Metal Ions in Biology

We shall begin a brief examination of metal complexes in biology with a short overview of metals from the first row of the periodic d block found in biomolecules, and follow this with more detail from selected examples. Although the focus below is on d-block elements, 

Nickel is an essential element in small amounts, is a component of the important enzymes urease, carbon monoxide dehydrogenase, hydrogenase and methyl-S-coenzyme M reductase, and lies at their active sites. Nickel can exist under physiological conditions in oxidation states I, II and III, but the higher two seem most relevant. 

Copper occurs in almost all life forms and it plays a role at the active site of a large number of enzymes. Copper is the third most abundant transition metal in the human body after iron and zinc. Enzymes of copper include superoxide dismutase, tyrosinase, nitrite reductase and cytochrome c oxidase. Most copper proteins and enzymes have roles as electron transfer agents and in redox reactions, as Cu(II) and Cu(I) are accessible. 

Zinc is recognized as essential to all forms of life, and is the most common transition metal in the body after iron. There are 2 to 3 g of zinc in adults, compared with 4 to 6 g of iron and 0.25 g of copper. Enzymes containing zinc include carbonic anhydrase and carboxypeptidase, the first two metalloenzymes detected - now there are over 300 zinc enzymes known. Zinc serves an important structural role in DNA binding proteins, stabilizing the correct binding site. Zinc reserves are stored in the metallothionine proteins. 

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H. Siegel, ed.. Metal Ions in Biological Systems, Vol. 12, Properties of Copper, Marcel Dekker, New York, 1981, p. 384. 

Tu, A. J., Heller, M. J. Structure and Stability of Metal-Nucleoside Phosphate Complexes, in Metal Ions in Biological Systems Vol. 1 (ed. Sigel, H.), p. 1, Marcel Dekker, Inc. New York 1974... 

Lewis Acid-Base Behavior in Aqueous Solution Some Implications for Metal Ions in Biology Robert D. Hancock and Arthur E. Martell... 

Petering DH (1973) In Sigel H (ed) Metal ions in biological systems, vol 11, Marcel Dekker, New York... 

Bodaly RA, St. Louis VL, Paterson MJ, Fudge RJP, Hall BD, Rosenberg DM, Rudd JWM. 1997. Bioaccumulation of mercury in the aquatic food chain in newly flooded areas. In Sigel A, Sigel H, editors, Metal ions in biological systems, Vol. 34 Mercury and its effects on environment and biology. New York (NY) Marcel Dekker Inc., p. 259-287. 

The nitrogen donor commonly found coordinated to metal ions in biology is the imidazole group from histidine. It is found coordinated... 

Yano, S. Otsuka, K. In Metal Ions in Biological Systems, Sigel, A. Sigel, H., Eds. Marcel Dekker New York, 1996 p 27. 

Sposito G., Page A.L. In Circulation of Metal Ions in the Environment Metal Ions in Biological Systems. New York Marcel Dekker, 1984. 

In the course of evolution, the importance of particular metal ions in biological systems has ebbed and flowed, as a function of environmental conditions. Before the arrival of photosynthesis, when there was no oxygen, elements like Fe and Ni were extremely important, whereas, for example, Cu was virtually inaccessible for reasons of solubility. With the arrival of an oxidizing environment, Ni virtually disappeared from the equation, Cu became bioavailable, and Fe, although it was now insoluble and poorly available, had proved of such fundamental importance in biological catalysis that specific systems were developed for its uptake from the environment, such that it continues to play a key role in life as we know it today. 

Fig. 2.7. Characteristic rate constants (s 1) for substitution of inner-sphere H20 of various aqua ions. Note The substitution rates of water in complexes ML(H20)m will also depend on the symmetry of the complex (adapted from Frey, C.M. and Stuehr, J. (1974). Kinetics of metal ion interactions with nucleotides and base free phosphates in H. Sigel (ed.), Metal ions in biological systems (Vol. 1). Marcel Dekker, New York, p. 69).

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