Final answer:
Fatty acid synthesis is regulated by Acetyl-CoA carboxylase, activated by dephosphorylation influenced by insulin, and allosterically by citrate. Inhibition occurs through phosphorylation by enzymes like AMP Kinase, allosterically by Palmitoyl-CoA, and feedback from the synthesis of cholesterol. Insulin and gastric inhibitory peptide also play key roles in the process.
Step-by-step explanation:
Fatty acid synthesis is stimulated and inhibited by various factors. The central enzyme in this process is Acetyl-CoA carboxylase, which converts acetyl-CoA to malonyl-CoA and is regulated by both allosteric control and covalent modification. It is activated (dephosphorylated) by insulin and inhibited (phosphorylated) by AMP Kinase and Protein Kinase A. Citrate serves as an allosteric activator, while Palmitoyl-CoA allosterically inactivates it.
Regulation of cholesterol biosynthesis occurs mainly at the HMG-CoA reductase step, with feedback inhibition by cholesterol and its intermediate mevalonate. Hormones such as glucagon and epinephrine can inactivate the enzyme through phosphorylation, whereas dephosphorylation, which activates it, can be influenced by insulin. In general, the process of fatty acid oxidation contrasts with synthesis by requiring activation of fatty acids into an energy-rich fatty acyl-CoA derivative, a step catalyzed by acyl-CoA synthetase. This utilizes ATP, underscoring the different energetic requirements between lipid synthesis and degradation.
Insulin not only activates Acetyl-CoA carboxylase but also influences several pathways by inhibiting lipolysis, stimulating the HMP shunt pathway for NADPH production required for lipid synthesis, and enhancing protein synthesis. Additionally, gastric inhibitory peptide is secreted in response to fatty foods, slowing the digestion process to allow for better lipid absorption.