Preface to Volume 15 Sustaining Life on Planet Earth: Metalloenzymes Mastering Dioxygen and Other Chewy Gases

In this volume of the Metal Ions in Life Sciences series the mastering of dioxygen (O2), methane (CH4), and ammonia (NH3) by mainly manganese-, ironand copper-dependent metalloenzymes and their biomimetic complexes is discussed. It is closely related to Volume 14, The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment, which deals with the biogeochemistry of gases including dihydrogen (H2), carbon monoxide (CO), acetylene (HC CH), dinitrogen (N2), nitrous oxide (N2O), hydrogen sulfide (H2S), and dimethylsulfide (CH3-S-CH3). The accumulation of O2 in the atmosphere forever changed the surface chemistry of the Earth. Dioxygen, as electron acceptor, is used in the respiration of numerous different organisms that conduct a wide variety of chemically complex metabolisms. To produce O2, and to conserve energy by activating and transforming O2, CH4, or NH3, sophisticated metal-dependent enzymes had to be evolved by Nature. These catalysts can overcome unusually high activation barriers of kinetically inert molecules, still a tremendous challenge in the chemical laboratory today. In the first chapter, the reader is shortly introduced to several aspects and properties of this special molecule “dioxygen” and its extraordinary impact on our current Earth. Just think of water (H2O), perhaps the most important compound containing oxygen, a superb solvent for numerous biomolecules, and the main source of O2 in the atmosphere. Carl Zimmer reports in his article entitled “The Mystery of Earth’s Oxygen” (The New York Times, October 3, 2013) about the work of the geochemist D. E. Canfield from the University of Southern Denmark: “There’s something astonishing in every breath we take. What is even more astonishing is that the Earth started out with an oxygen-free atmosphere, it took billions of years before there was enough of it to keep animals like us alive”. Clearly, oxygen must be considered one of the most important elements on Earth,