Biochemistry, 4th Edition P11. 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 Complex Equilibria Involved in ATP Hydrolysis 63 O O -O P O- H2O -O P O- OH O Enolase O H2C---------CH COO- 2-Phosphoglycerate Mg2 AG kJ mol h2c c coo- Phosphoenolpyruvate PEP O II -O P O- I O I H2C C coo- ATP H ADP I Pyruvate kinase O Mg2 K H3C C COO- Phosphoenolpyruvate AG kJ mol PEP Pyruvate CENGAGENOW ANIMATED FIGURE Phosphoenolpyruvate PEP is produced by the enolase reaction in glycolysis see Chapter 18 and in turn drives the phosphorylation of ADP to form ATP in the pyruvate kinase reaction. See this figure animated at login O II -O P O- I O I H2C C COO- h2o ----------------i AG kJ mol O II -O P O- I OH OH I h2c c COO- Pyruvate unstable enol form Tautomerization AG kJ mol O II H3C C COO Pyruvate stable keto PEP CENGAGENOW ANIMATED FIGURE Hydrolysis and the subsequent tautomerization account for the very large AG of PEP. See this figure animated at login tants also contribute to the large negative AGvalues. The value of AGdepends on the pKa values of the starting anhydride and the product phosphoric and carboxylic acids and of course also on the pH of the medium. Enol Phosphates Are Potent Phosphorylating Agents The largest value of AG in Table belongs to phosphoenolpyruvate or PEP an example of an enolic phosphate. This molecule is an important intermediate in carbohydrate metabolism and due to its large negative AG it is a potent phosphorylating agent. PEP is formed via dehydration of 2-phosphoglycerate by enolase during fermentation and glycolysis. PEP is subsequently transformed into pyruvate upon transfer of its phosphate to ADP by pyruvate kinase Figure . The very large negative value of AGfor the latter reaction is to a large extent the result of a secondary reaction of the enol form of pyruvate. Upon hydrolysis the unstable enolic form of pyruvate immediately converts to the keto form with a resulting large negative AG Figure . Together the hydrolysis and .
