Biochemistry, 4th Edition P86. 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. | 26 Synthesis and Degradation of Nucleotides ESSENTIAL QUESTION Virtually all cells are capable of synthesizing purine and pyrimidine nucleotides. These compounds then serve as essential intermediates in metabolism and as the building blocks for DNA and RNA synthesis. How do cells synthesize purines and pyrimidines Nucleotides are ubiquitous constituents of life actively participating in the majority of biochemical reactions. Recall that ATP is the energy currency of the cell that uracil nucleotide derivatives of carbohydrates are common intermediates in cellular transformations of carbohydrates see Chapter 22 and that biosynthesis of phospholipids proceeds via cytosine nucleotide derivatives see Chapter 24 . In Chapter 30 we will see that GTP serves as the immediate energy source driving the endergonic reactions of protein synthesis. Many of the coenzymes such as coenzyme A NAD NADP and FAD are derivatives of nucleotides. Nucleotides also act in metabolic regulation as in the response of key enzymes of intermediary metabolism to the relative concentrations of AMP ADP and ATP PFK is a prime example here see also Chapter 18 . Furthermore cyclic derivatives of purine nucleotides such as cAMP and cGMP have no other role in metabolism than regulation. Last but not least nucleotides are the monomeric units of nucleic acids. Deoxynucleoside triphosphates dNTPs and nucleoside triphosphates NTPs serve as the immediate substrates for the biosynthesis of DNA and RNA respectively see Part 4 . Can Cells Synthesize Nucleotides Nearly all organisms can make the purine and pyrimidine nucleotides via so-called de novo biosynthetic pathways. De novo means anew a less literal but more apt translation might be from scratch because de novo pathways are metabolic sequences that form complex end products from rather simple precursors. Many organisms also have salvage pathways to recover purine and pyrimidine compounds obtained in the diet or released during nucleic acid turnover and
