tailieunhanh - Biochemistry, 4th Edition P82

Biochemistry, 4th Edition P82. 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. | Which Metabolic Pathways Allow Organisms to Live on Inorganic Forms of Nitrogen 773 FIGURE Ribbon diagram of nitrogenase reductase the Fe-protein blue nitrogenase FeMo protein green Fe-protein iron-sulfur cluster is shown in yellow bound ADP in nitrogenase FeMo cofactor is shown in cyan the P-cluster in red pdb id 1N2C . tase so it no longer binds to nitrogenase. The ADP oxidized nitrogenase reductase complex dissociates making way for another ATP reduced nitrogenase reductase complex to bind to nitrogenase. Interestingly nitrogenase reductase is a member of the G-protein family G proteins are molecular switches whose operation is driven by NTP hydrolysis. Nitrogenase is a rather slow enzyme Its optimal rate of e transfer is about 12 e pairs per second per enzyme molecule that is it reduces only three molecules of nitrogen gas per second. Because its activity is so weak nitrogen-fixing cells maintain large amounts of nitrogenase so that their requirements for reduced N can be met. As much as 5 of the cellular protein may be nitrogenase. The Regulation of Nitrogen Fixation To a first approximation two regulatory controls are paramount Figure 1 ADP inhibits the activity of nitrogenase thus as the ATP ADP ratio drops nitrogen fixation is blocked. 2 NH4 represses N2 8 10 H À- S b nif gene expression i 2 NH4 Nitrogenase 16 ATP 16 Pi 16 ADP - H a Active ADP-ribosyl group FIGURE Regulation of nitrogen fixation. a ADP inhibits nitrogenase activity. b NH4 represses nif gene expression. c In some organisms the nitrogenase complex is regulated by covalent modification. ADP-ribosylation of nitrogenase reductase leads to its inactivation. 774 Chapter 25 Nitrogen Acquisition and Amino Acid Metabolism the expression of the nif genes the genes that encode the proteins of the nitrogenfixing system. To date some 20 nif genes have been identified with the nitrogen fixation process. Repression of nif gene expression by ammonium the primary .