Beta oxidation and fatty acid biosynthesis are prevented from conflicting in a futile cycle mainly by compartmentation and tissue specialization. Beta oxidation is mitochondrial, with high activity is slow-twitch muscle, fatty acid biosynthesis is cytoplasmic and primarily in the liver.
Hormonal signals activate fatty acid release from adipose tissue.
Liver initiates fatty acid biosynthesis when citrate production exceeds the needs of energy metabolism. Aspartate transaminase gathers amino groups from other amino acids, converting oxaloacetate to aspartate. Aspartate tends to be exported to the cytoplasm, where it is ligated to to citrulline. Elimination of fumarate leaves the amino group in the form of arginine. In 1990, it was not known that purine biosynthesis involves multienzyme complexes, and that the C 1 donors were integrated with this complexes in the form of specific serine hydroxymethyl transferase subunits.