## 6 Metal Buffering, Muffling and Hormonal Control  
A fundamental fact is that each metal ion must be controlled and buffered in a limited range of concentrations in order to avoid interference with other metal ions, which could lead to mismetalation when putting the wrong metal ion into a functional site of a protein [38]. The underlying principle for this control is the affinity of the metal ion to its ligands, with alkali and alkaline earth metal ions of the s-block in the periodic table having low affinities and metal ions of the d-block having higher affinities that follow the Irving-Williams series [39]. In general, the higher the affinity to the protein the lower the concentration of non-protein bound metal ions. The principle of metal ion buffering is analogous to hydrogen ion/proton (pH) buffering. In the case of metal ion buffering, the metal ion concentration is related to the affinity of the complex for the metal ion and the ratio of metal ion-bound ligand to free ligand. The metal buffering establishes a range of free metal ions covering well over 14 orders of magnitude from millimolar concentrations of alkali metal ions to attomolar concentrations of cuprous (Cu(I)) ions. Thus, in contrast to pH, where the scale of 14 orders of magnitude (pH 0-14) is not realized in biology, a large scale of concentrations of different metal ions indeed is realized in biology and allows the control of biological processes by employing different metal ions. Since buffering is a thermodynamic concept and kinetic factors are also important in controlling biological metal ions, the concept of muffling was introduced [40; 41]. Membrane transporters increase or decrease metal ion concentrations and are kinetic factors involved in muffling. They also underlie biological regulation. In addition to transcriptional regulation, membrane metal transporter proteins are endocytosed, recycled and eventually escorted to lysosomes or extracted from membranes for degradation [42].  
In addition to the cellular and subcellular control of metal ions and its dynamics there are often systemic and hormonal control at several levels, in the blood, organs and cells. Control of Ca\({}^{2+}\) by the peptide hormones, parathyroid hormone and calcitonin, and the ensuing generation of vitamin D metabolites serving as hormones, is well established and so now the peptide hepcidin serves as a hormone in the regulation of iron metabolism. Hepcidin interacts with the iron exporter, ferroportin, to decrease the release of iron from cells [9; 43].  
Environmental factors (diet, drugs, pollutants) can perturb metal metabolism and result in deficiency, overload or changed distribution of metal ions. This dyshomeostasis has been found to be associated with neurodegeneration and chronic diseases, such as cancer and diabetes, for some metal ions. One functional component in the potential to cause disease is the redox activity of some metal ions and the indirect effects of other, redox-inert metal ions such as calcium and zinc on redox homeostasis and redox stress [29]. Degenerative, metabolic and infectious/communicable diseases are all connected to the dyshomeostasis of metal metabolism, often in a causal rather than an effectual way. Zinc, in particular, is critically important for innate and adaptive immunity and hence adequacy and proper control are major factors for maintaining optimal immunity and regulating inflammation [44]. Zinc, therefore, has become a focus in the susceptibility to viral infections and their progression, as further evident in the present discussions of its role in the COVID-19 pandemic and in therapeutic approaches to the disease, though some of the underlying observations had been made many years ago [45; 46].