Chapter 26: RNA Metabolism: Transcription, RNA Processing, and Catalytic RNA

Section 26.1 focuses on the formation of phosphatidic acid (1,2-diacylglycerol-3-phosphate) via the sequential acylation of glycerol-3-phosphate by acyl-CoA molecules. This intermediate is a key branch point in lipid metabolism. Section 26.2 explains how phosphatidic acid is converted into either triacylglycerols or glycerophospholipids. TAGs are formed through the dephosphorylation of phosphatidic acid to diacylglycerol (DAG), followed by the addition of a third fatty acid. This occurs primarily in the liver and adipose tissue and is hormonally regulated based on energy status. For glycerophospholipid biosynthesis, DAG can be activated by CDP-choline or CDP-ethanolamine to form phosphatidylcholine (PC) and phosphatidylethanolamine (PE), respectively. Alternatively, CDP-diacylglycerol can react with inositol or glycerol to produce phosphatidylinositol (PI) or phosphatidylglycerol (PG). Section 26.3 describes the remodeling of membrane phospholipids, including desaturation, chain length changes, and head group exchange, which fine-tune membrane fluidity and function. It also covers the role of phospholipases (A1, A2, C, and D) in lipid signaling and membrane turnover. The chapter concludes with the synthesis and biological significance of ether lipids (e.g., plasmalogens) and platelet-activating factor (PAF), as well as cardiolipin in mitochondria and sphingolipids in specialized membranes. Overall, the chapter underscores how lipid biosynthesis is intricately linked to cellular energy status, signaling pathways, and membrane dynamics. By detailing the enzymatic steps, regulatory mechanisms, and physiological relevance of TAG and phospholipid biosynthesis, this chapter connects lipid metabolism to broader biochemical systems involved in membrane biology, energy storage, and intracellular communication.