Fatty acid synthesis is regulated by the enzyme acetyl-CoA carboxylase (ACC), which is the sole regulatory enzyme involved in fatty acid synthesis. This enzyme is regulated both allosterically and through covalent modification. It is also phosphorylated by both AMP kinase and PKA. ACC is allosterically activated by high levels of citrate and inhibited by fatty acyl-CoA. It may also be inhibited through increased levels of glucagon, AMP-activated protein kinase phosphorylation, and epinephrine. Insulin functions to dephosphorylate and activate the enzyme so that it polymerizes, while phosphorylation reverts the process.