Fatty acid synthesis

Biological synthesis of fatty acids, also called lipogenesis, is an anabolic, assimilating metabolic process, are prepared in the fatty acids ( for example, for the purpose of storage of energy ). It passes through the successive cultivation of malonyl-CoA to a known initial acetyl group, which is linked to coenzyme A. In contrast to the breakdown of fatty acids of the β - oxidation, which takes place in the mitochondrial matrix, the structure of which will take place in animals and fungi in the cytosol.

Fatty acid synthesis in plants

In plants, fatty acid synthesis runs only in the plastids from in green plant cells in the chloroplasts, or in the chromoplasts, leucoplasts or proplastids. It run from the same reactions as in the cytosol, but only fatty acids are synthesized to C18. This can get a maximum of a double bond by a soluble desaturase of the stroma. After transport to the smooth endoplasmic reticulum ( SER ), the chain extension occurs in the SER more double bonds can be incorporated by membrane-bound desaturases. The degradation of the fatty acids does not take place in the mitochondria, but only in peroxisomes in plants.

Long-chain fatty acids in plants are the starting point for the biosynthesis of wax and cutin, both of which are indispensable for the protection from the environment.

General reaction sequence

For the exact biosynthesis including structural formulas, see section External links

It is remarkable that the fatty acid is currently under construction remains bound to a multifunctional enzyme, called fatty acid synthase in mammals and fungi to final completion, which bears all seven enzyme functions. It has a peripheral ( distal ) SH group, and a central ( proximal ) to the SH group of a subunit of the complex, the acyl carrier protein domain (ACP). The individual enzyme functions and ACP are distributed in plants and bacteria on different proteins that assemble into a protein complex. Also exists in all eukaryotes a second Fettsäuresyntheseweg in mitochondria, wherein the enzyme activities are located on individual proteins.

The reaction sequence is as follows:

This process is repeated until completion, which mostly palmitic acid is hydrolytically cleaved by a thiolase. It should be noted that the fatty acid is first released. The key enzyme in fatty acid synthesis Acetyl -CoA carboxylase, is regulated by both hormonal and allosterically.

The chain extension of the fatty acids is catalyzed in the plant by Elongases.

This pathway plays under normal feeding conditions a minor role, since sufficient fats are absorbed by the food already. As a result, there is no need to build fatty acids from carbohydrates. In animals, fatty acid synthesis has a greater role, since they must form a substantial fat reserves for the winter, for example.

Odd-numbered fatty acids

Fatty acid chains with an odd number of carbon atoms arise when propionyl -CoA is used instead of acetyl -CoA as a starter molecule. The chain extension is then, as already described in the even-numbered fatty acids, by successive reactions with malonyl -CoA instead.

Branched-chain fatty acids

Branching on the alkyl end of the fatty acid obtained if the synthesis begins with a branched molecule. This is called the presence of an additional methyl group on the penultimate carbon atom of the main chain of a fatty acid and isobutyl on the antepenultimate of a anteiso - fatty acid. Iso fatty acids are formed when the synthesis of isovaleryl -CoA and isobutyryl -CoA begins. These compounds are derived from the branched-chain amino acids leucine or valine. On the other hand is produced by the synthesis starting with 2- methylbutyryl -CoA, which is derived from isoleucine, an anteiso fatty acid. More in the fatty acid chain methyl branching but can also be produced during the synthesis. This occurs when the chain extension is not carried out with malonyl -CoA, but with methylmalonyl -CoA.

For some types of bacteria such as mycobacteria is the synthesis of branched-chain fatty acids is of great importance, since chain branches present an opportunity for the regulation of membrane fluidity. The biosynthesis of the longest natural fatty acids, the mycolic acids, is so extensive that it can not be presented in detail here.