Metals in living systems

The metal ions of major biological significance are indicated in Figure 1, which shows part of the Periodic Table. Some information on the distribution and concentration levels of these metals in living systems is shown in Table 1. The transition metals and zinc are usually regarded as trace elements, as they are present in very small amounts. Of the transition elements, iron is the most abundant metal, and probably the most well studied. Iron is essential for all living systems with the exception of certain members of the lactic acid bacteria, which grow in environments notoriously low in iron, such as milk. Lactic acid bacteria are devoid of cytochromes, peroxidases... 

The most complicated speciation analysis is in plants and biological samples. Besides the compounds and ions already mentioned metals in living systems participate in a lot more complicated species. They may be bound to amino acids, proteins, peptides and the separation methods may influence their original distribution. For the determination of organo-arsenic, selenium, lead and tin hydride generation combined with different preconcentration steps may be used (Dedina and Tsalev, 1995). 

Bioinorganic Chemistry Finding Metals in Living Systems... 

Aromatic alkylations occur in numerous biological pathways, although there is of course no MCI3 present in living systems to catalyze the reaction. Instead, the carbocation electrophile is usually formed by dissociation of an organodiphosphate, as we saw in Section 11.6. The dissociation is typically assisted by complexation to a divalent metal cation such as Mg2+ to help neutralize charge. 

Because the low-lying spectroscopic transitions of these species commonly lie in the visible region of the spectrum, coordinated transition metals underlie the colors of many natural and synthetic dyestuffs. Coordinated transition metals are also conspicuous features of the active sites of metalloproteins and play an important role in living systems. 

Besides metal containing Lewis acids, non-metal additives have also found application in catalysis. These studies are quite pertinent to the development of artificial enzymelike catalysts. As there is a large number of Lewis basic sites in living systems able to be involved in hydrogen bonds, the analysis of the catalytic activity of hydrogen bonding additives would give some indication as to the existence of Diels-Alder reactions... 

Magnesium is perhaps the most versatile metal cation found in living systems. It can and does interact with an extremely wide variety of biomolecules, thus giving rise to multiple biological roles of fundamental importance in life processes. A comprehensive discussion of all biosystems that have an absolute dependency on Mg + as a cofactor would currently have to include hundreds of unrelated examples and more are being discovered all the time. 

Most biochemical reactions in living systems are catalyzed by enzymes - that is, biocatalysts - which includes proteins and, in some cases, cofactors and coenzymes such as vitamins, nucleotides, and metal cations. Enzyme-catalyzed reactions generally proceed via intermediates, for example. 

Looking at the literature in the field of biomineralization, one notices, that the majority of articles is descriptive in nature. On the basis of electron micrographs or thin section studies, the intricate relationships between mineral phase and organic matrix are investigated. Other papers deal with the chemical composition of the mineralized tissue and the minerals. Only a few authors address themselves to the question of metal ion transport mechanisms in cellular systems and the solid state principles involved in mineral deposition on organic substrates. All three sets of information, however, are essential to understand calcification processes. It appears, therefore, that information on the functionality of metal ions in living systems and their role in mineral deposition are particularly desired in this area of research. 

In living systems that use metal ions in several places, transport of metal ions is an important process, for which efficient systems are in operation. Well-known examples are transferrin for iron transport in humans, albumin for copper transport, and ferritin for iron storage. In addition to these natural transporting proteins, nature makes use of other systems to remove excess of toxic metal ions. 

---