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26.11 Steroids: Cholesterol

26.11 Steroids: Cholesterol. Structure of Cholesterol. Fundamental framework of steroids is the tetracyclic unit shown. CH 3. CH 3. CH 3. CH 3. CH 3. H. H. H. HO. Structure of Cholesterol. Cholesterol has the fundamental steroid skeleton modified as shown. Structure of Cholesterol.

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26.11 Steroids: Cholesterol

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  1. 26.11Steroids: Cholesterol

  2. Structure of Cholesterol • Fundamental framework of steroids is the tetracyclic unit shown.

  3. CH3 CH3 CH3 CH3 CH3 H H H HO Structure of Cholesterol • Cholesterol has the fundamental steroid skeleton modified as shown.

  4. Structure of Cholesterol CH3 CH3 CH3 • Some parts of the cholesterol molecule are isoprenoid. But other parts don't obey the isoprene rule. Also, cholesterol has 27 carbons, which is not a multiple of 5. CH3 CH3 H H H HO

  5. Biosynthesis of Cholesterol • Cholesterol is biosynthesized from the triterpene squalene. In the first step, squalene is converted to its 2,3-epoxide.

  6. Biosynthesis of Cholesterol • Cholesterol is biosynthesized from the triterpene squalene. In the first step, squalene is converted to its 2,3-epoxide. O2, NADH, enzyme O

  7. O Biosynthesis of Cholesterol • To understand the second step, we need to look at squalene oxide in a different conformation, one that is in a geometry suitable for cyclization.

  8. O O Biosynthesis of Cholesterol • To understand the second step, we need to look at squalene oxide in a different conformation, one that is in a geometry suitable for cyclization.

  9. O + H Biosynthesis of Cholesterol • Cyclization is triggered by epoxide ring opening.

  10. O + H Biosynthesis of Cholesterol • Cyclization is triggered by epoxide ring opening. + HO

  11. •• OH2 •• H HO H Biosynthesis of Cholesterol H • Loss of a proton is accompanied by a series of hydride shifts and methyl migrations. + HO H H

  12. H HO H Biosynthesis of Cholesterol • The product of this rearrangement is a triterpene called lanosterol. A number of enzyme-catalyzed steps follow that convert lanosterol to cholesterol.

  13. Cholesterol • Cholesterol is the biosynthetic precursor to a large number of important steroids: • Bile acidsVitamin DCorticosteroidsSex hormones

  14. 26.12Vitamin D

  15. CH3 CH3 CH3 CH3 CH3 H H H HO Cholesterol • Cholesterol is the precursor to vitamin D. • Enzymes dehydrogenate cholesterol to introduce a second double bond in conjugation with the existing one. The product of this reaction is called 7-dehydrocholesterol.

  16. 7-Dehydrocholesterol CH3 CH3 • Sunlight converts 7-dehydrocholesterol on the skin's surface to vitamin D3. CH3 CH3 CH3 H H HO

  17. Vitamin D3 CH3 CH3 • Insufficient sunlight can lead to a deficiency of vitamin D3, interfering with Ca2+ transport and bone development. Rickets can result. CH3 CH3 H HO

  18. 26.13Bile Acids

  19. CH3 CH3 CH3 CH3 CH3 H H H HO Cholesterol • Oxidation in the liver degrades the cholesterol side chain and introduces OH groups at various positions on the steroid skeleton. Cholic acid is the most abundant of the bile acids.

  20. O CH3 HO OH CH3 CH3 H H H HO OH H Cholic Acid • Salts of cholic acid amides (bile salts), such as sodium taurocholate, act as emulsifying agents to aid digestion.

  21. O CH3 HO NHCH2CH2SO3Na CH3 CH3 H H H HO OH H Sodium Taurocholate

  22. 26.14Corticosteroids

  23. CH3 CH3 CH3 CH3 CH3 H H H HO Cholesterol • Enzymatic degradation of the side chain and oxidation of various positions on the steroid skeleton convert cholesterol to corticosteroids.

  24. Cortisol O OH CH3 • Cortisol is the most abundant of the corticosteroids. Enzyme-catalyzed oxidation of cortisol gives cortisone. HO OH CH3 H H H O

  25. Cortisone O OH CH3 • Corticosteroids are involved in maintaining electrolyte levels, in the metabolism of carbohydrates, and in mediating the allergic response. O OH CH3 H H H O

  26. 26.15Sex Hormones

  27. OH H3C H3C H H H O Testosterone • Testosterone is the main male sex hormone.

  28. Estradiol OH H3C • Estradiol is a female sex hormone involved in regulating the menstrual cycle and in reproduction. H H H HO

  29. O H3C H3C H H H O Progesterone • Supresses ovulation during pregnancy.

  30. 26.16Carotenoids

  31. Carotenoids • Carotenoids are naturally occurring pigments. • Structurally, carotenoids are tetraterpenes. They have 40 carbons. Two C20 units are linked in a tail-to-tail fashion. • Examples are lycopene and b-carotene.

  32. Carotenoids Lycopene (tomatoes) b-Carotene (carrots)

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