Fatty acid synthesis runs in the cytosol of liver and fat cells after a carbohydrate-rich meal, fed by the citrate shuttle: acetyl-CoA leaves the mitochondria as citrate, and ATP-citrate lyase frees it. Its rate-limiting step, acetyl-CoA to malonyl-CoA, is catalyzed by biotin-dependent acetyl-CoA carboxylase, and fatty acid synthase then uses 14 NADPH per palmitate.
The NADPH comes from the pentose phosphate pathway and from malic enzyme, the last step of the citrate shuttle. Insulin and citrate turn acetyl-CoA carboxylase (ACC) on; glucagon, epinephrine and AMP-activated protein kinase (AMPK) turn it off by phosphorylation, and palmitoyl-CoA inhibits it by feedback.
ACC's product, malonyl-CoA, also blocks carnitine palmitoyltransferase 1 (CPT-1), so a liver that is making fat does not burn it at the same time.
The citrate shuttle for fatty acid synthesis
Pyruvate dehydrogenase makes acetyl-CoA inside the mitochondria, but fatty acid synthase works in the cytosol, and acetyl-CoA cannot cross the mitochondrial membranes. So the acetyl group travels inside citrate:
- Citrate synthase (matrix) joins acetyl-CoA to oxaloacetate. When ATP and NADH are high, the TCA cycle slows and citrate is exported instead of oxidized.
- The citrate carrier (SLC25A1, inner membrane) swaps citrate going out for malate coming in.
- ATP-citrate lyase (cytosol) spends 1 ATP to split citrate into acetyl-CoA and oxaloacetate. This acetyl-CoA feeds both fatty acid and cholesterol synthesis.
- Cytosolic malate dehydrogenase reduces oxaloacetate to malate, regenerating NAD⁺ for glycolysis.
- Malic enzyme turns malate into pyruvate + CO₂, making 1 NADPH. Pyruvate re-enters the mitochondria, where biotin-dependent pyruvate carboxylase rebuilds oxaloacetate; alternatively, malate returns in exchange for the next citrate.
In rat liver, normal citrate carrier activity requires insulin, while starvation and polyunsaturated fat lower it.
Memory hook: citrate carries acetyl units out for synthesis; carnitine carries long-chain acyl groups in for β-oxidation.
The whole pathway in five steps:
| Step | Where | What happens | Enzyme and cofactors |
|---|---|---|---|
| 1. Make acetyl-CoA | Mitochondrial matrix | Pyruvate → acetyl-CoA | Pyruvate dehydrogenase (thiamine pyrophosphate) |
| 2. Export | Inner membrane, then cytosol | Acetyl units leave as citrate and are freed | Citrate carrier; ATP-citrate lyase (ATP) |
| 3. Commit | Cytosol | Acetyl-CoA + HCO₃⁻ → malonyl-CoA | ACC (biotin, ATP); rate-limiting |
| 4. Build | Cytosol | Seven cycles of 2 carbons give palmitate | Fatty acid synthase (NADPH) |
| 5. Remodel and ship | Endoplasmic reticulum | Elongation, first double bond, VLDL | ELOVL6, stearoyl-CoA desaturase 1 |
Step 1 is drilled in the pyruvate dehydrogenase cofactors mnemonic.
Rate-limiting step of fatty acid synthesis: acetyl-CoA carboxylase
The first and rate-limiting step is the carboxylation of acetyl-CoA by cytosolic ACC1:
Acetyl-CoA + HCO₃⁻ + ATP → malonyl-CoA + ADP + Pi
Its biotin carboxylase domain uses ATP to attach a carboxyl group from bicarbonate to covalently bound biotin, and its carboxyltransferase domain moves that carboxyl onto acetyl-CoA. ACC1 works as a dimer, the biotin of one subunit reaching the acetyl-CoA site of its partner, and citrate assembles it into filaments, its most active form. Phosphorylation blocks dimer formation and switches it off.
- Cofactors: biotin, ATP and bicarbonate. Biotin deficiency also hits pyruvate, propionyl-CoA and 3-methylcrotonyl-CoA carboxylases; see biotin (vitamin B7) and its 4 carboxylases.
- The bicarbonate carbon does not stay: it leaves as CO₂ at each condensation, so all 16 carbons of palmitate come from acetyl-CoA.
Every pathway's control point is lined up in the rate-limiting enzymes table.
Is fatty acid synthesis in the cytosol? Where it happens
Yes. ATP-citrate lyase, ACC1, fatty acid synthase and malic enzyme are cytosolic; the mitochondria supply the acetyl-CoA, and the endoplasmic reticulum finishes the job.
| Compartment | What happens there | Enzymes |
|---|---|---|
| Mitochondrial matrix | Acetyl-CoA and citrate made; oxaloacetate rebuilt | Pyruvate dehydrogenase, citrate synthase, pyruvate carboxylase |
| Inner mitochondrial membrane | Citrate out, malate in | Citrate carrier |
| Cytosol | Palmitate built; NADPH made | ATP-citrate lyase, ACC1, fatty acid synthase, malic enzyme, G6PD |
| Outer mitochondrial membrane | Local malonyl-CoA blocks fatty acid entry | ACC2, CPT-1 |
| Endoplasmic reticulum | Elongation, first double bond, VLDL assembly | ELOVL6, stearoyl-CoA desaturase 1 |
Which tissues? In principle every cell can make fatty acids for its membranes, but synthesis is most active in the liver, adipose tissue and the lactating mammary gland. The liver turns glucose into fat once its glycogen stores are full (roughly 5% of liver mass) and exports it as triglyceride in VLDL; see lipoprotein metabolism. In rodents the liver dominates; in humans overfed carbohydrate, adipose tissue may make more than the liver. Skeletal muscle normally makes little. In healthy people, newly made fatty acids are under 5% of VLDL triglyceride after an overnight fast and peak around 23% within 4 hours of a meal.
Does fatty acid synthesis use NADPH? Yes, 14 per palmitate
Each of the seven cycles on fatty acid synthase contains two reductions, and both use NADPH:
Acetyl-CoA + 7 malonyl-CoA + 14 NADPH → palmitate + 7 CO₂ + 8 CoA + 14 NADP⁺ (protons and water omitted)
ACC also spends 1 ATP per malonyl-CoA (7 per palmitate), and ATP-citrate lyase 1 more for each acetyl-CoA it frees.
| Source of NADPH | Reaction | Notes |
|---|---|---|
| Pentose phosphate pathway | Oxidative phase: 2 NADPH per glucose-6-phosphate | Rate-limiting enzyme G6PD; more active in the fed, high-carbohydrate state |
| Malic enzyme | Malate + NADP⁺ → pyruvate + CO₂ + NADPH | Citrate shuttle; at most 1 per acetyl unit exported, so at most 8 of the 14 |
| Cytosolic isocitrate dehydrogenase | Isocitrate + NADP⁺ → α-ketoglutarate + CO₂ + NADPH | A further cytosolic source |
G6PD and the tissues that depend on the pathway are covered in the pentose phosphate pathway visual hack.
NADPH, not NADH. β-Oxidation runs the other way and produces NADH and FADH₂.
How ACC is regulated: citrate, insulin, glucagon, AMPK and palmitoyl-CoA
ACC is controlled within minutes by allosteric effectors and phosphorylation, and over hours by how much enzyme is made.
| Signal | Effect | How | When |
|---|---|---|---|
| Citrate | On | Feed-forward allosteric activation; drives polymerization into active filaments | Fed, energy-rich |
| Insulin | On | Fast: dephosphorylation (insulin inhibits AMPK and activates a phosphatase) and more polymerized enzyme. Slow: SREBP-1c induces ACC and fatty acid synthase | Fed |
| Glucose | On | ChREBP induces ACC, fatty acid synthase, ELOVL6 and SCD1 | High-carbohydrate meals |
| Palmitoyl-CoA | Off | End-product feedback; breaks up the dimer | Fatty acids plentiful |
| AMPK | Off | Phosphorylates ACC1 and ACC2, blocking dimer formation | Low energy (high ADP-to-ATP ratio) |
| Glucagon, epinephrine | Off | Phosphorylation, mainly through AMPK; protein kinase A can phosphorylate ACC in vitro | Fasting, stress |
| Polyunsaturated fat | Off | Suppresses ACC, fatty acid synthase and the citrate carrier | High-PUFA diet |
Rule of thumb: phosphorylated ACC is off. When energy returns, protein phosphatase 2A removes the AMPK phosphate. Metformin, an AMPK activator, promotes ACC phosphorylation.
| Feature | ACC1 (ACACA) | ACC2 (ACACB) |
|---|---|---|
| Location | Cytosol | Outer mitochondrial membrane |
| Main tissues | Liver, adipose tissue, lactating mammary gland | Heart, skeletal muscle; also liver and adipose tissue |
| Its malonyl-CoA | Substrate for fatty acid synthase | Local brake on CPT-1 |
HMG-CoA reductase, the rate-limiting enzyme of cholesterol synthesis, follows the same energy logic: endoplasmic reticulum membrane, 2 NADPH, induced by SREBP-2 when sterols are low, degraded when sterols are high, inactivated by AMPK, blocked by statins. See the cholesterol synthesis pathway.
The fed-fasted switch: malonyl-CoA blocks CPT-1
In 1977, McGarry and colleagues showed that malonyl-CoA, the substrate of fatty acid synthesis, also inhibits CPT-1, the rate-limiting enzyme of β-oxidation. One molecule decides whether long-chain fatty acids are built or burned.
- Fed: insulin keeps ACC active, malonyl-CoA rises and CPT-1 is blocked, so fatty acids are esterified into triglyceride instead of being oxidized to ketone bodies.
- Fasting: glucagon signaling activates AMPK, which phosphorylates both ACCs and activates malonyl-CoA decarboxylase. Malonyl-CoA falls, CPT-1 opens, and the liver, the only ketone-producing organ, starts ketogenesis.
- Untreated type 1 diabetes: the same switch plus unrestrained lipolysis drives ketoacidosis; see ketogenesis and DKA vs AKA.
- Fine print: liver CPT-1 needs about ten times more malonyl-CoA for inhibition than the muscle enzyme, and medium-chain fatty acids such as octanoate enter mitochondria without CPT-1.
| Feature | Fatty acid synthesis | β-Oxidation |
|---|---|---|
| Location | Cytosol | Mitochondrial matrix |
| Hormonal state | Fed: insulin | Fasting: glucagon, epinephrine |
| Crossing the membrane | Citrate carries acetyl units out | Carnitine carries long-chain acyl groups in |
| Rate-limiting enzyme | ACC | CPT-1 |
| Chain carrier | Acyl carrier protein | Coenzyme A |
| Two-carbon unit | Added from malonyl-CoA | Removed as acetyl-CoA |
| Electron carrier | Uses NADPH | Makes FADH₂ and NADH |
| Product | Palmitate (16:0) | Acetyl-CoA |
Transport and oxidation are in the carnitine shuttle steps and beta-oxidation steps.
Fatty acid synthase: seven cycles to palmitate
Human fatty acid synthase is a homodimer. Each subunit is one large polypeptide with seven catalytic activities and an acyl carrier protein (ACP) domain, whose phosphopantetheine arm, made from pantothenic acid (vitamin B5), swings the growing chain between active sites. The malonyl/acetyl transferase loads the acetyl primer and a malonyl group onto ACP, and the primer moves to a cysteine of the condensing enzyme; each cycle then runs four steps:
| Step | Domain | Reaction | Uses or releases |
|---|---|---|---|
| 1. Condensation | β-Ketoacyl synthase | Acyl + malonyl-ACP → β-ketoacyl-ACP | Releases CO₂ |
| 2. Reduction | β-Ketoacyl reductase | → β-hydroxyacyl-ACP | NADPH |
| 3. Dehydration | Dehydratase | → trans-enoyl-ACP | Releases H₂O |
| 4. Reduction | Enoyl reductase | → acyl-ACP, 2 carbons longer | NADPH |
Release: the thioesterase prefers 16-carbon chains, so it frees palmitate (16:0), the main product; some stearate (18:0) also forms. Palmitate is then activated to palmitoyl-CoA.
The arithmetic: 2 primer carbons + 7 × 2 malonyl carbons = 16; 7 cycles × 2 NADPH = 14 NADPH.
Control: mainly by how much enzyme is made (insulin through SREBP-1c, glucose through ChREBP); ACC supplies the minute-to-minute control.
After palmitate: elongation, desaturation and essential fatty acids
Endoplasmic reticulum enzymes take over:
- Elongation: ELOVL6 condenses palmitoyl-CoA with malonyl-CoA, the rate-limiting step of a four-step elongation cycle, making stearate (18:0).
- Desaturation: stearoyl-CoA desaturase 1 (SCD1) inserts a cis double bond at Δ9, using O₂ and electrons from cytochrome b5: stearate becomes oleate (18:1 n-9), palmitate becomes palmitoleate (16:1).
- The limit: humans lack the Δ12 and Δ15 desaturases, so linoleic acid (18:2 n-6) and α-linolenic acid (18:3 n-3) are essential.
- Downstream: Δ6 (FADS2) and Δ5 (FADS1) desaturases plus elongases make arachidonic acid from linoleic acid and EPA and DHA from α-linolenic acid. Conversion is poor (in young men about 8% to EPA and 0–4% to DHA), so long-chain omega-3s are sometimes called conditionally essential.
| Fatty acid | Shorthand | Made by humans? |
|---|---|---|
| Palmitic, stearic | 16:0, 18:0 | Yes |
| Oleic | 18:1 n-9 | Yes (SCD1) |
| Linoleic, α-linolenic | 18:2 n-6, 18:3 n-3 | No: essential |
| Arachidonic | 20:4 n-6 | Yes, from linoleic acid |
| DHA | 22:6 n-3 | Poorly, from α-linolenic acid |
Clinical tie-ins
Biotin deficiency impairs ACC and the other biotin carboxylases.
- Causes: raw egg white (avidin binds biotin; cooking denatures it), long-term antibiotics, anticonvulsants (phenytoin, carbamazepine, phenobarbital), isotretinoin, parenteral nutrition without biotin, chronic alcohol use.
- Signs: thinning hair, then alopecia; a scaly red rash around the eyes, nose, mouth and perineum, attributed to impaired fatty acid metabolism; conjunctivitis; depression, lethargy, hallucinations and paresthesias in adults; hypotonia and seizures in infants; ketolactic acidosis. Best marker: raised urinary 3-hydroxyisovaleric acid.
- Inherited: biotinidase deficiency (autosomal recessive; biotin is not recycled) and holocarboxylase synthetase deficiency (biotin is not attached) cause multiple carboxylase deficiency; both are found on newborn screening and treated with lifelong biotin.
Essential fatty acid deficiency follows fat-free parenteral nutrition (biochemical signs in as little as 7–10 days, because continuous dextrose keeps insulin high and traps stored essential fatty acids in fat), severe fat malabsorption or cystic fibrosis. Expect dry scaly dermatitis, alopecia, poor wound healing, slow growth in children and infections; a triene:tetraene (mead acid to arachidonic acid) ratio above 0.2 indicates it.
MASLD and MASH are the 2023 multisociety names for NAFLD and NASH. In a stable-isotope study of 9 patients with NAFLD, 59% of liver triglyceride fatty acids came from plasma free fatty acids, 26% from new synthesis and 15% from the diet, and synthesis stayed high even when fasting. Fructose is especially lipogenic because it enters glycolysis below phosphofructokinase-1. ACC inhibitors tested in fatty liver lower liver fat but can raise plasma triglycerides, through SREBP-1c activation and more VLDL secretion.
Alcohol: ethanol oxidation raises the liver's NADH/NAD⁺ ratio, which inhibits fatty acid oxidation and gluconeogenesis and promotes fat accumulation; steatosis occurs in more than 90% of people with alcohol use disorder and is potentially reversible.
Von Gierke disease (GSD I): trapped glucose-6-phosphate is pushed toward acetyl-CoA and malonyl-CoA, so fatty acid synthesis rises and lipid clearance falls (hypertriglyceridemia, milky plasma, eruptive xanthomas, pancreatitis risk), while malonyl-CoA blocks CPT-1 and lowers ketone production; the mechanism is not completely understood. See the von Gierke disease overview.
Bempedoic acid inhibits ATP-citrate lyase, upstream of HMG-CoA reductase. Its activating enzyme is high in the liver and low in muscle. It lowers LDL cholesterol, reduces cardiovascular events in adults who cannot take statins, and can cause hyperuricemia, gout and tendon rupture.
USMLE traps
| Trap | What's actually true |
|---|---|
| Fatty acid synthase is rate-limiting | ACC is the first and rate-limiting step; fatty acid synthase is controlled mainly by how much is made (some reviews also call it rate-limiting) |
| Citrate inhibits ACC | Citrate activates ACC; it inhibits phosphofructokinase-1 |
| ACC needs pantothenic acid; fatty acid synthase needs biotin | Reversed |
| The reductant is NADH | NADPH; β-oxidation makes NADH and FADH₂ |
| Acetyl-CoA leaves the mitochondria on carnitine | It leaves as citrate; carnitine brings long-chain acyl groups in |
| Phosphorylated ACC is active | Phosphorylation inactivates it |
| Glucagon turns ACC off through protein kinase A | Mainly through AMPK; protein kinase A phosphorylates ACC in vitro, but its role in the body is unclear. Either way, fasting means phosphorylated, inactive ACC |
| Malonyl-CoA inhibits CPT-2 | It inhibits CPT-1, which medium-chain fatty acids bypass |
| Fatty acid synthase makes oleate or linoleate | It makes palmitate; SCD1 makes oleate; humans cannot make linoleate |
| Statins block fatty acid synthesis | Statins block HMG-CoA reductase; bempedoic acid blocks ATP-citrate lyase, which feeds both pathways |
Worked USMLE-style vignettes
Vignette 1: the raw-egg shake. A 24-year-old bodybuilder has drunk a shake made with 6 raw egg whites daily for 8 months. He has thinning hair, a scaly red rash around his eyes, nose and mouth, conjunctivitis and tingling feet; urinary 3-hydroxyisovaleric acid is raised. Reduced activity of which enzyme most directly lowers his hepatic malonyl-CoA?
Answer: Acetyl-CoA carboxylase. Avidin in raw egg white binds biotin, ACC's cofactor, and blocks its absorption.
Why the others are wrong:
- ATP-citrate lyase: supplies acetyl-CoA; uses ATP, not biotin.
- Malic enzyme: NADP⁺-dependent; makes NADPH.
- Fatty acid synthase: consumes malonyl-CoA; its ACP arm needs pantothenic acid.
- Pyruvate dehydrogenase: uses thiamine pyrophosphate.
- Pyruvate carboxylase: also biotin-dependent and impaired, but it makes oxaloacetate, not malonyl-CoA.
Vignette 2: the switch in DKA. A 17-year-old with 3 weeks of polyuria and weight loss has glucose 520 mg/dL, pH 7.12 and high β-hydroxybutyrate. Which hepatic change most directly lets long-chain fatty acids enter mitochondria for ketogenesis?
Answer: A fall in malonyl-CoA. With no insulin and high glucagon, ACC is phosphorylated and inactive, and CPT-1 is released from inhibition.
Why the others are wrong:
- More citrate activating ACC: would raise malonyl-CoA, the fed pattern.
- Dephosphorylation of ACC: the insulin effect; it turns ACC on.
- More fatty acid synthase: induced by insulin; it falls without insulin.
- Carnitine deficiency: reduces fatty acid entry and causes hypoketotic hypoglycemia.
- More HMG-CoA reductase: cholesterol synthesis; ketogenesis uses HMG-CoA synthase and lyase.
Vignette 3: fat-free parenteral nutrition. A 52-year-old man has received parenteral dextrose and amino acids without lipid for 4 weeks after small-bowel resection. He develops dry scaly dermatitis, patchy hair loss and a nonhealing wound; his triene:tetraene ratio is 0.5. Which fatty acid can he not make, and why?
Answer: Linoleic acid (and α-linolenic acid): humans lack the Δ12 and Δ15 desaturases, and high insulin from continuous dextrose traps stored essential fatty acids in fat.
Why the others are wrong:
- Palmitic acid: made by fatty acid synthase from glucose-derived acetyl-CoA.
- Stearic acid: made by elongating palmitate.
- Oleic acid: made by SCD1 at Δ9.
- Arachidonic acid: made from linoleic acid; low only because its precursor is missing.
- Mead acid: the triene that rises in this deficiency.
Vignette 4: the statin-intolerant patient. A 64-year-old woman with coronary disease and LDL cholesterol 165 mg/dL had muscle aches on three statins. She starts a prodrug activated in the liver that inhibits the cytosolic enzyme that cleaves citrate. Which enzyme is inhibited, and what should be monitored?
Answer: ATP-citrate lyase (bempedoic acid), which supplies acetyl-CoA to cholesterol and fatty acid synthesis. Monitor uric acid and stop the drug at the first sign of tendon rupture.
Why the others are wrong:
- HMG-CoA reductase: the statin target.
- Acetyl-CoA carboxylase: uses the acetyl-CoA that ATP-citrate lyase frees; not part of cholesterol synthesis.
- Citrate synthase: mitochondrial; makes citrate.
- NPC1L1: the intestinal cholesterol transporter that ezetimibe blocks.
- PCSK9: targeted by drugs that reduce LDL receptor degradation.
Key takeaways
- Where and when: cytosol of liver, adipose tissue and lactating mammary gland, in the fed state; the endoplasmic reticulum elongates and desaturates.
- Citrate shuttle: acetyl-CoA leaves as citrate; ATP-citrate lyase frees it; malic enzyme makes NADPH.
- Rate-limiting step: ACC (biotin). On: citrate, insulin. Off: glucagon, epinephrine and AMPK (phosphorylation), palmitoyl-CoA.
- Fatty acid synthase: seven cycles, 14 NADPH, 7 ATP at ACC; product palmitate.
- The switch: malonyl-CoA inhibits CPT-1, so synthesis and β-oxidation do not run together.
- Clinical: biotin deficiency, essential fatty acid deficiency, MASLD, von Gierke and bempedoic acid. More pathways are on the Biochemistry hub.
Sources
- Wang Y, et al. Acetyl-CoA Carboxylases and Diseases — Frontiers in Oncology (2022)
- Sanders FWB, Griffin JL. De novo lipogenesis in the liver in health and disease — Biological Reviews (2016)
- Song Z, et al. Regulation and Metabolic Significance of De Novo Lipogenesis in Adipose Tissues — Nutrients (2018)
- Postic C, Girard J. Contribution of de novo fatty acid synthesis to hepatic steatosis and insulin resistance — J Clin Invest (2008)
- Foster DW. Malonyl-CoA: the regulator of fatty acid synthesis and oxidation — J Clin Invest (2012)
- McGarry JD, et al. A possible role for malonyl-CoA in the regulation of hepatic fatty acid oxidation and ketogenesis — J Clin Invest (1977)
- Zara V, et al. Multiple roles played by the mitochondrial citrate carrier in cellular metabolism and physiology — Cell Mol Life Sci (2022)
- Supplee JG, et al. Acetyl-CoA-dependent processes are preferentially supported by local metabolite synthesis — Science Advances (2026)
- Donnelly KL, et al. Sources of fatty acids stored in liver and secreted via lipoproteins in patients with nonalcoholic fatty liver disease — J Clin Invest (2005)
- Duan Y, et al. Regulation of cholesterol homeostasis in health and diseases — Signal Transduction and Targeted Therapy (2022)
- Berg JM, et al. Biochemistry, 8th ed., Chapter 22 (acetyl CoA carboxylase regulation) — Macmillan Learning
- Tymoczko JL, et al. Biochemistry: A Short Course, 3rd ed., Chapter 28 answers — Macmillan Learning
- Biochemistry, Fatty Acid Oxidation — StatPearls (2023)
- Biochemistry, Ketogenesis — StatPearls (2025)
- Biochemistry, Hexose Monophosphate Pathway — StatPearls (2023)
- Biochemistry, Nutrients — StatPearls (2023)
- Biotin Deficiency — StatPearls (2025)
- Biotinidase Deficiency — StatPearls (2025)
- Nutrition: Macronutrient Intake, Imbalances, and Interventions — StatPearls (2023)
- Glycogen Storage Disease Type I — GeneReviews (2021)
- Froissart R, et al. Glucose-6-phosphatase deficiency — Orphanet Journal of Rare Diseases (2011)
- Essential Fatty Acids — Linus Pauling Institute, Oregon State University (2019)
- Biotin: Fact Sheet for Health Professionals — NIH Office of Dietary Supplements (2022)
- Pantothenic Acid: Fact Sheet for Health Professionals — NIH Office of Dietary Supplements (2026)
- Alcohol-Related Liver Disease — Merck Manual Professional Edition (2025)
- New MASLD Nomenclature — American Association for the Study of Liver Diseases (2023)
- NEXLETOL (bempedoic acid) prescribing information — DailyMed (2026)
- UniProtKB Q13085, acetyl-CoA carboxylase 1 — UniProt (2026)
- UniProtKB O00763, acetyl-CoA carboxylase 2 — UniProt (2026)
- UniProtKB P49327, fatty acid synthase — UniProt (2026)
- UniProtKB P53396, ATP-citrate lyase — UniProt (2026)
- UniProtKB P53007, mitochondrial citrate carrier — UniProt (2026)
- UniProtKB P48163, NADP-dependent malic enzyme — UniProt (2026)
- UniProtKB O75874, cytosolic isocitrate dehydrogenase — UniProt (2026)
- UniProtKB Q9H5J4, elongase ELOVL6 — UniProt (2026)
- UniProtKB O00767, stearoyl-CoA desaturase — UniProt (2026)
- UniProtKB P04035, HMG-CoA reductase — UniProt (2026)
Frequently Asked Questions
Is fatty acid synthesis in the cytosol?
Yes. Fatty acid synthesis takes place in the cytosol, mainly in the liver, adipose tissue and the lactating mammary gland after a carbohydrate-rich meal. Acetyl-CoA is made in the mitochondria and reaches the cytosol as citrate, where ATP-citrate lyase, acetyl-CoA carboxylase and fatty acid synthase turn it into palmitate. Elongation and the first double bond are then added on the endoplasmic reticulum, while beta-oxidation runs in the mitochondrial matrix.
What is the citrate shuttle in fatty acid synthesis?
Acetyl-CoA cannot cross the mitochondrial membranes, so citrate synthase joins it to oxaloacetate and the citrate carrier exports the citrate in exchange for malate. In the cytosol, ATP-citrate lyase spends one ATP to split citrate back into acetyl-CoA and oxaloacetate. The oxaloacetate is reduced to malate, and malic enzyme converts malate to pyruvate while making NADPH; pyruvate carboxylase then rebuilds oxaloacetate inside the mitochondria.
What is the rate-limiting step of fatty acid synthesis?
The rate-limiting step is the carboxylation of acetyl-CoA to malonyl-CoA by acetyl-CoA carboxylase (ACC1, in the cytosol), which uses biotin, ATP and bicarbonate. Citrate and insulin activate it; glucagon, epinephrine and AMP-activated protein kinase inactivate it by phosphorylation, and palmitoyl-CoA inhibits it by feedback. Fatty acid synthase, the next enzyme, is controlled mainly by how much of it the cell makes, through SREBP-1c and ChREBP.
Does fatty acid synthesis use NADPH?
Yes. Each of the seven cycles on fatty acid synthase includes two NADPH-dependent reductions, so one palmitate needs 14 NADPH, plus 7 ATP spent by acetyl-CoA carboxylase to make the malonyl-CoA. The NADPH comes from the oxidative pentose phosphate pathway, whose rate-limiting enzyme is G6PD, from malic enzyme at the end of the citrate shuttle, and from cytosolic isocitrate dehydrogenase. Beta-oxidation instead produces NADH and FADH2.
Why does malonyl-CoA inhibit CPT-1?
Malonyl-CoA, the product of acetyl-CoA carboxylase, inhibits carnitine palmitoyltransferase 1, the rate-limiting enzyme of beta-oxidation, so newly made fatty acids are stored as triglyceride rather than burned. In fasting or untreated type 1 diabetes, glucagon activates AMPK, which phosphorylates acetyl-CoA carboxylase and activates malonyl-CoA decarboxylase. Malonyl-CoA falls, CPT-1 opens and the liver oxidizes fatty acids to ketone bodies. ACC2, on the mitochondria, makes the malonyl-CoA that regulates CPT-1.
Which vitamins does fatty acid synthesis depend on?
Biotin is the cofactor of acetyl-CoA carboxylase, so deficiency from large amounts of raw egg white, anticonvulsants, long-term antibiotics or biotinidase deficiency impairs malonyl-CoA production. Pantothenic acid (vitamin B5) is needed for coenzyme A and for the phosphopantetheine arm of the acyl carrier protein on fatty acid synthase. Thiamine matters upstream, because thiamine-dependent pyruvate dehydrogenase makes the acetyl-CoA that leaves the mitochondria as citrate.