Metabolismo De Lipideos |verified| -

Inside the cell, FFAs are activated to fatty acyl-CoA by acyl-CoA synthetase. The critical entry step into the mitochondria, where β-oxidation occurs, is mediated by the carnitine shuttle. The enzyme carnitine palmitoyltransferase I (CPT1) is the rate-limiting, regulated step; it converts fatty acyl-CoA to acylcarnitine, which is transported across the inner mitochondrial membrane by translocase and then reconverted to acyl-CoA by CPT2. Malonyl-CoA, the first intermediate in fatty acid synthesis, allosterically inhibits CPT1—a prime example of reciprocal regulation between catabolism and anabolism.

Introduction

When energy and carbohydrate intake exceed immediate needs, the liver and adipose tissue convert excess acetyl-CoA into fatty acids via . This pathway occurs in the cytoplasm. The key regulated enzyme is acetyl-CoA carboxylase (ACC), which converts acetyl-CoA to malonyl-CoA. ACC is activated by citrate (a sign of abundant energy) and insulin, and inhibited by AMPK (energy stress) and glucagon. The fatty acid synthase (FAS) complex, a large multienzyme protein, then uses NADPH (supplied primarily by the pentose phosphate pathway) to extend the malonyl-CoA-derived two-carbon units into palmitate. Further elongation and desaturation (introducing double bonds via desaturases like SCD1) yield the diverse spectrum of cellular fatty acids. metabolismo de lipideos

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). Inside the cell, FFAs are activated to fatty

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. Malonyl-CoA, the first intermediate in fatty acid synthesis,

These newly synthesized fatty acids are esterified to glycerol-3-phosphate to form TAGs for storage. They are also incorporated into membrane phospholipids via the Kennedy pathway or by remodeling existing phospholipids (Lands’ cycle). Cholesterol synthesis (isoprenoid pathway) is another critical anabolic component, beginning with HMG-CoA reductase—the target of statin drugs. Cholesterol is essential for membrane fluidity, lipid rafts, and steroid hormone synthesis. The coordinated regulation of lipogenesis, TAG assembly, and cholesterol synthesis by insulin, SREBP (sterol regulatory element-binding proteins), and ChREBP (carbohydrate response element-binding protein) ensures that excess carbon is stored efficiently.

Dysregulation of these pathways underlies major diseases. results from chronic positive energy balance, with hypertrophied adipocytes becoming insulin-resistant and releasing excess FFAs (lipotoxicity). Atherosclerosis is driven by retention of apoB-containing lipoproteins (LDL) in artery walls, where they become oxidized, triggering inflammation and plaque formation. NAFLD arises from ectopic TAG accumulation in the liver due to increased lipogenesis and reduced VLDL export, often in the context of insulin resistance. The carnitine shuttle defects cause hypoketotic hypoglycemia and cardiomyopathy in infants. Understanding these pathways has led to effective therapies: statins (HMG-CoA reductase inhibitors), fibrates (PPAR-α activators that enhance fatty acid oxidation), and emerging inhibitors of ACC or SCD1 for NAFLD.