Fatty Acid Metabolism

Fatty Acid Metabolism

Fatty acids play four major physiological roles: 1. Stored as triacylglycerols, they are fuel molecules. Comparing 38 kJ per gram of energy to 17 kJ per gram in carbohydrates and proteins. 2. They are building blocks for phospholipids and glycolipids - amphipathic molecules that are essential components of a plasmalemma. 3. Some proteins are covalently modified with fatty acids, targeting them to membrane locations. 4. Fatty acid derivatives serve as hormones and intracellular messengers.

Fatty acids destined for metabolism do not exist free but linked via a thioester bond to Coenzyme A. Fatty acid synthesis and degradation are the mirrors of each other, yet distinct. In synthesis, acetyl CoA (coenzyme A bonded to a two-carbon chain) and malonyl CoA (four-carbon unit) condense to form butyryl CoA. The carbonyl group is reduced to a methylene group in three steps: a reduction, a dehydration and another reduction. In degradation, an acyl CoA (depending upon the fatty acid chain length) is oxidised to introduce a double bond, which is then hydrated to a hydroxyl group, oxidised again to a ketone and cleaved by another CoA molecule to yield acetyl CoA, the common currency of the citric acid cycle and an acyl CoA two carbon groups shorter. The general process is oxidation, hydration, oxidation, cleavage; the reverse of synthesis. However, each pathway contains distinct enzymes, and a diferent mechanistically, exemplifying that synthetic and degradative pathways are almost always distinct.