tailieunhanh - Biochemistry, 4th Edition P57

Biochemistry, 4th Edition P57. 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. | What Experiments Can Be Used to Elucidate Metabolic Pathways 523 NADPH Provides the Reducing Power for Anabolic Processes Whereas catabolism is fundamentally an oxidative process anabolism is by its contrasting nature reductive. The biosynthesis of the complex constituents of the cell begins at the level of intermediates derived from the degradative pathways of catabolism or less commonly biosynthesis begins with oxidized substances available in the inanimate environment such as carbon dioxide. When the hydrocarbon chains of fatty acids are assembled from acetyl-CoA units activated hydrogens are needed to reduce the carbonyl C O carbon of acetyl-CoA into a CH2 at every other position along the chain. When glucose is synthesized from CO2 during photosynthesis in plants reducing power is required. These reducing equivalents are provided by NADPH the usual source of high-energy hydrogens for reductive biosynthesis. NADPH is generated when NADP is reduced with electrons in the form of hydride ions. In heterotrophic organisms these electrons are removed from fuel molecules by NADP -specific dehydrogenases. In these organisms NADPH can be viewed as the carrier of electrons from catabolic reactions to anabolic reactions Figure . In photosynthetic organisms the energy of light is used to pull electrons from water and transfer them to NADP O2 is a by-product of this process. Reduced fuel Oxidized product Reductive biosynthetic reactions Reductive biosynthetic product Oxidized precursor FIGURE Transfer of reducing equivalents from catabolism to anabolism via the NADPH cycle. Coenzymes and Vitamins Provide Unique Chemistry and Essential Nutrients to Pathways In addition to NAD and NADPH a variety of other small molecules are essential to metabolism. Some of these are essential nutrients called vitamins. The name was coined by Kazimierz Funk who discovered thiamine as a cure for beriberi in 1912 and termed it a vital amine. He later proposed that other .