Lipid Metabolism

The chapter focuses on the metabolic pathways for synthesis and degradation of various classes of lipids. Lipids are available to the body either from ingested diet or synthesized de novo mainly in the liver. Lipids ingested in the form of triacylglycerols are broken down into free fatty acids (FFAs). The FFAs undergo β-oxidation in the mitochondria to produce acetyl CoA, utilized in the tricarboxylic acid cycle or ketogenesis to generate energy. FFAs may also be channeled into different biosynthetic pathways to produce membrane lipids such as phospholipids, glycolipids, sphingolipids, or cholesterol. Again, signaling molecules such as prostaglandins, leukotrienes, and thromboxanes can be synthesized from the essential fatty acids, mostly arachidonic acid. Similarly, these "formed lipids" can be hydrolyzed by phospholipases or glycohydrolases into basic components as part of the body's homeostasis and recycling mechanisms. Moreover, the excess dietary carbohydrates or proteins can be converted to triacylglycerols and stored in the adipose tissue for use during periods of energy deprivation. These lipid metabolic pathways involve highly regulated enzyme-catalyzed reactions, and hence defects in any of them may be associated with a wide range of health problems.