Metal Ions in Biology

Metals occur in several forms in biology largely in environments that we would recognize as those of coordination compounds. Functionally the most advanced class met are the metal-containing enzymes, or biological catalysts. Around 2000 enzymes are known, and a large number are metalloenzymes with one or several metal ions present coordinated to donor groups that are part of a biopolymer. The metal ions in metalloenzymes and other Metallo biomolecules are commonly the lighter, more abundant and more reactive metals of the s or d block of the Periodic Table, but not exclusively. Rarer and heavier elements (such as molybdenum) can be used. The metal ion is also not present just by accident– it has a specific role. In many cases it is found coordinated at the active site in enzymes (i.e. the site of catalysis) where the chemical reaction catalysed by the enzyme occurs. 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 it should be remembered that s-block elements in particular, are also important; for example apart from its structural role in bones calcium has a key role in triggering muscle action and magnesium appears in several transferase phosphohydrase and polymerase enzymes. While metals are met usually in trace amounts, balance is important in biosystems - both a deficiency and an excess of metals can cause disease. Fortunately, a balanced diet provides sufficient amounts of essential metals.

Vanadium in extremely small amounts is a nutritional requirement for many organisms, including higher animals. Some marine organisms (tunicates) accumulate vanadium, whereas some lichens and fungi contain vanadium in the active site of some enzymes. Vanadium appears in one form of the important enzyme nitrogenase (which converts dinitrogen to ammonium ion), and as vanadium(V) in halo peroxidase (which coordinates hydrogen peroxide and then oxidizes halides).

Chromium as Cr(III) is an essential trace element in mammals, and participates in glucose and lipid metabolism. Chromium may help in maintaining normal glucose tolerance by regulating insulin action. There is no evidence of significant chromium (III) toxicity, but chromium (VI) and chromium(V) are toxic (mutagenic and carcinogenic). Cr(V) is believed to damage DNA by promoting cleavage and DNA-protein cross-linking. Manganese is an essential element and appears in a wide range of organisms including humans. It occurs in important enzymes and processes, such as in photosynthesis in plants. It can have a redox role, using particularly its Mn (II) and Mn (III) oxidation states, as in superoxide dismutase, whose role is to destroy undesirable superoxide ion through conversion to oxygen and peroxide ion.

Iron is truly ubiquitous in living systems, being found in all life forms from bacteria through to humans. It is at the active centre of molecules responsible for oxygen transport, for electron transport and in a vast range of enzymes. In the human body, hemoglobin and myoglobin are vital components for oxygen transport, and represent 65% and 6% re- spectively of all iron in the body. There is a wide range of other human iron-containing proteins, however, with mainly redox roles, making use of accessible oxidation states notably Fe(II) and Fe(III). The iron proteins tend to be classified as either hemes (featuring cyclic aromatic nitrogen ligands) or nonhemes. Iron is so significant that a special iron storage protein, ferritin, is used to allow rapid access to iron when required. Cobalt is an essential element in small amounts. Cobalt complexes of macrocyclic nitrogen- donor ligands are found in many organisms, including humans, who contain about 5 mg of these cobalamins. Vitamin B12 is a cobalamin coenzyme required in humans for its key role in promoting several molecular transformations. Recently, a range of enzymes containing inert cobalt(III) has been discovered, distinguished by their unusual low coordination number, which provides labile reaction sites despite the inherent inertness of this ion.

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 re- ductase 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 1 umber 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 to3gofzincinadults, 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.

Heavier elements can also appear in proteins. Molybdenum, found geologically together with iron, has found use in a range of enzymes. Its prime role seems to be to facilitate oxygen atom transfer to a substrate, via a MoVI=O species, leading to a MoIV compound and an oxygenated substrate. Enzymes with Mo present include aldehyde oxidase nitrate reductase, sulfite oxidase, xanthine oxidase formate dehydrogenase and DMSO reductase. Tungsten-containing enzymes are also known; in fact, Mo and W appear to f ind more roles in proteins than their triad parent chromium.

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