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Biochemistry, 4th Edition P63

Biochemistry, 4th Edition P63. 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. | 19.7 What Are the Anaplerotic or Filling Up Reactions 583 C COO- I ch3 Pyruvate ADP Pi Pyruvate carboxylase C COO- H2C fOO- Oxaloacetate 2-O3PO c COO- II ch2 Phosphoenolpyruvate PEP H2O PEP carboxylase C COO- I _ h2c .OO- Oxaloacetate Pyruvate C COO- I ch3 H NADPH enzyme NADP OH I H C COO- h2c J.oo- L-Malate FIGURE 19.17 Pyruvate carboxylase phosphoenolpyruvate PEP carboxylase and malic enzyme catalyze anaplerotic reactions replenishing TCA cycle intermediates. A DEEPER LOOK Fool s Gold and the Reductive Citric Acid Cycle The First Metabolic Pathway How did life arise on the planet Earth It was once supposed that a reducing atmosphere together with random synthesis of organic compounds gave rise to a prebiotic soup in which the first living things appeared. However certain key compounds such as arginine lysine and histidine the straight-chain fatty acids porphyrins and essential coenzymes have not been convincingly synthesized under simulated prebiotic conditions. This and other problems have led researchers to consider other models for the evolution of life. One of these alternative models postulated by Gunter Wachtershauser involves an archaic version of the TCA cycle running in the reverse reductive direction. Reversal of the TCA cycle results in assimilation of CO2 and fixation of carbon as shown. For each turn of the reversed cycle two carbons are fixed in the formation of isocitrate and two more are fixed in the reductive transformation of acetyl-CoA to oxaloacetate. Thus for every succinate that enters the reversed cycle two succinates are returned making the cycle highly autocatalytic. Because TCA cycle intermediates are involved in many biosynthetic pathways see Section 19.6 a reversed TCA cycle would be a bountiful and broad source of metabolic substrates. A reversed reductive TCA cycle would require energy input to drive it. What might have been the thermodynamic driving force for such a cycle Wachtershauser hypothesizes that the anaerobic reaction