Protein selenium-containing, table

In all forms the selenium occurs in a position that usually has sulfur, a result that is not too surprising given their relative positions in the periodic table. In most cases, however, the reason for the replacement is unknown, although we speculate below about the function of selenocysteine in two of the enzymes. The role of selenium in those proteins that contain selenomethionine is particularly perplexing because the selenomethionine is not present in stoichiometric amounts at one particular position in the amino acid sequence rather it is distributed throughout several methionine sites . ... 

Ionic, zwitterionic, and neutral selenium species are present in natural systems. Table 2 shows selenium compounds of biological and clinical interest and species in living organisms are listed in Table 3. Selenoproteins contain selenocysteinyl residues but proteins that contain selenomethionyl residues are not formally classified as selenoproteins. 

Table 4. y-Glutamyl Derivatives of Non-Protein, Sulphur- or Selenium-Containing Amino Acids, Identified in Plants... 

Selenocysteine was identified in 1976 (57) in a protein produced by Clostridium stricklandii, and it is thought to be the form in which selenium is incorporated, stoichiometricaHy, into proteins. Studies with rats show that over 80% of the dietary selenium given them is incorporated into proteins, thus selenocysteine takes on metaboHc importance. Selenoproteins having known enzymatic activities contain selenocysteine at the active sites. Two other forms of metabohc selenium are recognized methylated selenium compounds are synthesized for excretion, and selenium is incorporated into some transfer ribonucleic acids (tRNAs) in cultured cells (58). Some of the more important seleno-compounds are Hsted in Table 4. Examples of simple ring compounds are shown in Eigure 4. 

Formate dehydrogenases from many bacteria contain molybdopterin and also often selenium (Table 15-4).664/665 A membrane-bound Mo-containing formate dehydrogenase is produced by E. coli grown anaerobically in the presence of nitrate. Under these circumstances it is coupled to nitrate reductase via an electron-transport chain in the membranes which permits oxidation of formate by nitrate (Eq. 18-26). This enzyme is also a multisubunit protein.665 666 Two other Mo- and Se- containing formate dehydrogenases are produced... 

Selenium plays a special role in development and protection of spermatozoa (Chapter 15). Tire selenoprotein phospholipid hydroperoxide glutathione peroxidase (PHGPx Eq. 15-58, Table 15-4) has a high activity in the testis and in spermatids. However, in mature spermatozoa it forms an enzymatically inactive oxidatively crosslinked capsular material around the midpiece of the cell perhaps providing mechanical stability.268 A similar 34-kDa selenoprotein is present in sperm nuclei and may be essential for condensation of DNA.269 Sperm tails contain specialized cytoskele-tal proteins which form "outer dense fibers."270 In contrast to mammalian spermatozoa, nematode sperm move by ameboid motility that depends upon a specialized actin-like molecule.271 Sperm cells are unusually rich in polyamines, most of which are bound to RNA and DNA (Chapter 24). 

The U. S. Department of Agriculture maintains the USDA National Nutrient Database for Standard Reference, which contains over 7000 food items with data on the energy content, minerals, vitamins, and other properties of nutritional interest. The table here includes about 600 common foods extracted from that database. The properties listed are the energy content (in effect, the enthalpy of combustion) the content of carbohydrates, proteins, and lipids (fats) the cholesterol content and the amount of sodium, potassium, calcium, magnesium, iron, copper, zinc, manganese, phosphorus, and selenium. AU values are given for a 100 gram sample of the food. 

Selenium is used by animals primarily in cellular antioxidant systems and also a few specialized enzymes. Most plants contain no metabolic systems that include selenium. But remember the Periodic Table Elements in the same column of this table have similar chemical characteristics. Look at selenium. Directly above selenium in the Periodic Table, in the same column, is sulfur. Sulfur, of course, is an integral atom of the two essential amino acids methionine and cysteine, and plants require relatively large amounts of sulfur. Since selenium chemically resembles sulfur, plants don t differentiate them very well. Plants will absorb selenium from the soil along with sulfur and incorporate it into amino acids to form selenomethionine and selenocysteine. The plants then insert these amino acids into various proteins instead of the regular sulfur-containing amino acids. Although those proteins may not function as well, these aberrant amino acids do no real harm because their levels are so low. 

Pyl is rare, occurring only in some ancient bacteria. Sec, containing the side chain CH2SeH, is widespread in fact, it occurs in at least two dozen human proteins that rely on the reactivity of the essential trace element selenium for their function. The pAT, for the SeH group in Sec is 5.2. (For comparison, the pATa for the SH in Cys is 8.2.) (a) Draw the structure of Sec at pH 7 compare it with that of Cys at the same pH. (b) Determine p/ for Sec. (c) Given the relationship between sulfur and selenium in the periodic table, and the pATa differences between SH and SeH, how do you expect the chemical reactivities of Sec and Cys to compare ... 

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