ReviewBiologically active compounds of semi-metals
Graphical abstract
The present state of knowledge in the chemistry of organic compounds of semi-metals from different organisms is reviewed. They include, e.g., the boron-containing antibiotics or the silicon compounds present in “silicate” bacteria. Arsenic is incorporated into arsenosugars, arsenobetaines or volatile methylated arsenic compounds, selenium is incorporated into selenocysteine that is found in some proteins. Other metalloids, i.e. the rare and toxic tellurium and the radioactive short-lived astatine, have no biological significance.
Introduction
In the periodic table of elements, semi-metals (metalloids) are found along the line that distinguishes metals from nonmetals. Together with metals and nonmetals, metalloids form one of the three categories of chemical elements as classified by ionization and bonding properties (Rochow, 1966, Venugopal and Luckey, 1978). They have some of the qualities of both metals and nonmetals. Unlike the true metals, metalloids are usually semiconductors rather than conductors and have therefore received intensive attention from the computer and electronics industries. Their relative abundance in the environment is summarized in Table 1.
When involved in chemical bonding, the metalloids again exhibit intermediate qualities. They are capable of taking electrons from most metals and will readily lose electrons to most nonmetals. Their electronegativity values are also mid-range. They usually establish covalent bonding, ionic bonding being rather rare.
The reactivity of the metalloids depends on the counter-element. Boron acts as a nonmetal when reacting with sodium yet as a metal when reacting with fluorine. In organic compounds, they can substitute other elements such as sulfur. Boron and arsenic form compounds with lipids, sugars, phenols, organic acids and some polymers. Organic arsenic compounds are components of the food chains of many organisms including humans and may play important biological roles in them. Selenium is found in living organisms in its soluble inorganic forms and as protein-bound and free seleno-amino acids and volatile organoselenium compounds. Most small organic selenium compounds in living cells are isologues of sulfur amino acids or their derivatives. The interactions of silicon with living matter are vitally important for some bacteria, which release silicon from aluminosilicates through the secretion of organic acids, or for unicellular algae – diatoms for which it serves as the basic building block of their bodies. Tellurium is highly toxic and is not thought to be required by biological systems; the highly radioactive astatine is so rare in nature that the existence of biological astatine compounds is highly improbable.
Section snippets
Boron
Boron is a ubiquitous element in rocks, soil, and water, its average concentration ranging from ∼1 mg/kg in fresh water to ∼100 mg/kg in rocks (Steinberg, 1964).
Tetrahedral borate or boronate complexes have been shown to be involved in enzyme inhibition. Serine proteases were proposed to be inhibited by boric acid (Antonov et al., 1968), and simple borates have been patented as protease stabilizers in liquid detergent formulations (Hora and Kivits, 1981, Severson, 1985).
Serine hydrolase enzymes
Silicon
Silicon, tetravalent metalloid, is less reactive than its chemical analogue carbon. It is the second most abundant element in the Earth’s crust, making up 25.7% of it by weight. Elemental silicon is not found in nature. It occurs mainly in the form of silicon dioxide – silica – and silicates that contain silicon, oxygen, and metals. Amethyst, agate, quartz, rock crystal, flint, jasper, and opal are some of the forms in which the oxide appears. Granite, asbestos, feldspar, clay, hornblende, and
Arsenic
Arsenic, the twentieth most abundant element in the Earth’s crust, is widely distributed in the environment, mainly associated with sulfide minerals. Its content in soils is 0.5–35 mg/kg (Yan-Chu, 1994, Walsh and Keeney, 1975, Leonard, 1991). The most important anthropogenic arsenic sources are the smelting of Cu, Ni, Pb, and Zn ores and the burning of fossil fuels in households and power plants. Coal burning causes the emission of arsenic by volatilization of As4O6, which condenses in the flue
Selenium
The relative proportion of selenium in the environment is very low. In the Earth’s crust, selenium is present in a concentration of 0.05–0.09 mg/kg. Its level in sea water is usually 0.45 μg/kg, in stream water 0.2 μg/kg. In compounds, selenium is present as Se2−, Se2+, Se4+, and Se6+. In general, it is present in the environment in elemental form or in the form of selenide (Se2−), selenate , or selenite . In soils, the identity and amounts of the various oxidation state species depend
Tellurium
The principal source of tellurium is the anode sludges produced during the electrolytic refining of blister copper. It is also a component of dusts from blast furnace refining of lead. Tellurium is a relatively rare element that has no significant biological role. Tellurium and tellurium compounds are considered to be toxic and need to be handled with care. Organotellurium compounds damage cells, e.g., by oxidizing sulfhydryl groups and depleting endogenous reduced glutathione in a variety of
Astatine
It has been estimated that the whole Earth’s crust contains less than 44 mg astatine and this element can thus be considered one of the rarest naturally occurring elements on Earth. All isotopes of this radioactive element have short half-lives and are products of several radioactive decay series. Because of the very low half-life of its isotopes, the radioactive astatine has no significance for living systems.
Acknowledgement
The review was written within the frames of the Institutional Research Concept AV0Z50200510.
Tomáš Řezanka was born in Prague, graduated from the Charles University, Prague, Czech Republic in 1979. In 1980, he begun to work in the Institute of Microbiology, and he obtained the Ph.D. degree in Microbiology in 1984. His main area of research is the study of the chemistry of natural products from unusual sources where he focuses on the structure elucidation via physical methods, especially NMR and MS, and other chemical–physical methods.
He has published widely on microbial lipids,
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Tomáš Řezanka was born in Prague, graduated from the Charles University, Prague, Czech Republic in 1979. In 1980, he begun to work in the Institute of Microbiology, and he obtained the Ph.D. degree in Microbiology in 1984. His main area of research is the study of the chemistry of natural products from unusual sources where he focuses on the structure elucidation via physical methods, especially NMR and MS, and other chemical–physical methods.
He has published widely on microbial lipids, including very long chain fatty acids from blue-green algae and he is the author and co-author of more than 180 scientific papers, including six books. He is currently a Senior Principal Scientist in the Institute of Microbiology where he continues with the endeavors into the Chemistry of Natural Products.
Karel Sigler graduated from the Czech Technical University in Prague, Czech Republic, in 1964. Though he majored in nuclear chemistry, his interests took him to biological sciences. In 1965, he joined the Institute of Microbiology, Czechoslovak Academy of Sciences, obtaining the Ph.D. degree in biology in 1970 and the D.Sc. degree in biological sciences in 1990. His main area of research has been cell biology and biophysics, with special reference to bioenergetics and membrane phenomena. His interest in xenobiotics and other substances affecting cellular processes brought him to the study of natural products and their impact on living organisms. He has published widely on membrane transport phenomena in different types of cells (plants, animals, yeast) and on the effect of chemical and other stressors on cellular processes. He is the author and co-author of more than 130 scientific papers, 1 patent and 1 book. He is currently a Senior Scientist at the Institute of Microbiology where he pursues his biophysical studies and has started exploring the realm of chemistry and biological effects of natural products.