REPLACEMENT OF SULFUR BY SELENIUM IN IRON-SULFUR PROTEINS

This chapter describes the replacement of sulfur by selenium in iron–sulfur proteins. By its chemical properties, selenium is most similar to sulfur and occurs in the same valence states: −2, 0, +2, +4, and +6. However, the two elements display noteworthy differences relevant to the biochemistry of selenium. Selenium tends to be more stable than sulfur in its intermediate oxidation states and less stable in the extreme ones. Accordingly, selenate and selenite are relatively easy to reduce to the element, and selenides are more reactive (reducing) than sulfides. Selenium is most often found in biological systems in compounds such as selenols, diselenides, and selenoethers, which are usually more reactive than their sulfur counterparts, because of the greater polarity and lower strength of the C–Se, N–Se and O–Se bonds. Selenols are more acidic (usually ionized at neutral pH), are better nucleophiles, better leaving groups, and are more reducing than the corresponding thiols. © 1992 Academic Press, Inc.