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Synthesis of Lipids in Smooth Endoplasmic Reticulum
The smooth endoplasmic reticulum (SER) is a soft membrane system in the cytoplasm of eukaryotic cells. It differs from the rough ER because it does not have ribosomes attached to it, whereas rough ER has ribosomes. It is responsible for the synthesis of proteins and lipids such as phospholipids. It is also involved in protein modification reactions such as glycosylation. Cells that are heavily involved in lipid metabolism, such as muscle cells and secreting glands, have a lot of smooth ER.
The ER is one of the largest and most complex cellular structures. It has a vast variety of functions that include synthesis and folding of secreted proteins, calcium storage and membrane lipid biogenesis. It also is implicated in cholesterol, plasmalogen and sphingomyelin biosynthesis. It is in contact with most cellular organelles including mitochondria, peroxisomes, Golgi apparatus, lipid droplets and the plasma membrane. This allows for the coordinated action of internal and external signals that drive lipid synthesis within the ER.
Synthesis of Lipids in Golgi Cells
Cells are constantly adjusting the composition of their membranes. Most of the lipids that comprise membranes are synthesized in the Golgi apparatus. They are transported from the Golgi by transport vesicles to other compartments within the cell, including endosomes and lysosomes.
The structure of the Golgi apparatus is a complex network of flattened sacs connected by tubules in the cytoplasm of eukaryotic cells. The ER and Golgi apparatus are part of the endomembrane compartment, which also includes mitochondria.
The synthesis of phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn) is regulated by changes in cellular energy status. The lipid levels of the membrane are adjusted by de novo synthesis at designated cellular sites, by targeted transport to new locations, and by localized remodeling reactions. The lipids are modified by enzymes that remove fatty acid head groups or add them in the opposite direction (a Lands cycle). The N-linked oligosaccharides added to proteins in the ER are processed in a series of reactions within the Golgi apparatus.
Synthesis of Lipids in Mitochondria
The phospholipids PE and CL are essential for mitochondrial functions including the formation of the cristae and stabilization of respiratory complexes, but how they are synthesised, transported into and out of mitochondria, and remodeled to maintain homeostasis remains poorly understood. High levels of mitochondrial PE can be generated by a membrane-resident enzyme, PS decarboxylase 1, within the inner mitochondrial membrane (IMM).
Alternatively, acetyl-CoA produced in mitochondria from carbohydrate or amino acid catabolism can cross the mitochondrial inner membrane and enter the cytosol to initiate de novo fatty acid synthesis via the citrate/malate shuttle. The latter pathway depends on the Ups2-Mdm35 lipid-shuttle complex, which facilitates the transfer of acyl groups between the two compartments.
In addition to lipoic acid, the mtFAS pathway produces a number of other products, with their metabolic roles and cellular functions remaining unknown. For example, mtFAS mutant cells exhibit reduced protein lipoylation, but do not show impaired assembly of ETC complexes, suggesting that a distinct function is associated with the other, as yet unidentified, products of mtFAS.
Synthesis of Lipids in Capillaries
The biosynthesis of glycerophospholipids, sphingolipids and sterols is well established but insights into how these molecules are delivered to their target membranes are less comprehensive. Lipid transport mechanisms vary according to the nature of the lipid.
Glycerophospholipids are the most abundant class of lipids in mammalian cells. Their de novo synthesis starts in the ER through a series of acylation and reduction reactions that produce phosphatidic acid. This can be converted into phosphatidylcholine, phosphatidylethanolamine, or phosphatidylserine by exchanging the head group with a polar molecule.
The phosphoinositide (PI) head group is incredibly versatile, with three of its five hydroxyls being able to be phosphorylated resulting in seven different phosphoinositides (Figure 7). In order for the phosphoinositide to be transported between leaflets of the bilayer it must first be flipped across the membrane. There are three categories of inter-leaflet lipid transport proteins: scramblases, flippases and floppases. Each has its own distinct domain that recognises a specific lipid head group.