CHAPTER TWENTY-SIX Lipids This phase of fatty acid biosynthesis concludes with the transfer of the acyl group from acyl carrier protein to coenzyme A. The resulting acyl coenzyme a molecules car then undergo a number of subsequent biological transformations. One such transforma tion is chain extension, leading to acyl groups with more than 16 carbons. Another is the introduction of one or more carbon-carbon double bonds. A third is acyl transfer from sulfur to oxygen to form esters such as triacylglycerols. The process by which acyl coenzyme A molecules are converted to triacylglycerols involves a type of intermediate alled a phospholipid and is discussed in the following section 26. 4 PHOSPHOLIPIDS Triacylglycerols arise, not by acylation of glycerol itself, but by a sequence of steps in which the first stage is acyl transfer to L-glycerol 3-phosphate(from reduction of dihy droxyacetone 3-phosphate, formed as described in Section 25. 21). The product of this stage is called a phosphatidic acid. CH,OH O o CH,OCR H RCSCoA +R'cSCoa RCO H 2HSCOA CH,,H CH,OPO3H2 Glycer Two acyl coenzyme a molecules Phosphated Coenzyme a 3- (R and r may be the same or they may be different PROBLEM 26.4 What is the absolute configuration(R or S)of L-glycerol 3- phosphate? What must be the absolute configuration of the naturally occurring phosphatidic acids biosynthesized from it? Hydrolysis of the phosphate ester function of the phosphatidic acid gives a diacylglycerol, which then reacts with a third acyl coenzyme A molecule to produce a triacylglycerol O CH,OCR O CH,OCR O CHOCR RCO >RCO R CSCOA H →RCO+H CH,OPO3H2 CHOH CH,OCR Phosphatidic acid Diacylglycerol Triacylglycerol Phosphatidic acids not only are intermediates in the biosynthesis of triacylglycerols but also are biosynthetic precursors of other members of a group of compounds called phosphoglycerides or glycerol phosphatides. Phosphorus-containing derivatives of lipids are known as phospholipids, and phosphoglycerides are one type of phospholipid. nt to prevent One important phospholipid is phosphatidylcholine, also called lecithin. Phos- from sepa- phatidylcholine is a mixture of diesters of phosphoric acid. One ester function is derived from a diacylglycerol, whereas the other is a choline [-OCH2CH2N(CH3)3) unit Back Forward Main MenuToc Study Guide ToC Student o MHHE Website1022 CHAPTER TWENTY-SIX Lipids This phase of fatty acid biosynthesis concludes with the transfer of the acyl group from acyl carrier protein to coenzyme A. The resulting acyl coenzyme A molecules can then undergo a number of subsequent biological transformations. One such transforma￾tion is chain extension, leading to acyl groups with more than 16 carbons. Another is the introduction of one or more carbon–carbon double bonds. A third is acyl transfer from sulfur to oxygen to form esters such as triacylglycerols. The process by which acyl coenzyme A molecules are converted to triacylglycerols involves a type of intermediate called a phospholipid and is discussed in the following section. 26.4 PHOSPHOLIPIDS Triacylglycerols arise, not by acylation of glycerol itself, but by a sequence of steps in which the first stage is acyl transfer to L-glycerol 3-phosphate (from reduction of dihy￾droxyacetone 3-phosphate, formed as described in Section 25.21). The product of this stage is called a phosphatidic acid. PROBLEM 26.4 What is the absolute configuration (R or S) of L-glycerol 3- phosphate? What must be the absolute configuration of the naturally occurring phosphatidic acids biosynthesized from it? Hydrolysis of the phosphate ester function of the phosphatidic acid gives a diacylglycerol, which then reacts with a third acyl coenzyme A molecule to produce a triacylglycerol. Phosphatidic acids not only are intermediates in the biosynthesis of triacylglycerols but also are biosynthetic precursors of other members of a group of compounds called phosphoglycerides or glycerol phosphatides. Phosphorus-containing derivatives of lipids are known as phospholipids, and phosphoglycerides are one type of phospholipid. One important phospholipid is phosphatidylcholine, also called lecithin. Phos￾phatidylcholine is a mixture of diesters of phosphoric acid. One ester function is derived from a diacylglycerol, whereas the other is a choline unit. [±OCH2CH2N(CH3)3] H O RCO CH2OPO3H2 CH2OCR O Phosphatidic acid H O RCO CH2OH CH2OCR O Diacylglycerol H O RCO CH2OCR CH2OCR O O Triacylglycerol H2O RCSCoA O X Lecithin is added to foods such as mayonnaise as an emulsifying agent to prevent the fat and water from sepa￾rating into two layers. HO H CH2OPO3H2 CH2OH L-Glycerol 3-phosphate O RCSCoA O RCSCoA Two acyl coenzyme A molecules (R and R may be the same or they may be different) H O RCO CH2OPO3H2 CH2OCR O Phosphatidic acid 2HSCoA Coenzyme A Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website