Biochemistry, 4th Edition P78. Continuing Garrett and Grisham's innovative conceptual and organizing framework, "Essential Questions," BIOCHEMISTRY guides students through course concepts in a way that reveals the beauty and usefulness of biochemistry in the everyday world. Streamlined for increased clarity and readability, this edition also includes new photos and illustrations that show the subject matter consistently throughout the text. New end-of-chapter problems, MCAT practice questions, and the unparalleled text/media integration with the power of CengageNOW round out this exceptional package, giving you the tools you need to both master course concepts and develop critical problem-solving skills you can draw upon. | How Are Fatty Acids Synthesized 733 C16 Fatty Acids May Undergo Elongation and Unsaturation Additional Elongation As seen already palmitate is the primary product of the fatty acid synthase. Cells synthesize many other fatty acids. Shorter chains are easily made if the chain is released before reaching 16 carbons in length. Longer chains are made through special elongation reactions which occur both in the mitochondria and at the surface of the endoplasmic reticulum ER . The ER reactions are actually quite similar to those we have just discussed addition of two-carbon units at the carboxyl end of the chain by means of oxidative decarboxylations involving malonyl-CoA. As was the case for the fatty acid synthase this decarboxylation provides the thermodynamic driving force for the condensation reaction. The mitochondrial reactions involve addition and subsequent reduction of acetyl units. These reactions Figure are essentially a reversal of fatty acid oxidation with the exception that NADPH is utilized in the saturation of the double bond instead of FADH2. Introduction of a Single cis Double Bond Both prokaryotes and eukaryotes are capable of introducing a single cis double bond in a newly synthesized fatty acid. Bacteria such as E. coli carry out this process in an O2-independent pathway whereas eukaryotes have adopted an O2-dependent pathway. There is a fundamental chemical difference between the two. The O2-dependent reaction can occur anywhere in the fatty acid chain with no additional need to activate the desired bond toward dehydrogenation. However in the absence of O2 some other means must be found to activate the bond in question. Thus in the bacterial reaction dehydrogenation occurs while the bond of interest is still near the 8-carbonyl or -hydroxy group and the thioester group at the end of the chain. O CH3 C SCoA O R CH2 C SCoA HSCoA Thiolase R CH2 C CH2 C SCoA O II O J-Ketoacyl-CoA Acyl-CoA NADH H NAD H L-jB-hydroxyacyl-CoA dehydrogenase O R .
