Biochemistry, 4th Edition P60. 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 Are the Metabolic Fates of NADH and Pyruvate Produced in Glycolysis 553 a Alcoholic fermentation CHO I H C OH I CH2OPO - d - Glyceraldehyde-3-phosphate O COPO2- H C OH I ch2opo - b Lactic acid fermentation CH3CH2OH Ethanol 1 3-BPG CHO I H C OH I CH2OPO - D-Glyceraldehyde- 3-phosphate O COPO2- H C OH CH2OPO - 1 3-BPG NAD V NADH H NAD NADH H Alcohol Acetaldehyde dehydrogenase OH I CH3 C COO- I H Lactate dehydrogenase Pyruvate Lactate FIGURE a Pyruvate reduction to ethanol in yeast provides a means for regenerating NAD consumed in the glyceraldehyde-3-P dehydrogenase reaction. b In oxygen-depleted muscle NAD is regenerated in the lactate dehydrogenase reaction. process. Pyruvate is decarboxylated to acetaldehyde by pyruvate decarboxylase in an essentially irreversible reaction. Thiamine pyrophosphate see page 568 is a required cofactor for this enzyme. The second step the reduction of acetaldehyde to ethanol by NADH is catalyzed by alcohol dehydrogenase Figure . At pH 7 the reaction equilibrium strongly favors ethanol. The end products of alcoholic fermentation are thus ethanol and carbon dioxide. Alcoholic fermentations are the basis for the brewing of beers and the fermentation of grape sugar in wine making. Lactate produced by anaerobic microorganisms during lactic acid fermentation is responsible for the taste of sour milk and for the characteristic taste and fragrance of sauerkraut which in reality is fermented cabbage. Lactate Accumulates Under Anaerobic Conditions in Animal Tissues In animal tissues experiencing anaerobic conditions pyruvate is reduced to lactate. Pyruvate reduction occurs in tissues that normally experience minimal access to blood flow for example the cornea of the eye and also in rapidly contracting skeletal muscle. When skeletal muscles are exercised strenuously the available tissue oxygen is consumed and the pyruvate generated by glycolysis can no longer be oxidized in the TCA cycle. Instead excess pyruvate is reduced
