Endoplasmic reticulum, cholesterol

Although lanosterol may appear similar to cholesterol in structure, another 20 steps are required to convert lanosterol to cholesterol (Figure 25.35). The enzymes responsible for this are all associated with the endoplasmic reticulum. The primary pathway involves 7-dehydroeholesterol as the penultimate intermediate. An alternative pathway, also composed of many steps, produces the intermediate desmosterol. Reduction of the double bond at C-24 yields cholesterol. Cholesterol esters—a principal form of circulating cholesterol—are synthesized by acyl-CoA cholesterol acyltransferases (ACAT) on the cytoplasmic face of the endoplasmic reticulum. 

HDL and VLDL are assembled primarily in the endoplasmic reticulum of the liver (with smaller amounts produced in the intestine), whereas chylomicrons form in the intestine. LDL is not synthesized directly, but is made from VLDL. LDL appears to be the major circulatory complex for cholesterol and cholesterol esters. The primary task of chylomicrons is to transport triacylglycerols. Despite all this, it is extremely important to note that each of these lipoprotein classes contains some of each type of lipid. The relative amounts of HDL and LDL are important in the disposition of cholesterol in the body and in the development of arterial plaques (Figure 25.36). The structures of the various... 

A little more than half the cholesterol of the body arises by synthesis (about 700 mg/d), and the remainder is provided by the average diet. The liver and intestine account for approximately 10% each of total synthesis in humans. Virtually all tissues containing nucleated cells are capable of cholesterol synthesis, which occurs in the endoplasmic reticulum and the cytosol. 

Step 5—Formation of Cholesterol The formation of cholesterol from lanosterol takes place in the membranes of the endoplasmic reticulum and involves changes in the steroid nucleus and side chain (Figure 26-3). The methyl groups on C,4 and C4 are removed to form 14-desmethyl lanosterol and then zymosterol. The double bond at 03—C9 is subsequently moved to Cj-Cg in two steps, forming desmosterol. Finally, the double bond of the side chain is reduced, producing cholesterol. The exact order in which the steps described actually take place is not known with certainty. 

Biosynthesis of cholesterol from acetyl-CoA proceeds, assisted by the enzymes of endoplasmic reticulum and hyaloplasm, in many tissues and organs. This pro-cess is especially active in the liver of adult humans. 

The lipid compositions of plasma membranes, endoplasmic reticulum and Golgi membranes are distinct 26 Cholesterol transport and regulation in the central nervous system is distinct from that of peripheral tissues 26 In adult brain most cholesterol synthesis occurs in astrocytes 26 The astrocytic cholesterol supply to neurons is important for neuronal development and remodeling 27 The structure and roles of membrane microdomains (rafts) in cell membranes are under intensive study but many aspects are still unresolved 28... 

Because membranes components participate in nearly every cell activity their structures are also dynamic and far from the equilibrium states that are most readily understood in biophysical terms. Newly synthesized bilayer lipids are initially associated with endoplasmic reticulum (Ch.3) whereas phospholipids initially insert into the cytoplasmic leaflet while cholesterol and sphingolipids insert into the luminal endoplasmic reticulum (ER) leaflet. Glycosylation of ceramides occurs as they transit the Golgi compartments, forming cerebrosides and gangliosides in the luminal leaflet. Thus, unlike model systems, the leaflets of ER membranes are asymmetric by virtue of their mode of biosynthesis. 

Lipids are transported between membranes. As indicated above, lipids are often biosynthesized in one intracellular membrane and must be transported to other intracellular compartments for membrane biogenesis. Because lipids are insoluble in water, special mechanisms must exist for the inter- and intracellular transport of membrane lipids. Vesicular trafficking, cytoplasmic transfer-exchange proteins and direct transfer across membrane contacts can transport lipids from one membrane to another. The best understood of such mechanisms is vesicular transport, wherein the lipid molecules are sorted into membrane vesicles that bud out from the donor membrane and travel to and then fuse with the recipient membrane. The well characterized transport of plasma cholesterol into cells via receptor-mediated endocytosis is a useful model of this type of lipid transport. [9, 20]. A brain specific transporter for cholesterol has been identified (see Chapter 5). It is believed that transport of cholesterol from the endoplasmic reticulum to other membranes and of glycolipids from the Golgi bodies to the plasma membrane is mediated by similar mechanisms. The transport of phosphoglycerides is less clearly understood. Recent evidence suggests that net phospholipid movement between subcellular membranes may occur via specialized zones of apposition, as characterized for transfer of PtdSer between mitochondria and the endoplasmic reticulum [21]. 

Cholesterol epoxide hydrolase, which is expressed in the endoplasmic reticulum and catalyzes the trans-addition of H20 to cholesterol 5,6a-ox-ide and cholesterol 5,6/3-oxide, as well as to a number of other steroid 5,6-oxides. The products are the corresponding vicinal diols [54][55],... 

Hydroxy-3-methylglutaryl (HMG)-CoA reductase on the smooth endoplasmic reticulum (SER) is the rate-limiting enzyme. Insulin acth"ates the enzyme (dephosphorylation), and glucagon inhibits it. Mevalonate is the product, and the statin drugs competitively inhibit the enzyme. Cholesterol represses the expression o the HMG-CoA reductase gene and also increases degradation of the enzyme. 

Formation of mevalonate. The conversion of acetyl CoA to acetoacetyl CoA and then to 3-hydroxy-3-methylglutaryl CoA (3-HMG CoA) corresponds to the biosynthetic pathway for ketone bodies (details on p. 312). In this case, however, the synthesis occurs not in the mitochondria as in ketone body synthesis, but in the smooth endoplasmic reticulum. In the next step, the 3-HMG group is cleaved from the CoA and at the same time reduced to mevalonate with the help of NADPH+H 3-HMG CoA reductase is the key enzyme in cholesterol biosynthesis. It is regulated by repression of transcription (effectors oxysterols such as cholesterol) and by interconversion... 

The endergonic biosynthetic pathway described above is located entirely in the smooth endoplasmic reticulum. The energy needed comes from the CoA derivatives used and from ATP. The reducing agent in the formation of mevalonate and squalene, as well as in the final steps of cholesterol biosynthesis, is NADPH+H ... 

The fatty acids could be carried by proteins by a process similar to the way in which serum albumin binds fatty acid in the bloodstream of mammals. Other types of lipid might be formed into complexes analogous to low-density lipoproteins of the type found in animal tissues, where the lipid core of the lipoprotein is surrounded by a hydrophilic cortex made up of protein, phospholipid, and cholesterol (87). This allows the lipid to be moved in an aqueous environment. The protein of the lipoprotein shell could also act as possible ligands for particular receptors at the membrane of the cell at which the export occurs. The lipoproteins, if they are present, would probably be formed within the endomembrane lumen and would receive the proteins at the endoplasmic reticulum. 

The endoplasmic reticulum is composed of a convoluted network of channels and so has a large surface area. Apart from cytochromes P-450, the endoplasmic reticulum has many enzymes and functions, besides the metabolism of foreign compounds. These include the synthesis of proteins and triglycerides and other aspects of lipid metabolism and fatty acid metabolism. Specific enzymes present on the endoplasmic reticulum include cholesterol esterase, azo reductase, glucuronosyl transferase, NADPH cytochromes P-450 reductase and NADH cytochrome b5 reductase and cytochrome b5. A FAD-containing monooxygenase is also found in the endoplasmic reticulum, and this is discussed later in this chapter. 

Cellular site of choles terol synthesisr Cholesterol is synthesized in the cytoplasm, with enzymes in both the cytosol and the membrane of the endoplasmic reticulum. 

In addition to their plasma membrane eukaryotic cells also contain internal membranes that define a variety of organelles (fig. 17.2). Each of these organelles is specialized for particular functions The nucleus synthesizes nucleic acids, mitochondria oxidize carbohydrates and lipids and make ATP, chloroplasts carry out photosynthesis, the endoplasmic reticulum and the Golgi apparatus synthesize and secrete proteins, and lysosomes digest proteins. Additional membranes divide mitochondria and chloroplasts into even finer, more specialized subcompartments. Like the plasma membrane, organellar membranes act as barriers to the leakage of proteins, metabolites, and ions they contain transport systems for import and export of materials, and they are the sites of enzymatic activities as diverse as cholesterol biosynthesis and oxidative phosphorylation. 

Lipoproteins Are Made in the Endoplasmic Reticulum of the Liver and Intestine Chylomicrons and Very-Low-Density Lipoproteins (VLDLs) Transport Cholesterol and Triacylglycerol to Other Tissues Low-Density Lipoproteins (LDLs) Are Removed from the Plasma by the Liver, Adrenals, and Adipose Tissue... 

The thiolase and HMG-CoA synthase exhibit some regulatory properties in rat liver (cholesterol feeding causes a decrease in these enzyme activities in the cytosol but not in the mitochondria). However, the primary regulation of cholesterol biosynthesis appears to be centered on the HMG-CoA reductase reaction. HMG-CoA reductase is found on the endoplasmic reticulum, has a molecular weight of 97,092, and consists of 887 amino acids in a single polypeptide chain. The sequence of the enzyme was deduced by Michael Brown and Joseph Goldstein from the sequence of a piece of complimentary DNA (cDNA) derived from mRNA that codes for the reductase. The enzyme... 

The last sequence of reactions in the biosynthesis of choles-terol involves approximately 20 enzymatic steps, starting with lanosterol. In mammals the major route involves a series of double-bond reductions and demethylations (fig. 20.10). The sequence of reactions involves reduction of the A24 double bond, the oxidation and removal of the 14a methyl group followed by the oxidation and removal of the two methyl groups at position 4 in the sterol. The final reaction is a reduction of the A7 double bond in 7-dehydro-cholesterol. An alternative pathway from lanosterol to cholesterol also exists. The enzymes involved in the transformation of lanosterol to cholesterol are all located on the endoplasmic reticulum. 

Of the various lipid components of the lipoproteins, only the biosynthesis of cholesteryl esters has not yet been mentioned. Cholesteryl ester is the storage form of cholesterol in cells. It is synthesized from cholesterol and acyl-CoA by acyl-CoA cholesterol acyltransferase (ACAT) (fig. 20.13), which is located on the cytosolic surface of hepatic endoplasmic reticulum. Acylation of the 3 hydroxyl group of cholesterol eliminates the polarity of cholesterol and facilitates the packing of cholesterol as its ester in the core of the lipoprotein or for storage in lipid droplets within cells. 

The synthesis of the apoproteins takes place on ribosomes that are bound to the endoplasmic reticulum. As we mentioned previously, the biosynthesis of the other lipids in lipoproteins (cholesterol, triacylglycerols, and phospholipids) also occurs on the endoplasmic reticulum. 

Biosynthesis of cholesteryl esters. The acyl-CoA cholesterol acyltransferase involved in cholesteryl ester synthesis is located on the cytosolic surface of liver endoplasmic reticulum. 

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