22. What Shuts Down Fat Burning? Too Much Energy.

This lesson covers the regulation of beta-oxidation. The primary regulation of beta-oxidation occurs at the mitochondrial membrane, where fatty acids are transported into the mitochondrion. Acetyl CoA carboxylase governs both the formation of fatty acids from non-carbohydrate precursors and the transport of fatty acids into the mitochondrion. Its product, malonyl CoA, is a substrate for fatty acid synthesis in the cytosol but a regulator of fatty acid transport in the mitochondrion. Thus, there are two isoforms of acetyl CoA carboxylase that are regulated similarly. The cytosolic isoform plays a direct role in fatty acid synthesis and the mitochondrial isoform regulates beta-oxidation. This ensures that the two processes are regulated reciprocally, so that one is shut down to the extent the other is activated, thereby preventing wasteful futile cycling. The primary regulator of acetyl CoA carboxylase activity is, as you might expect by this point, energy status. When a cell needs more energy, it lets fatty acids into the mitochondrion. When it has too much, it shuts down fat-burning.