Is fatty acid synthesis simply the reverse of beta-oxidation?

No. Although both add or remove two-carbon units, synthesis occurs in the cytosol on fatty acid synthase using malonyl-CoA and NADPH, whereas oxidation occurs in mitochondria with separate enzymes using NAD+ and FAD; keeping them distinct allows independent regulation.

Where does the carbon for new fatty acids come from?

Mostly from acetyl-CoA derived from glucose and other fuels, exported to the cytosol as citrate; this is why excess dietary carbohydrate can be converted to fat through de novo lipogenesis.

Fatty Acid and Lipid Synthesis

Fatty acid synthesis, or de novo lipogenesis, is the cytosolic anabolic pathway that builds long-chain fatty acids from acetyl-CoA, using NADPH as reducing power. The fatty acids are then esterified to glycerol to form the triacylglycerols and phospholipids that the body stores for energy and uses to build membranes; the pathway is most active in liver and adipose tissue in the fed, insulin-rich state.

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Definition

Fatty acid synthesis is the reductive, cytosolic pathway in which acetyl-CoA is first carboxylated to malonyl-CoA by acetyl-CoA carboxylase and then assembled into a long-chain saturated fatty acid (typically palmitate) by the multifunctional fatty acid synthase, consuming NADPH; the products are esterified to glycerol-3-phosphate to form triacylglycerols and other complex lipids.

Scope

The entry covers the committed carboxylation of acetyl-CoA to malonyl-CoA, the iterative chain-elongation chemistry of fatty acid synthase, the supply of building blocks and NADPH, the subsequent assembly of triacylglycerols, and the transcriptional and allosteric control of the pathway. It is a biochemical reference and does not address the management of metabolic disease.

Core questions

Which reaction commits carbon to fatty acid synthesis and how is it regulated?
How does fatty acid synthase elongate a chain two carbons at a time?
Where do the acetyl-CoA and NADPH for synthesis come from?
How is lipogenesis switched on in the fed state and off during fasting?

Key concepts

Acetyl-CoA carboxylase and malonyl-CoA
Fatty acid synthase (FAS) and acyl carrier protein
NADPH supply from the pentose phosphate pathway and malic enzyme
Citrate shuttle for cytosolic acetyl-CoA
Triacylglycerol and phospholipid assembly
SREBP-1c and insulin signaling
Chain elongation and desaturation

Key theories

Acetyl-CoA carboxylase as the committed, regulated step: Acetyl-CoA carboxylase catalyzes the irreversible, rate-limiting formation of malonyl-CoA and is controlled by citrate activation, long-chain acyl-CoA inhibition, and inactivating phosphorylation by AMP-activated protein kinase, making it the master switch for lipogenesis.
SREBP-driven transcriptional program: Insulin and high carbohydrate availability activate sterol regulatory element-binding protein-1c, which transcriptionally induces the enzymes of fatty acid synthesis, coordinating the pathway with nutritional state.

Mechanisms

Cytosolic acetyl-CoA, exported from mitochondria via the citrate shuttle, is carboxylated by acetyl-CoA carboxylase to malonyl-CoA in the committed step. Fatty acid synthase, a large multifunctional enzyme, then carries out repeated rounds of condensation, reduction, dehydration, and a second reduction, each adding a two-carbon unit and consuming two NADPH, until palmitate (16 carbons) is released. NADPH is supplied largely by the pentose phosphate pathway and malic enzyme. Palmitate can be further elongated and desaturated, then esterified to glycerol-3-phosphate to build triacylglycerols for storage or phospholipids for membranes. The pathway is reciprocally linked to oxidation because the malonyl-CoA it generates inhibits CPT1, and it is regulated acutely by AMP-activated protein kinase and over the longer term by the insulin-driven SREBP-1c transcriptional program.

Clinical relevance

De novo lipogenesis is central to how the body converts excess dietary carbohydrate into stored fat and is studied in the context of fatty liver, obesity, and the metabolic effects of insulin. This entry describes the normal biosynthetic pathway and its regulation for reference and education and is not a basis for clinical decisions.

History

The elucidation of fatty acid synthesis in the 1950s and 1960s, including the discovery of acetyl-CoA carboxylase and the malonyl-CoA pathway by Salih Wakil and the characterization of acyl carrier protein by Vagelos, established that synthesis is chemically distinct from the reverse of beta-oxidation. The later identification of SREBP-1c by Goldstein, Brown, and Horton clarified how insulin and nutrient status transcriptionally drive the lipogenic program.

Key figures

Salih Wakil
Feodor Lynen
P. Roy Vagelos
Jay Horton

Seminal works

wakil-2009
horton-2002

Frequently asked questions

Is fatty acid synthesis simply the reverse of beta-oxidation?: No. Although both add or remove two-carbon units, synthesis occurs in the cytosol on fatty acid synthase using malonyl-CoA and NADPH, whereas oxidation occurs in mitochondria with separate enzymes using NAD+ and FAD; keeping them distinct allows independent regulation.
Where does the carbon for new fatty acids come from?: Mostly from acetyl-CoA derived from glucose and other fuels, exported to the cytosol as citrate; this is why excess dietary carbohydrate can be converted to fat through de novo lipogenesis.