Abstract
The understanding of sulfur bonding is undergoing change. Old theories on hypervalency of sulfur and the nature of the chalcogen-chalcogen bond are now questioned. At the same time, there is a rapidly expanding literature on the effects of sulfur in regulating biological systems. The two fields are inter-related because the new understanding of the thiosulfoxide bond helps to explain the newfound roles of sulfur in biology. This review examines the nature of thiosulfoxide (sulfane, S0) sulfur, the history of its regulatory role, its generation in biological systems, and its functions in cells. The functions include synthesis of cofactors (molybdenum cofactor, iron-sulfur clusters), sulfuration of tRNA, modulation of enzyme activities, and regulating the redox environment by several mechanisms (including the enhancement of the reductive capacity of glutathione). A brief review of the analogous form of selenium suggests that the toxicity of selenium may be due to over-reduction caused by the powerful reductive activity of glutathione perselenide.
Highlights
Sulfur BondingSome long-held theories on sulfur bonding have been called into question by the use modern physico-chemical technology and enhanced computing ability
It should be noted that the proposed agents are less appropriate as “therapeutic” agents than are mercaptoethanol disulfide and cystamine, which have already been tested in animals [29,30,31]
The first two are “dedicated” sulfane sulfur carriers in which the sulfur atom is carried as a persulfide on a cysteine residue in a specific domain called the rhodanese homology domain (RHOD) [74]
Summary
Some long-held theories on sulfur bonding have been called into question by the use modern physico-chemical technology and enhanced computing ability. Chalcogen atoms (group 16 of the periodic table) have six electrons in the valence shell providing these atoms with special bonding possibilities. Carbon (group 14), with four valence electrons, catenates in three dimensional lattices, but sulfur, with six valence electrons, forms chains of atoms bonded by 2-electron dative bonds (Figure 1). It was thought that sulfur, unlike oxygen, was able to accommodate more than eight Lewis electrons in its valence shell. Some sulfur atoms in the structures are shown in the classical (4-electron) format but other bonds are shown as 2-electron bonds when the chemical and biological evidence supports this representation
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