Metabolites fatty acid synthesis regulation

Regulation is also accomplished by compartmentaliza-tion of enzyme systems involved in anabolic and catabolic pathways into different cell organelles. For example, fatty acid synthesis occurs in the soluble fraction of the cytoplasm, whereas fatty acid oxidation takes place in mitochondria. Heme synthesis begins and is completed in mitochondria, but some of the intermediate reactions take place in the cytosol. Heme catabolism is initiated in the smooth endoplasmic reticulum. Transport of key metabolites across an organelle membrane system is also a form of regulation. 

Regulation of enzyme level serves as a coarse control over fatty acid synthesis. In response to changes in physiological state, the levels of the enzymes of fatty acid synthesis fluctuate coordinately. Fatty acid synthesis is also regulated by the direct action of metabolite effectors on key enzymes in the pathway. This means of control is more responsive to sudden alterations in cellular fatty acid requirements. In the case of the committed acetyl-CoA carboxylation step, citrate has been shown to be a positive feed-forward allosteric effector. Since this is the rate-determining step, activation by citrate can effectively adjust the rate of fatty acid synthesis to momentary fluctuations in cellular needs. 

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