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R-HSA-556833
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Metabolism of lipids |
Lipids are hydrophobic but otherwise chemically diverse molecules that play a wide variety of roles in human biology. They include ketone bodies, fatty acids, triacylglycerols, phospholipids and sphingolipids, eicosanoids, cholesterol, bile salts, steroid hormones, and fat-soluble vitamins. They function as a major source of energy (fatty acids, triacylglycerols, and ketone bodies), are major constituents of cell membranes (cholesterol and phospholipids), play a major role in their own digestion and uptake (bile salts), and participate in numerous signaling and regulatory processes (steroid hormones, eicosanoids, phosphatidylinositols, and sphingolipids) (Vance & Vance 2008 - URL).<p>The central steroid in human biology is cholesterol, obtained from animal fats consumed in the diet or synthesized de novo from acetyl-coenzyme A. (Vegetable fats contain various sterols but no cholesterol.) Cholesterol is an essential constituent of lipid bilayer membranes and is the starting point for the biosyntheses of bile acids and salts, steroid hormones, and vitamin D. Bile acids and salts are mostly synthesized in the liver. They are released into the intestine and function as detergents to solubilize dietary fats. Steroid hormones are mostly synthesized in the adrenal gland and gonads. They regulate energy metabolism and stress responses (glucocorticoids), salt balance (mineralocorticoids), and sexual development and function (androgens and estrogens). At the same time, chronically elevated cholesterol levels in the body are associated with the formation of atherosclerotic lesions and hence increased risk of heart attacks and strokes. The human body lacks a mechanism for degrading excess cholesterol, although an appreciable amount is lost daily in the form of bile salts and acids that escape recycling.<p>Aspects of lipid metabolism currently annotated in Reactome include lipid digestion, mobilization, and transport; fatty acid, triacylglycerol, and ketone body metabolism; peroxisomal lipid metabolism; phospholipid and sphingolipid metabolism; cholesterol biosynthesis; bile acid and bile salt metabolism; and steroid hormone biosynthesis.
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R-HSA-199220
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Vitamin B5 (pantothenate) metabolism |
Vitamin B5 ((R)-pantothenate, PanK), is an essential precursor for the synthesis of the metabolic cofactor Coenzyme A (CoA-SH) (Robishaw and Neely 1985) and is the prosthetic group of acyl carrier protein (ACP) (Joshi et al. 2003). The name pantothenate is from the Greek “pantothen”, "from everywhere". Both pantothenate and CoA-SH are found in nearly every foodstuff and in the gut microbiome. CoA-SH itself is readily degraded in the gut and in extracellular fluids within the body. No processes are known to transport it across plasma membranes. Instead, individual cells take up PanK, which is stable in the extracellular environment, to synthesize CoA-SH for their own use. Within a cell, distinct groups of CoA-SH-requiring reactions occur in the cytosol, mitochondrial matrix, and peroxisomes, and controlling CoA pool size in each location plays a major role in regulating and integrating cellular metabolic processes. Control is achieved by selective degradation, synthesis, and transport of CoA within a cell (Cavestro et al. 2023, Naquet et al. 2020). The reactions annotated here provide an incomplete description of these processes, as key steps remain incompletely understood.
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R-HSA-199991
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Membrane Trafficking |
The secretory membrane system allows a cell to regulate delivery of newly synthesized proteins, carbohydrates, and lipids to the cell surface, a necessity for growth and homeostasis. The system is made up of distinct organelles, including the endoplasmic reticulum (ER), Golgi complex, plasma membrane, and tubulovesicular transport intermediates. These organelles mediate intracellular membrane transport between themselves and the cell surface. Membrane traffic within this system flows along highly organized directional routes. Secretory cargo is synthesized and assembled in the ER and then transported to the Golgi complex for further processing and maturation. Upon arrival at the trans Golgi network (TGN), the cargo is sorted and packaged into post-Golgi carriers that move through the cytoplasm to fuse with the cell surface. This directional membrane flow is balanced by retrieval pathways that bring membrane and selected proteins back to the compartment of origin.
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