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Biochemistry, 4th Edition P65

Biochemistry, 4th Edition P65. 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. | 20.3 How Is the Electron-Transport Chain Organized 603 FIGURE 20.8 The fatty acyl-CoA dehydrogenase reaction emphasizing that the reaction involves reduction of enzyme-bound FAD indicated by brackets . electrons to UQ including mitochondrial sn-glycerophosphate dehydrogenase an inner membrane-bound shuttle enzyme and the fatty acyl-CoA dehydrogenases three soluble matrix enzymes involved in fatty acid oxidation Figure 20.8 also see Chapter 23 . The path of electrons from succinate to UQ is shown in Figure 20.7. Complex III Mediates Electron Transport from Coenzyme Q to Cytochrome c In the third complex of the electron-transport chain reduced coenzyme Q UQH2 passes its electrons to cytochrome c via a unique redox pathway known as the Q cycle. UQ-cytochrome creductase UQ-cyt c reductase as this complex is known involves three different cytochromes and an Fe-S protein. In the cytochromes of these and similar complexes the iron atom at the center of the porphyrin ring cycles between the reduced Fe2 ferrous and oxidized Fe3 ferric states. Cytochromes were first named and classified on the basis of their absorption spectra Figure 20.9 which depend upon the structure and environment of their heme groups. The b cytochromes contain iron protoporphyrin IX Figure 20.10 the same heme found in hemoglobin and myoglobin. The c cytochromes contain heme c derived from iron protoporphyrin IX by the covalent attachment to cysteine residues from the associated protein. One other heme variation heme a contains a 15-carbon isoprenoid chain on a modified vinyl group and a formyl group in place of one of the methyls see Figure 20.10 . Cytochrome a is found in two forms in Complex IV of the electron-transport chain as we shall see. UQ-cyt c reductase Figure 20.11 contains a b-type cytochrome of 30 to 40 kD with two different heme sites and one c-type cytochrome. The two hemes on the b cytochrome polypeptide in UQ-cyt c reductase are distinguished by their reduction potentials and the .