the growing chain never floats free.
The C of CRDR. The units join into a four-carbon β-ketoacyl chain, and the malonyl CO₂ — the one ACC just added — is released. That decarboxylation is the engine: two acetyl units cannot condense directly because the reaction is uphill, so the cell spends a CO₂ to pull it forward, then takes it right back.
The R of CRDR. The keto group is reduced to a hydroxyl using NADPH. Synthesis makes the D-isomer here; β-oxidation runs through the L-isomer — same atoms, opposite stereochemistry, one more sign the two pathways are genuinely separate machines.
The D of CRDR. Water leaves, creating a trans double bond. The chain is now one reduction away from being fully saturated.
The second R of CRDR. A second NADPH saturates the double bond, finishing one turn with a chain two carbons longer. The cycle then repeats: each turn adds one malonyl-CoA (two carbons) and spends two NADPH. Seven turns build C16.
After seven CRDR cycles the chain reaches sixteen carbons and thioesterase cleaves it free as palmitate. The balanced ledger: 8 acetyl-CoA + 7 ATP + 14 NADPH + 7 HCO₃⁻ → palmitate + 7 CO₂ + 8 CoA + 14 NADP⁺ + 7 ADP + 7 Pᵢ + 6 H₂O. Elongation past C16 and double bonds are added later, on the endoplasmic reticulum.
Clinical. Mammals lack the desaturases for the omega-6 and omega-3 positions, so linoleate and linolenate are essential fatty acids that must come from the diet.
Stage 3: the assembly line, two carbons at a time
The actual chain is built by fatty acid synthase (FAS), in mammals a single giant polypeptide folded into a homodimer that carries every catalytic activity it needs: a condensing enzyme, a couple of reductases, a dehydratase, a transferase, and the swinging ACP arm. It is a one-stop assembly line.
The chain grows by a four-step cycle, and the mnemonic is CRDR:
- Condensation joins an acetyl group and a malonyl group into a four-carbon beta-ketoacyl chain, releasing CO2.
- Reduction uses NADPH to turn the keto group into a hydroxyl.
- Dehydration removes water, leaving a double bond.
- Reduction uses a second NADPH to saturate that double bond.
After cycle one you have butyryl-ACP, four carbons. The cycle then repeats, adding two carbons each turn, seven turns in all, until the chain reaches sixteen carbons and a thioesterase clips it free as palmitate.
The cleverest piece is the condensation. Two acetyl groups cannot simply join, because that reaction is uphill and will not go on its own. So the cell first spends an ATP to bolt CO2 onto acetyl CoA, making malonyl CoA, and then releases that same CO2 during condensation. The decarboxylation pulls the reaction forward. CO2 is a leaving group here, not a building block. It is the quarter you drop into a vending machine: you do not get the quarter back, but it makes the snack come out. (One more tell that synthesis and degradation are different machines: synthesis makes D-hydroxyacyl intermediates, while beta-oxidation runs through the L-isomers.)
Count it all up and the ledger is clean:
8 acetyl CoA + 7 ATP + 14 NADPH + 7 HCO3⁻ → palmitate + 7 CO2 + 8 CoA + 6 H2O.
After palmitate, and why some fats are essential
FAS stops at sixteen carbons. Longer chains and any double bonds are added afterward on the endoplasmic reticulum: elongases extend the chain, and desaturases (using O2, NADH, and cytochrome b5) install double bonds. Mammals can make a double bond at carbon 9 but lack the enzymes to place one past it, at the omega-6 and omega-3 positions. That gap is why linoleate and linolenate are essential fatty acids you must eat. Burr and Burr proved it in 1929 by raising rats on fat-free diets and watching them fail. Downstream, arachidonate becomes the eicosanoids: cyclooxygenase makes prostaglandins and thromboxanes, and aspirin works by permanently acetylating that enzyme, which is why a single dose thins your blood for a week.
Regulation: the see-saw between building and burning
ACC is controlled three ways, and they all converge on one idea: do not build fat and burn fat at the same time.
Phosphorylation. AMPK, the cell’s low-fuel sensor, phosphorylates ACC and shuts it off. PP2A removes the phosphate and turns it on. Allostery. Citrate (abundant fuel) makes ACC polymerize into active filaments; palmitoyl CoA (the end product) depolymerizes them, a clean feedback brake. Hormones. Insulin activates ACC; glucagon and epinephrine inhibit it.
The reciprocal piece is the elegant part. Malonyl CoA, the product of ACC, also blocks CPT-I, the gate that lets fat into the mitochondrion for burning. So the moment the cell commits to synthesis, it slams the door on oxidation. This is why metformin works in diabetes: it inhibits Complex I, the AMP-to-ATP ratio climbs, AMPK switches on, synthesis falls, and oxidation rises. (Metformin descends from the French lilac, a folk remedy for high blood sugar.)
When malonyl-CoA is high, synthesis runs and that same malonyl-CoA plugs CPT-1, so fat cannot enter the mitochondrion to be burned. The cell builds.
ACC is the hinge. Insulin activates it (build); glucagon and the low-energy sensor AMPK shut it off (burn). Because its product malonyl-CoA both feeds synthesis and blocks the burn gate, one signal flips the whole cell between storing and spending fat.
Alcohol breaks the see-saw from a different angle. Metabolizing ethanol floods the liver with NADH, which stalls the TCA cycle and shunts acetyl CoA toward fat and ketone bodies, producing the fatty liver of heavy drinking.
Several Nobel Prizes touch this pathway and its neighbors. Next, in chapter 29, we follow palmitate and its cousins back the other way, into beta-oxidation, and you will see exactly how the cell keeps the two directions from ever running at once.
How we measure it
Radioisotope and stable-isotope tracing
Feed an animal acetate labeled with deuterium or carbon-14 and find the label inside its fat and cholesterol. This is how Schoenheimer showed body lipids are in constant turnover and how Bloch proved animals build sterols from two-carbon acetate. The same logic, run with a labeled bicarbonate, demonstrates that the CO2 added by ACC is the one released during condensation, never incorporated into the chain.
Avidin-biotin affinity capture
Egg-white avidin binds biotin with a dissociation constant near 10 to the minus 15 molar, one of the tightest non-covalent interactions known. Biochemists exploit it to pull any biotin-tagged molecule cleanly out of a mixture. Historically the same binding starved rats fed raw egg white of their biotin, the 1927 Wisconsin observation that revealed biotin as a vitamin and, later, as ACC's CO2 carrier.
Enzyme assay of a regulated polymer
ACC is active only as a long filament and inactive as a dissociated dimer. Adding citrate to the purified enzyme drives polymerization and switches it on; adding palmitoyl CoA depolymerizes it and switches it off. Watching velocity rise and fall as you titrate one effector at a time is how the allosteric controls on the committed step were mapped.