## 2 Metallomics  
The field of inorganic biochemistry, a.k.a. bioinorganic chemistry or biological inorganic chemistry, has attempted to address some of the deficiencies resulting from the artificial dichotomy of separating organic and inorganic chemistry with regard to biochemistry. Quite a bit of excitement has been generated about the coordination environments of metal ions in biomolecules, which in many cases revealed an unprecedented and \"elegant\" chemistry that had millions of years to evolve for specific biological functions, such as enzymatic catalysis. It has stimulated investigations into biomimetic chemistry and synthetic biology by learning from and applying the principles embodied by inorganic biochemistry. However, for the most part, these bioinorganic advances developed almost independently of mainstream biochemistry and never fully bridged the gap to it. In recent years, with the development of analytical omics-based tools for a global profiling of metabolites and macromolecules within biological matrices, the term \"metallomics\" has been introduced to reflect the technical capacity to measure multiple biometals and metalloprotein within cells and tissues simultaneously and with a remarkable spatial, and sometimes temporal, resolution with methods for metal imaging [3]. Metallomics, in parallel to and in combination with genomics, proteomics and metabolomics, provides new tools and approaches to fully integrate bioinorganic chemistry into the life and applied sciences, bridging the gap that has developed between these disciplines. Without specific mention of metallomics as an emerging discipline, an article focusing on the chemistry of iron exchange among proteins in the biological milieu emphasized the necessity of merging rather than juxtaposing disciplines with the expected outcome of a higher level of understanding of many important questions in the basic and applied biosciences [4]. Quantity vs. Quality of the Chemical Elements, Homeostatic Control and Interactions among the Chemical Elements in Life Processes  
One factor that has led to the present underappreciation of some biometals is confusing quantity with quality and thus treating chemical elements that are essential at lower quantities, referred to as trace elements, as less important than the bulk non-metal elements. Historically, the term \"trace elements\" is rooted in the early work in the field when analytical methods were inadequate to determine their concentrations precisely [5]. In addition, rather minute quantities are needed on a daily basis. For example, while the human body has about 2-3 g of zinc, the daily requirement is only one thousandth of this amount [6]. Moreover, the cellular concentrations of zinc are in the range of hundreds of micromolar, in the order of the concentrations of ATP, which would hardly qualify as a trace [7]. There is a wide spectrum of elemental concentrations within a typical human, from 43 kg oxygen to 3 mg cobalt, a difference of seven orders of magnitude. Yet, although only two human enzymes require it as part of the vitamin B\({}_{12}\) prosthetic group, a corrin, and the recommended daily allowance of vitamin B\({}_{12}\) is only 2.4 \(\upmu\)g, cobalt is as essential as oxygen. Additional proteins are needed for handling the corrin, the classic example being the intrinsic factor, a glycoprotein synthesized in the stomach and responsible for the uptake of vitamin B\({}_{12}\) in the gut [8]. Its deficiency leads to pernicious anaemia, demonstrating the importance of the minute amounts of cobalt for handling the much larger amounts of iron with three orders of magnitude difference in concentrations; however, there are yet other proteins, e.g., transcobalamins, that are responsible for the transport and delivery of vitamin B\({}_{12}\). Thus at least three ancillary proteins are necessary, illustrating a critically important fact, namely that not only the element itself (the extrinsic factor) but also its homeostatic control in biological space and time (intrinsic factors) are important.