Lipid metabolism is exquisitely controlled by hormonal and nutritional signals. Insulin promotes anabolism (lipogenesis, TAG storage) and suppresses catabolism (inhibits HSL, activates ACC). Glucagon and epinephrine do the opposite, activating lipolysis and β-oxidation. The AMPK (AMP-activated protein kinase) system acts as a cellular fuel gauge: low energy (high AMP) activates AMPK, which shuts down energy-consuming anabolic pathways (e.g., ACC, HMG-CoA reductase) and turns on catabolic ones (e.g., fatty acid uptake and oxidation).
The journey of dietary lipids begins in the gastrointestinal tract. The hydrophobic nature of triglycerides (TAGs), phospholipids, and cholesterol esters necessitates emulsification by bile salts in the small intestine. Pancreatic lipase, along with its cofactor colipase, then cleaves TAGs into free fatty acids (FFAs) and 2-monoacylglycerols. Phospholipase A2 acts on phospholipids, while cholesterol esterase hydrolyzes cholesterol esters. These breakdown products are incorporated into mixed micelles, which diffuse to the enterocyte brush border for absorption. metabolismo de lipideos
When energy demands rise or glucose is scarce (e.g., fasting, exercise), fatty acids become the primary fuel. Hormone-sensitive lipase (HSL) in adipose tissue is activated by glucagon and epinephrine, liberating FFAs into the bloodstream. FFAs, bound to serum albumin, are transported to oxidative tissues like heart, skeletal muscle, and liver. Lipid metabolism is exquisitely controlled by hormonal and
Once inside the mitochondrial matrix, β-oxidation proceeds as a four-step cycle (dehydrogenation, hydration, dehydrogenation, thiolysis) that shortens the fatty acid chain by two carbons (acetyl-CoA) per turn. For a saturated 16-carbon palmitate, this yields 8 acetyl-CoA, 7 FADH2, and 7 NADH. The acetyl-CoA enters the TCA cycle for complete oxidation to CO2 and water, generating substantial ATP via oxidative phosphorylation. In times of prolonged fasting or uncontrolled diabetes, however, the liver produces acetyl-CoA in excess of the TCA cycle’s capacity. This surplus is channeled into —the synthesis of ketone bodies (acetoacetate, β-hydroxybutyrate, and acetone). Ketone bodies serve as a water-soluble, alternative fuel for the brain, heart, and muscle, preserving glucose for obligate users like red blood cells. Pathological overproduction leads to ketoacidosis, a life-threatening condition. The AMPK (AMP-activated protein kinase) system acts as