Lipogenesis - Fatty Acid Synthesis - Control and Regulation - Acetyl-CoA Carboxylase

Acetyl-CoA Carboxylase

Insulin affects ACC in a similar way to PDH. It leads to its dephosphorylation which activates the enzyme. Glucagon has an antagonistic effect and increases phosphorylation, deactivation, thereby inhibiting ACC and slowing fat synthesis.

Affecting ACC affects the rate of acetyl-CoA conversion to malonyl-CoA. Increased malonyl-CoA level pushes the equilibrium over to increase production of fatty acids through biosynthesis. Long chain fatty acids are negative allosteric regulators of ACC and so when the cell has sufficient long chain fatty acids, they will eventually inhibit ACC activity and stop fatty acid synthesis.

AMP and ATP concentrations of the cell act as a measure of the ATP needs of a cell and as ATP levels get low it activates the ATP synthetase which in turn phosphorylates ACC. When ATP is depleted, there is a rise in 5'AMP. This rise activates AMP-activated protein kinase, which phosphorylates ACC, thereby inhibits fat synthesis. This is a useful way to ensure that glucose is not diverted down a storage pathway in times when energy levels are low.

ACC is also activated by citrate. This means that, when there is abundant acetyl-CoA in the cell cytoplasm for fat synthesis, it proceeds at an appropriate rate.

Note: Research now shows that glucose metabolism (exact metabolite to be determined), aside from insulin's influence on lipogenic enzyme genes, can induce the gene products for liver's pyruvate kinase, acetyl-CoA carboxylase, and fatty acid synthase. These genes are induced by the transcription factors ChREBP/Mlx via high blood glucose levels and presently unknown signaling events. Insulin induction is due to SREBP-1c, which is also involved in cholesterol metabolism.

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