Fatty Acid Synthesis (De Novo Lipogenesis)

1. 1

   Acetyl-CoA + oxaloacetate → citrate

   Citrate synthase 2.3.3.1 ST-0060 Irreversible/directional ATP CoA-SH
   In Acetyl-CoA Oxaloacetate

   Out Citrate
   › Notes

   Mitochondrial citrate synthase condenses acetyl-CoA with oxaloacetate; it requires no ATP directly and releases CoA-SH. When energy is abundant, citrate accumulates and is exported through the tricarboxylate carrier. This is the initiating reaction of the citrate shuttle, not the committed step of fatty-acid synthesis.

2. 2

   Citrate (cytosol) + ATP + CoA-SH → acetyl-CoA + oxaloacetate + ADP + Pi

   ATP-citrate lyase (ACLY) 2.3.3.8 ST-0061 Irreversible/directional ATP CoA-SH Pi
   In Citrate (cytosol) ATP CoA-SH

   Out Acetyl-CoA Oxaloacetate ADP Pi
   › Notes

   ATP-citrate lyase cleaves exported citrate and consumes ATP. This reaction supplies cytosolic acetyl-CoA, which cannot cross the inner mitochondrial membrane directly.

3. 3

   Oxaloacetate + NADH + H+ → malate + NAD+

   Cytosolic malate dehydrogenase 1.1.1.37 ST-0062 Irreversible/directional NAD+ NADH
   In Oxaloacetate NADH H+

   Out Malate NAD+
   › Notes

   Cytosolic malate dehydrogenase reduces oxaloacetate, using NADH. Malate may return to mitochondria or be used to generate NADPH.

4. 4

   Malate + NADP+ → pyruvate + CO2 + NADPH + H+

   Cytosolic, NADP+-dependent malic enzyme 1.1.1.40 ST-0063 Irreversible/directional NADP+ NADPH
   In Malate NADP+

   Out Pyruvate CO2 NADPH H+
   › Notes

   Cytosolic, NADP+-dependent malic enzyme produces NADPH and pyruvate. Pyruvate returns to mitochondria and can be carboxylated to oxaloacetate by pyruvate carboxylase (ATP and biotin required), completing the shuttle; the pentose-phosphate pathway is another major NADPH source.

5. 5

   Acetyl-CoA + HCO3− + ATP → malonyl-CoA + ADP + Pi

   Acetyl-CoA carboxylase (ACC1) 6.4.1.2 ST-0064 Irreversible rate-limiting ATP Biotin (B7) Pi Bicarbonate/CO2
   In Acetyl-CoA HCO3- ATP

   Out Malonyl-CoA ADP Pi
   › Notes

   Acetyl-CoA carboxylase (ACC1) uses covalently bound biotin, ATP, and bicarbonate to carboxylate acetyl-CoA. This is the committed, rate-limiting, and irreversible regulatory step of cytosolic fatty-acid synthesis. The added carbon is later released as CO2 and thereby drives chain elongation.

6. 6

   Acetyl-CoA + ACP → acetyl-ACP + CoA-SH

   FAS malonyl/acetyl transferase (MAT) domain 2.3.1.85 ST-0065 Irreversible/directional CoA-SH ACP
   In Acetyl-CoA ACP (acyl carrier protein)

   Out Acetyl-ACP CoA-SH
   › Notes

   The acetyl group is transferred by the acetyl/malonyl transferase activity of the multifunctional fatty acid synthase complex to its acyl-carrier protein (ACP) phosphopantetheine thiol; the acetyl group is then transferred to the active-site cysteine of the β-ketoacyl synthase (KS) domain. ACP contains 4′-phosphopantetheine, derived from pantothenate.

7. 7

   Malonyl-CoA + ACP → malonyl-ACP + CoA-SH

   FAS malonyl/acetyl transferase (MAT) domain 2.3.1.85 ST-0066 Irreversible/directional CoA-SH ACP
   In Malonyl-CoA ACP (acyl carrier protein)

   Out Malonyl-ACP CoA-SH
   › Notes

   The FAS malonyl/acetyl transferase loads malonyl-CoA onto ACP. One acetyl primer and seven malonyl units are required to make palmitate.

8. 8

   Acetyl-KS + malonyl-ACP → acetoacetyl-ACP + CO2

   beta-Ketoacyl-ACP synthase (KS) domain of FAS 2.3.1.85 ST-0067 Irreversible/directional ACP
   In Acetyl-KS Malonyl-ACP

   Out Acetoacetyl-ACP CO2
   › Notes

   The β-ketoacyl-ACP synthase (KS) domain catalyzes decarboxylative condensation, producing a four-carbon β-ketoacyl-ACP. No ATP is consumed in this condensation; decarboxylation of malonyl-ACP provides the thermodynamic drive. In subsequent rounds, the growing acyl chain replaces acetyl-KS.

9. 9

   β-Ketoacyl-ACP + NADPH + H+ → D-β-hydroxyacyl-ACP + NADP+

   beta-Ketoacyl-ACP reductase (KR) domain of FAS 2.3.1.85 ST-0068 Irreversible/directional NADP+ NADPH ACP
   In beta-Ketoacyl-ACP NADPH H+

   Out D-beta-Hydroxyacyl-ACP NADP+
   › Notes

   β-Ketoacyl-ACP reductase (KR) carries out the first reducing reaction in each elongation cycle. NADPH is the obligatory electron donor.

10. 10

    D-β-Hydroxyacyl-ACP → trans-Δ2-enoyl-ACP + H2O

    beta-Hydroxyacyl-ACP dehydratase (DH) domain of FAS 2.3.1.85 ST-0069 Irreversible/directional H2O ACP
    In D-beta-Hydroxyacyl-ACP

    Out trans-Delta-2-Enoyl-ACP H2O
    › Notes

    β-Hydroxyacyl-ACP dehydratase (DH) removes water to create the trans double bond. No nucleotide cofactor is required.

11. 11

    trans-Δ2-Enoyl-ACP + NADPH + H+ → saturated acyl-ACP + NADP+

    Enoyl-ACP reductase (ER) domain of FAS 2.3.1.85 ST-0070 Irreversible/directional NADP+ NADPH ACP
    In trans-Delta-2-Enoyl-ACP NADPH H+

    Out Saturated acyl-ACP NADP+
    › Notes

    Enoyl-ACP reductase (ER) completes the second reduction of the cycle using NADPH. Steps 8–11 repeat seven times, adding two carbons per cycle and converting the primer ultimately to palmitoyl-ACP.

12. 12

    Palmitoyl-ACP + H2O → palmitate + ACP

    Thioesterase (TE) domain of FAS 3.1.2.14 ST-0071 Irreversible H2O ACP
    In Palmitoyl-ACP H2O

    Out Palmitate ACP (acyl carrier protein)
    › Notes

    The thioesterase (TE) domain of FAS hydrolyzes palmitoyl-ACP, terminating synthesis predominantly at C16. This is effectively irreversible by hydrolysis, but ACC—not thioesterase—is the pathway’s principal rate-limiting step.

13. 13

    Palmitate + CoA + ATP → palmitoyl-CoA + AMP + PPi

    Long-chain acyl-CoA synthetase (ACSL) ST-0072 Irreversible/directional ATP PPi
    In Palmitate CoA-SH ATP

    Out Palmitoyl-CoA AMP PPi
    › Notes

    An endoplasmic-reticulum/outer-mitochondrial-membrane long-chain acyl-CoA synthetase activates palmitate for elongation, desaturation, or glycerolipid synthesis. ATP is cleaved to AMP, costing two ATP equivalents.

Showing all 13 steps.

Overview

De novo lipogenesis converts excess carbon, principally carbohydrate-derived acetyl-CoA, into fatty acids for membrane lipid synthesis, triacylglycerol storage, and production of signaling lipids. In humans, the principal product of the fatty acid synthase system is palmitate (16:0), which can subsequently be elongated, desaturated, activated, or esterified. The pathway is favored in the well-fed, insulin-dominant state, when ATP, citrate, and reducing power are abundant.

Cellular location

The pathway spans the mitochondrial matrix and cytosol. Mitochondrial acetyl-CoA is exported indirectly as citrate; ATP-citrate lyase, acetyl-CoA carboxylase (ACC), and fatty acid synthase (FAS) act in the cytosol. It is especially active in liver, adipose tissue, lactating mammary gland, and, to a lesser extent, kidney and brain; hepatic fatty acids are commonly packaged into VLDL.

Net energetics

For the FAS reaction itself, the net synthesis of palmitate is: 8 acetyl-CoA + 7 ATP + 14 NADPH + 14 H+ → palmitate + 8 CoA-SH + 7 ADP + 7 Pi + 14 NADP+ + 6 H2O (the bicarbonate used by ACC is released during condensation). Thus, palmitate synthesis consumes 7 ATP and 14 NADPH after cytosolic acetyl-CoA is available. If all eight acetyl-CoA molecules are supplied by citrate export, ATP-citrate lyase costs an additional 8 ATP; activation of the released palmitate costs a further 2 ATP equivalents if palmitoyl-CoA is needed.

Clinical significance

Lipogenesis stores surplus caloric carbon efficiently and supplies fatty acids for phospholipids, sphingolipids, and protein acylation. Its reciprocal relationship with mitochondrial oxidation is enforced partly by malonyl-CoA inhibition of CPT-1, preventing simultaneous synthesis and mitochondrial import of newly made fatty acids. Increased hepatic DNL contributes to hypertriacylglycerolemia and metabolic dysfunction-associated steatotic liver disease; excessive ACC/FAS signaling is also a metabolic feature of several cancers. ACC and FAS are therefore pharmacologic and investigational metabolic targets.