tailieunhanh - Biochemistry, 4th Edition P75

Biochemistry, 4th Edition P75. 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 Broken Down 703 chain derivatives must first be converted to acylcarnitine derivatives as shown in Figure . Carnitine acyltransferase I associated with the outer mitochondrial membrane catalyzes the formation of the O-acylcarnitine which is then transported across the inner membrane by a translocase. At this point the acylcarnitine is passed to carnitine acyltransferase II on the matrix side of the inner membrane which transfers the fatty acyl group back to CoA to re-form the fatty acyl-CoA leaving free carnitine which can return across the membrane via the translocase. Several additional points should be made. First although oxygen esters usually have lower group-transfer potentials than thiol esters the O acyl bonds in acylcarnitines have high group-transfer potentials and the transesterification reactions mediated by the acyltransferases have equilibrium constants close to 1. Second note that eukaryotic cells maintain separate pools of CoA in the mitochondria and in the cytosol. The cytosolic pool is utilized principally in fatty acid biosynthesis see Chapter 24 and the mitochondrial pool is important in the oxidation of fatty acids and pyruvate as well as some amino acids. 704 Chapter 23 Fatty Acid Catabolism -Oxidation Involves a Repeated Sequence of Four Reactions For saturated fatty acids the process of -oxidation involves a recurring cycle of four steps as shown in Figure . The overall strategy in the first three steps is to create a carbonyl group on the -carbon by oxidizing the Ca C bond to form an olefin with subsequent hydration and oxidation. In essence this cycle is directly analogous to the sequence of reactions converting succinate to oxaloacetate in the TCA cycle. The fourth reaction of the cycle cleaves the 8-keto ester in a reverse Claisen condensation producing an acetate unit and leaving a fatty acid chain that is two carbons shorter than it began. Recall from Chapter 19 that Claisen condensations involve attack

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