Biosynthesis and Distribution of Lipids

Abstract

This chapter provides an overview of lipid metabolism in mammalian cells from the point of view of subcellular synthesis, transport, and targeting of lipid. It discusses the available data, which indicate that different subcellular membranes have, to some extent, different lipid compositions. The various biosynthetic pathways of fatty acid and phospholipid biosynthesis are described, with an emphasis on the subcellular locations of the biosynthetic enzymes. The principal product of de novo fatty acid biosynthesis in mammalian tissue is palmitic acid. De novo fatty acid biosynthesis takes place in the cytoplasm in a cycle of reactions catalyzed by the multifunctional fatty acid synthetase complex. Fatty acids can be modified by desaturation reactions that introduce cis-double bonds and by elongation reactions that lengthen the fatty acid from the carboxyl end by two carbons per step. The chapter also discusses the possible mechanisms responsible for the transport of lipid between subcellular membranes and the mechanisms by which cells produce and maintain the membranes of different lipid compositions. It describes three intracellular processes, which may be responsible for the transport and sorting of lipid within the cell: (1) synthesis of certain classes of lipid at the membranes in which they are found, (2) transport of lipid by phospholipid transfer proteins, and (3) transport of lipid by a process of membrane flow.