chapter 1 The Biogeochemistry of Molybdenum and Tungsten -- chapter 2 Transport, Homeostasis, Regulation, and Binding of Molybdate and Tungstate to Proteins -- chapter 3 Molybdenum Nitrogenases: A Crystallographic and Mechanistic View -- chapter 4 Chemical Dinitrogen Fixation by Molybdenum and Tungsten Complexes: Insights from Coordination Chemistry -- chapter 5 Biosynthesis of the Nitrogenase Iron-Molybdenum-Cofactor from Azotobacter vinelandii -- chapter 6 Molybdenum Enzymes Containing the Pyranopterin Cofactor: An Overview -- chapter 7 The Molybdenum and Tungsten Cofactors: A Crystallographic View -- chapter 8 Models for the Pyranopterin-Containing Molybdenum and Tungsten Cofactors -- chapter 9 Biosynthesis and Molecular Biology of the Molybdenum Cofactor (Moco) -- chapter 10 Molybdenum in Nitrate Reductase and Nitrite Oxidoreductase -- chapter 11 The Molybdenum-Containing Hydroxylases of Nicotinate, Isonicotinate, and Nicotine -- chapter 12 The Molybdenum-Containing Xanthine Oxidoreductases and Picolinate Dehydrogenases -- chapter 13 Enzymes of the Xanthine Oxidase Family: The Role of Molybdenum -- chapter 14 The Molybdenum-Containing Hydroxylases of Quinoline, Isoquinoline, and Quinaldine -- chapter 15 Molybdenum Enzymes in Reactions Involving Aldehydes and Acids -- chapter 16 Molybdenum and Tungsten Enzymes in C1 Metabolism -- chapter 17 Molybdenum Enzymes and Sulfur Metabolism -- chapter 18 Comparison of Selenium-Containing Molybdoenzymes -- chapter 19 Tungsten-Dependent Aldehyde Oxidoreductase: A New Family of Enzymes Containing the Pterin Cofactor -- chapter 20 Tungsten-Substituted Molybdenum Enzymes -- chapter 21 Molybdenum Metabolism and Requirements in Humans -- chapter 22 Metabolism and Toxicity of Tungsten in Humans and Animals.