26.11 Steroids: cholesterol Step 4: A series of enzyme-catalyzed reactions converts lanosterol to cholesterol. The three highlighted methyl roups in the structural formula of lanosterol are lost via separate multistep operations, the C-8 and C-24 double bonds are reduced. and a new double bond is introduced at C-5 →>HO FIGURE 26.10 Cont PROBLEM 26.12 The biosynthesis of cholesterol as outlined in Figure 26.10 is admittedly quite complicated. It will aid your understanding of the process if you consider the following questions (a)Which carbon atoms of squalene 2,3-epoxide correspond to the doubly bonded carbons of cholesterol? (b)Which two hydrogen atoms of squalene 2, 3-epoxide are the ones that migrate in step 3? (c) Which methyl group of squalene 2, 3-epoxide becomes the methyl group at the C, d ring junction of cholesterol? (d)What three methyl groups of squalene 2, 3-epoxide are lost during the con- version of lanosterol to cholesterol? SAMPLE SOLUTION (a)As the structural formula in step 4 of Figure 26. 10 indi- cates, the double bond of cholesterol unites C-5 and C-6(steroid numbering). The corresponding carbons in the cyclization reaction of step 2 in the figure may be dentified as C-7 and C-8 of squalene 2, 3-epoxide(systematic IUPAC numbering) Coiled form of squalene 2, 3-epoxide PROBLEM 26.13 The biosynthetic pathway shown in Figure 26.10 was devel oped with the aid of isotopic labeling experiments Which carbon atoms of cho- lesterol would you expect to be labeled when acetate enriched with C in its methyl group (CH3 COOH)is used as the carbon source? Once formed in the body, cholesterol can undergo a number of transformations. A very common one is acylation of its C-3 hydroxyl group by reaction with coenzyme A derivatives of fatty acids. Other processes convert cholesterol to the biologically impor tant steroids described in the following sections Back Forward Main MenuToc Study Guide ToC Student o MHHE Website26.11 Stereoids: Cholesterol 1037 PROBLEM 26.12 The biosynthesis of cholesterol as outlined in Figure 26.10 is admittedly quite complicated. It will aid your understanding of the process if you consider the following questions: (a) Which carbon atoms of squalene 2,3-epoxide correspond to the doubly bonded carbons of cholesterol? (b) Which two hydrogen atoms of squalene 2,3-epoxide are the ones that migrate in step 3? (c) Which methyl group of squalene 2,3-epoxide becomes the methyl group at the C, D ring junction of cholesterol? (d) What three methyl groups of squalene 2,3-epoxide are lost during the con￾version of lanosterol to cholesterol? SAMPLE SOLUTION (a) As the structural formula in step 4 of Figure 26.10 indi￾cates, the double bond of cholesterol unites C-5 and C-6 (steroid numbering). The corresponding carbons in the cyclization reaction of step 2 in the figure may be identified as C-7 and C-8 of squalene 2,3-epoxide (systematic IUPAC numbering). PROBLEM 26.13 The biosynthetic pathway shown in Figure 26.10 was devel￾oped with the aid of isotopic labeling experiments. Which carbon atoms of cho￾lesterol would you expect to be labeled when acetate enriched with 14C in its methyl group (14CH3COOH) is used as the carbon source? Once formed in the body, cholesterol can undergo a number of transformations. A very common one is acylation of its C-3 hydroxyl group by reaction with coenzyme A derivatives of fatty acids. Other processes convert cholesterol to the biologically impor￾tant steroids described in the following sections. O 1 3 4 2 7 6 5 8 9 11 12 10 15 16 17 14 13 19 18 20 22 24 21 23 Coiled form of squalene 2,3-epoxide HO 5 8 24 many steps 5 6 HO Step 4: A series of enzyme-catalyzed reactions converts lanosterol to cholesterol. The three highlighted methyl groups in the structural formula of lanosterol are lost via separate multistep operations, the C-8 and C-24 double bonds are reduced, and a new double bond is introduced at C-5. Lanosterol Cholesterol FIGURE 26.10 Cont. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website