Fatty Acid Metabolism

Fatty acid metabolism comprises the degradation of fatty acids to release energy and their synthesis from acetyl units, two opposing pathways with distinct chemistry and locations.

Definition

Fatty acid metabolism is the set of pathways that catabolize fatty acids by beta-oxidation to acetyl-CoA and that synthesize fatty acids from acetyl-CoA via malonyl-CoA, using distinct enzymes, carriers, and cellular compartments.

Scope

This topic covers activation of fatty acids, beta-oxidation that shortens chains by two carbons per cycle to yield acetyl-CoA and reduced cofactors, the energetics of fatty acid oxidation, and fatty acid synthesis by the fatty acid synthase system, highlighting why degradation and synthesis are kept separate.

Core questions

• How does beta-oxidation degrade a fatty acid chain?
• Why is fatty acid oxidation so energy-rich compared with carbohydrate oxidation?
• How does fatty acid synthesis differ from oxidation?
• Why are synthesis and oxidation compartmentally and chemically separated?

Key theories

Beta-oxidation spiral

Fatty acids are degraded by a repeating four-step sequence—oxidation, hydration, oxidation, thiolytic cleavage—that removes two carbons as acetyl-CoA each cycle while generating reduced electron carriers.

Mechanisms

A fatty acid is first activated to a fatty acyl-CoA and transported into the mitochondrion. Each beta-oxidation cycle oxidizes the beta carbon, removing an acetyl-CoA and producing FADH2 and NADH, until the chain is fully consumed; the acetyl-CoA enters the citric acid cycle. Synthesis runs in the cytosol on fatty acid synthase, building chains from acetyl-CoA and malonyl-CoA with NADPH as reductant—reductive chemistry that mirrors yet is distinct from the oxidative degradation pathway.

Clinical relevance

Fatty acid metabolism is a central example of metabolic reaction logic and energy yield in biochemistry and metabolic engineering. The treatment is descriptive and non-prescriptive.

History

Knoop's labeling experiments early in the twentieth century proposed oxidation at the beta carbon; Lynen and others later identified coenzyme A and the enzymes of the pathway, and the separate fatty acid synthase system was characterized in subsequent decades.

Key figures

• Franz Knoop
• Feodor Lynen
• Salih Wakil

Seminal works

• nelson2021
• lynen1964

Frequently asked questions

Why does fat store more energy than carbohydrate per gram?

Fatty acids are highly reduced and nearly anhydrous, so their oxidation releases more energy per gram than the more oxidized, hydrated carbohydrates.

Are fatty acid synthesis and breakdown simply the reverse of each other?

No; although chemically related, they use different enzymes, carriers, cofactors, and cellular compartments, which allows the cell to regulate the two processes independently.