Cholesterol

Chemistry and Functions of Cholesterol

1. STEROID CHOLESTEROL

2. Steroids are a third class of lipids found in the membranes of eukaryotes, and, very rarely, in bacteria. Steroids , along with lipid vitamins and terpenes , are classified as isoprenoids because their structures are related to the five carbon molecule isoprene. Steroids contain four fused rings: three six-carbon rings designated A, B, and C and a five-carbon D ring.

3. Cholesterol

4. Cholesterol, is derived from the Ancient Greek word Chole - bile and stereos - solid Followed by the chemical suffix - ol for an alcohol, is an organic molecule. It is a derived lipid molecule – a sterol or modified steroid

5. IUPAC name [hide] (3β)-cholest-5-en-3-ol Systematic name 2,15-dimethyl-14-(1,5-dimethylhexyl) tetracyclo [8.7.0.0 2,7 .0 11,15 ]heptacos-7-en-5-ol

6. Other names Cholesterin , Cholesteryl alcohol (10 R ,13 R )-10,13-dimethyl-17-(6-methylheptan-2-yl)-2,3,4,7,8,9,11,12,14,15,16,17-dodecahydro-1 H -cyclopenta[ a ]phenanthren-3-ol

7. Properties Molecular formula C 27 H 46 O Molar mass 386.65 g/mol Appearance white crystalline powder [2] Density 1.052 g/cm 3 Melting point 148–150 °C [2] Boiling point 360 °C (decomposes) Solubility in water 0.095 mg/L (30 °C) Solubility soluble in acetone , benzene , chloroform , ethanol , ether , hexane , isopropyl myristate , methanol

8. Production of Cholesterol in Vertebrates In vertebrates the hepatic cells typically produce greater amounts of Cholesterol than other cells. It is almost completely absent among prokaryotes ( bacteria and archaea ), although there are some exceptions such as Mycoplasma , which require cholesterol for growth .

9. BODY CHOLESTEROL CONTENT For a man of about 68 kg (150  lb ), typical total body-cholesterol synthesis is approximately 1 g (1,000 mg) per day , T otal body content of Cholesterol is approximately 35 g, primarily located within the membranes of all the cells of the body . Typical daily dietary intake of additional cholesterol, is 200–300 mg .

10. INGESTED CHOLESTEROL Most ingested cholesterol is esterified, and esterified cholesterol is poorly absorbed . The body also compensates for any absorption of additional cholesterol by reducing cholesterol synthesis . For these reasons, seven to ten hours after ingestion of cholesterol, blood levels will show little if any effect on total body cholesterol content or concentrations of cholesterol in the blood.

11. However, during the first seven hours after ingestion of cholesterol, the levels significantly increase Cholesterol is recycled. The liver excretes it in a non-esterified form (via bile) into the digestive tract . Typically about 50% of the excreted cholesterol is reabsorbed by the small bowel back into the bloodstream.

12. Functions of Cholesterol

13. Cholesterol is required to build and maintain membranes It modulates membrane fluidity over the range of physiological temperatures .

14. Cholesterol serves as a precursor for the biosynthesis of steroids hormones, bile acids, and vitamin D Cholesterol is the principal sterol synthesized by animals. All kinds of cells in animals can produce it . Recent studies show that vitamin D is a potent antioxidant, helping to detox the body and protect arteries too.

15. Cell MEMBRAIN INTEGRITY It is biosynthesized by all animal cells because it is an essential structural component of animal cell membranes It is required to maintain both the structural integrity and fluidity of the cell membrane

16. Cholesterol enables animal cells (a) To protect membrane integrity/cell-viability cholesterol enables animal cells not to need a cell wall like plants & bacteria And thus animal cells are able to ( b) change shape and ( c) move about (unlike bacteria and plant cells which are restricted by their cell walls).

17. The hydroxyl group on cholesterol interacts with the polar head groups of the membrane phospholipids and sphingolipids W hile the bulky steroid and the hydrocarbon chain are embedded in the membrane, alongside the nonpolar fatty-acid chain of the other lipids. Through the interaction with the phospholipid fatty-acid chains, cholesterol increases membrane packing, which reduces membrane fluidity.

18. The structure of the tetracyclic ring of cholesterol contributes to the decreased fluidity of the cell membrane as the molecule is in a trans conformation making all but the side chain of cholesterol rigid and planar . In this structural role, cholesterol reduces the permeability of the plasma membrane to neutral solutes , hydrogen ions, and sodium ions .

19. Within the cell membrane, cholesterol also functions in intracellular transport, cell signalling and nerve conduction . Cholesterol is essential for the structure and function of invaginated caveolae and clathrin-coated pits, including caveolae-dependent and clathrin-dependent endocytosi s The role of cholesterol in such endocytosis can be investigated by using methyl beta cyclodextrin (MβCD) to remove cholesterol from the plasma membrane.

20. Recently, cholesterol has also been implicated in cell signalling processes, assisting in the formation of lipid rafts in the plasma membrane. Lipid raft formation brings receptor proteins in close proximity with high concentrations of second messenger molecules. ] In many neurons, a myelin sheath, rich in cholesterol, since it is derived from compacted layers of Schwann cell membrane , provides insulation for more efficient conduction of impulses .

21. Cholesterol inserts itself into the bilayer and keeps the chains of the phospholipids from becoming entangled. They also work with the proteins to allow for movement around the cell surface.

22. On the top you can see how the cholesterol (yellow) breaks up the monotony of the phospholipids. The space makes it easy for proteins and lipids to flow – hence the fluid mosaic model . The Figure shows a lipid raft, made with phospholipids with longer tails, and similar proteins to do similar functions. These rafts can move around to where they are needed on the surface, break up and reform later, all because the elements of them are fluid. Thank you cholesterol.

23. Cholesterol serves as a precursor for Vitamin ‘ D’

24. Precursor of Steroid Hormones Cholesterol is the precursor to all steroid hormones, which account for numerous physiological functions your body needs to maintain a normal, healthy state Cholesterol is similar to a band aid used to repair wounds and irritations on the arteries. Pregnenolone  is also a precursor to all other steroid hormones.

25. Cholesterol is an important precursor molecule for the synthesis of Steroid hormones Including the adrenal gland hormones cortisol and aldosterone , As well as the sex hormones progesterone , estrogens , and testosterone , and their derivatives . Some research indicates cholesterol may act as an antioxidant

26. Glucocorticoids are necessary for blood sugar regulation, being extremely important for your power houses, the mitochondria . Mineralocorticoids are the key to balancing your minerals, maintaining a perfect and very sensitive homeostasis. This in turn adjust blood pressure quickly and counteracts a loss of minerals e.g. through sweating . Sex Hormones -All your sex hormones are made from cholesterol. If you are male, testosterone (an androgen) Ladies fight with  estrogens and progesterone on a monthly basis .

27. Synthesis of Bile , Bile Salts and Bile acids

28. Within cells, cholesterol is the precursor molecule in several biochemical pathways . In the liver, cholesterol is converted to bile , which is then stored in the gallbladder . Bile contains bile salts, which solubilize fats in the digestive tract and aid in the intestinal absorption of fat molecules as well as the fat-soluble vitamins, A , D , E , and K .

29. What are Bile salts ? The bile salts are conjugates of cholic acid and chenodeoxycholic acid with glycine and taurine . Normally the ratio of glycine : taurine conjugates is about 3 : 1. They are synthesised in the liver by hydroxylation and side-chain oxidation of cholesterol, followed by conjugation with glycine or taurine .

30. conjugated bilirubin 5.1 cholesterol 16.0 lecithin and other phospholipids 3.9 bile salts 145.0 What are the main constituents of bile? Apart from electrolytes, the main constituents of gall bladder bile are as follows ( mmol /L )

31. What is the function of the bile salts ? The function of the bile salts in the small intestine is to emulsify dietary lipids into mixed micelles that are small enough to be absorbed across the intestinal mucosa . .

32. The bile salts are amphiphilic molecules, with a hydrophobic planar hydrocarbon ring and a hydrophilic region provided by the hydroxyl groups and the glycine or taurine conjugated to the carboxylic acid group above the plane of the ring .

33. In an aqueous medium the bile salts will form micelles on their own. In bile they also emulsify the cholesterol that is secreted in the bile, so keeping it in solution . The phospholipids in bile also help to emulsify the cholesterol

34. Reabsorption of bile salts It seems likely that bile salts are reabsorbed from the small intestine and recycled . Interestingly, they are not absorbed from the micelles that contain monoacylglycerol , non-esterified fatty acids, phospholipids and cholesterol. Instead , as the other lipids are absorbed into intestinal epithelial cells from the micelles, the remnants for micelles that consist more or less completely of bile salt conjugates . These are absorbed in the terminal ileum .

35. Bile Acids Analysis of bile shows the presence of four bile acids, conjugated with glycine and taurine : C henodeoxycholic and C holic acids, which are synthesised from cholesterol by isolated hepatocytes , A nd D eoxycholic acid, as well as a small amount of L ithocholic acid, neither of which is synthesised by isolated hepatocytes.

36. When the glycine and taurine conjugates of chenodeoxycholic acid and cholic acid were incubated with a mixed culture of faecal bacteria, F ree chenodeoxycholic acid and cholic acid were found in the incubation medium, together with two new compounds: lithocholic acid and deoxycholic acid .

37. Intestinal bacteria seem to be able to deconjugate the conjugated bile acids, and metabolise them further, to lithocholic and deoxycholic acids . Although some 75 mmol of bile salts are secreted each day, the total body pool is only about 7.5 - 12.5 mmol ; each molecule is secreted and reabsorbed some 6 - 10 time daily. Chenodeoxycholic and cholic acid are generally referred to as primary bile salts, because they are synthesised in the liver; lithocholic and deoxycholic acids are referred to as secondary bile salts .

38. Thank You Dr.Geeta Jaiswal