1020 CHAPTER TWENTY-SIX Lipids PROBLEM 26.2 Using HSCoA and Hs-ACP as abbreviations for coenzyme a and acyl carrier protein, respectively, write a structural formula for the tetrahedral intermediate in the preceding reaction A second molecule of acetyl coenzyme A reacts with carbon dioxide(actually bicarbonate ion at biological pH) to give malonyl coenzyme A CH3 CSCoA HCO3 OCCH, CSCoA HO Malony Water coenzyme A coenzyme A Formation of malonyl coenzyme A is followed by a nucleophilic acyl substitution, which transfers the malonyl group to the acyl carrier protein as a thioester. OCCHCSCOA+HS一ACP OCCHCS-ACP+ HSCOA Acyl carrier S-Malonyl acyl When both building block units are in place on the acyl carrier protein, carbon-car- bon bond formation occurs between the a-carbon atom of the malonyl group and the carbonyl carbon of the acetyl group. This is shown in step I of Figure 26.3. Carbon-car- bon bond formation is accompanied by decarboxylation and produces a four-carbon ace- toacetyl (3-oxobutanoyl) group bound to acyl carrier protein. The acetoacetyl group is then transformed to a butanoyl group by the reaction equence illustrated in steps 2 to 4 of Figure 26.3. The four carbon atoms of the butanoyl group originate in two molecules of acetyl coenzyme A. Carbon dioxide assists the reaction but is not incorporated into the prod uct. The same carbon dioxide that is used to convert one molecule of acetyl coenzyme A to malonyl coenzyme A is regenerated in the decarboxylation step that accompanies carbon-carbon bond formation Successive repetitions of the steps shown in Figure 26.3 give unbranched acyl groups having 6, 8, 10, 12, 14, and 16 carbon atoms. In each case, chain extension occurs by reaction with a malonyl group bound to the acyl carrier protein. Thus, the biosyn- thesis of the 16-carbon acyl group of hexadecanoic (palmitic) acid can be represented by the overall equation oO CH3CS-ACP 7HOCCH, CS-ACP+ 14 NADPH 14 H3o S-Malonyl acyl Reduced form Hydronium carner protein carrier protein of coenzyme CH3(CH2)14CS-ACP+ 7C0,+ 7HS-ACP+ 14 NADP 21 H,O S-Hexadecanoyl acyl Carbon Acyl carrier Oxidized form Water dioxide of Back Forward Main MenuToc Study Guide ToC Student o MHHE Website1020 CHAPTER TWENTY-SIX Lipids PROBLEM 26.2 Using HSCoA and HS±ACP as abbreviations for coenzyme A and acyl carrier protein, respectively, write a structural formula for the tetrahedral intermediate in the preceding reaction. A second molecule of acetyl coenzyme A reacts with carbon dioxide (actually bicarbonate ion at biological pH) to give malonyl coenzyme A: Formation of malonyl coenzyme A is followed by a nucleophilic acyl substitution, which transfers the malonyl group to the acyl carrier protein as a thioester. When both building block units are in place on the acyl carrier protein, carbon–car￾bon bond formation occurs between the -carbon atom of the malonyl group and the carbonyl carbon of the acetyl group. This is shown in step 1 of Figure 26.3. Carbon–car￾bon bond formation is accompanied by decarboxylation and produces a four-carbon ace￾toacetyl (3-oxobutanoyl) group bound to acyl carrier protein. The acetoacetyl group is then transformed to a butanoyl group by the reaction sequence illustrated in steps 2 to 4 of Figure 26.3. The four carbon atoms of the butanoyl group originate in two molecules of acetyl coenzyme A. Carbon dioxide assists the reaction but is not incorporated into the prod￾uct. The same carbon dioxide that is used to convert one molecule of acetyl coenzyme A to malonyl coenzyme A is regenerated in the decarboxylation step that accompanies carbon–carbon bond formation. Successive repetitions of the steps shown in Figure 26.3 give unbranched acyl groups having 6, 8, 10, 12, 14, and 16 carbon atoms. In each case, chain extension occurs by reaction with a malonyl group bound to the acyl carrier protein. Thus, the biosyn￾thesis of the 16-carbon acyl group of hexadecanoic (palmitic) acid can be represented by the overall equation: 7HS ACP Acyl carrier protein 21 H2O Water 14 NADP Oxidized form of coenzyme 7CO2 Carbon dioxide S-Hexadecanoyl acyl carrier protein ACP O CH3(CH2)14CS 14 NADPH Reduced form of coenzyme 14 H3O Hydronium ion S-Acetyl acyl carrier protein ACP O CH3CS S-Malonyl acyl carrier protein ACP O 7HOCCH2CS O HS ACP Acyl carrier protein O O OCCH2CSCoA Malonyl coenzyme A HSCoA Coenzyme A S-Malonyl acyl carrier protein ACP O O OCCH2CS O CH3CSCoA Acetyl coenzyme A O O OCCH2CSCoA Malonyl coenzyme A H2O Water HCO3  Bicarbonate Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website