1032 CHAPTER TWENTY-SIX Lipids A. Loss of a proton from the bicyclic carbocation yields a-pinene and B-pinene. The pinenes are the most abundant of the monoterpenes. They are the main constituents a-Pinene B Capture of the carbocation by water, accompanied by rearrangement of the bicyclo- [3. 1. 1] carbon skeleton to a bicyclo[2. 2. 1] unit, yields borneol. Borneol is found in the essential oil of certain trees that grow in Indonesia. FIGURE 26.7 Two of the reaction pathways available to the Clo bicyclic carbocation formed from ryI pyrophosp location can lead to the bicyclo[3. 1.1] or the bicyclo[2 2.1] carbon skeleton especially the triterpenes, later in this chapter. For the moment, however, let's return to smaller molecules in order to complete the picture of how isoprenoid compounds arise from acetate 26.10 THE PATHWAY FROM ACETATE TO ISOPENTENYL PYROPHOSPHATE The introduction to Section 26.8 pointed out that mevalonic acid is the biosynthetic pre- cursor of isopentenyl pyrophosphate. The early steps in the biosynthesis of mevalonate from three molecules of acetic acid are analogous to those in fatty acid biosynthesis (Sec tion 26.3)except that they do not involve acyl carrier protein. Thus, the reaction of acetyl coenzyme A with malonyl coenzyme A yields a molecule of acetoacetyl coenzyme A O CH3 CSCoA O2CCH2CSCoA-CH3 CCH2 CSCoA CO2 Malonyl Carbon zyme a Carbon-carbon bond formation then occurs between the ketone carbonyl of acetoacetyl coenzyme A and the a carbon of a molecule of acetyl coenzyme A. Back Forward Main MenuToc Study Guide ToC Student o MHHE Website1032 CHAPTER TWENTY-SIX Lipids especially the triterpenes, later in this chapter. For the moment, however, let’s return to smaller molecules in order to complete the picture of how isoprenoid compounds arise from acetate. 26.10 THE PATHWAY FROM ACETATE TO ISOPENTENYL PYROPHOSPHATE The introduction to Section 26.8 pointed out that mevalonic acid is the biosynthetic pre￾cursor of isopentenyl pyrophosphate. The early steps in the biosynthesis of mevalonate from three molecules of acetic acid are analogous to those in fatty acid biosynthesis (Sec￾tion 26.3) except that they do not involve acyl carrier protein. Thus, the reaction of acetyl coenzyme A with malonyl coenzyme A yields a molecule of acetoacetyl coenzyme A. Carbon–carbon bond formation then occurs between the ketone carbonyl of acetoacetyl coenzyme A and the carbon of a molecule of acetyl coenzyme A. O CH3CSCoA Acetyl coenzyme A O O CH3CCH2CSCoA Acetoacetyl coenzyme A CO2 Carbon dioxide O O2CCH2CSCoA Malonyl coenzyme A O H H H OH Borneol O H H H -Pinene -Pinene A. Loss of a proton from the bicyclic carbocation yields -pinene and -pinene. The pinenes are the most abundant of the monoterpenes. They are the main constituents of turpentine. B. Capture of the carbocation by water, accompanied by rearrangement of the bicyclo- [3.1.1] carbon skeleton to a bicyclo[2.2.1] unit, yields borneol. Borneol is found in the essential oil of certain trees that grow in Indonesia. FIGURE 26.7 Two of the reaction pathways available to the C10 bicyclic carbocation formed from neryl pyrophosphate. The same carbocation can lead to monoterpenes based on either the bicyclo[3.1.1] or the bicyclo[2.2.1] carbon skeleton. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website