Amino acids selenium

There is also a significant occurrence of nonprotein D-amino acids in nature.These amino acids have attractive structural variations and remarkable biological properties. However, this chapter will not include details on nonprotein D-amino acids. Selenium-containing amino acids, such as selenocysteine and selenomethionine, are also considered as unusual amino acids. Recently, much interest in selenium-containing amino acids has emerged. More information about selenium-containing amino acids is available in Chapter 5.05. 

Also reported as commercial products are selenium amino acid chelate, selenium ascorbate, selenium aspartate, and selenium citrate triturations, as well as selenium dibromide, selenium dichloride, selenium lysinate, and selenium sulfide bentonite. 

Although it had been assumed that only hypoxanthine dehydrogenase is required for the conversion of hypoxanthine (6-hydroxypurine) into uric acid, in Clostridium purinolyti-cum, two enzymes, both of which contain a selenium cofactor, are required. The enzymes differ in the molecular mass of their subunits, in their terminal amino acid sequences, in their kinetic parameters, and in their specific activities for purines (Self and Stadman 2000). Purine hydroxylase converts purine into hypoxanthine and xanthine (2,6-dihy-droxypurine), which is then further hydroxylated to uric acid (2,6,8-trihydroxypurine) by xanthine dehydrogenase (Self 2002). 

Forman, H.J., Rotman, E.I. and Fisher, A.B. (1983). Roles of selenium and sulfur-containing amino acids in protection against oxygen toxicity. Lab. Invest. 49, 148-153. 

The building blocks of proteins are the alpha-amino acids, and exclusively those with the L-configuration. There are 20 that occur in na- ture. They too all consist of the four elements C, H, N, and 0 two amino acids additionally contain sulfur (cysteine and methionine). In certain, but vital, enzymes (the peroxidases), sulfur is replaced by selenium. 

Although it is toxic in large doses, selenium is an essential micronutrient in all known forms of life. It is a component of the unusual amino acids selenocys-teine and selenomethionine. In humans, selenium is a trace element nutrient. 

The addition of ammonia to the variety of acids derivable from either the breakdown of glucose, glycolysis, or of the pentose shunt reaction products, ribose and NADPH, and from the citrate cycle, gives the amino acids (see Table 4.7 and Figure 4.4) Polymerisation of amino acids in cells gives proteins. In some of the amino acids sulfur and selenium can be incorporated easily. We assume NH3 was present. (Note that Se is in a coded amino acid not in Table 4.7.) Some selective metal-binding properties can be seen in Table 4.7, but amino acid carboxylates can bind all. 

Before going on to describe the functions of the metals we observe that among heavier non-metals only selenium, chlorine and other halogens need any further comment. Selenium is found in some hydrogenases in even the most primitive life forms and may be it was required initially since it is a more effective catalytic centre than sulfur although much less available. (Compare tungsten with molybdenum later.) Its amino acid selenomethionine is coded in early DNA Later it is involved... 

In evolution parts of the code, even parts essential for independent functioning, have been lost so that higher organisms are dependent on vitamins, for example for coenzymes, and on amino acids, lipids and saccharides, or even on selenium incorporation, from other forms of life. This is selective loss in an environment of supportive organisms. Can this be random ... 

The importance of the selenium-analog of cysteine, selenocysteine (Se-cysteine), HSeCH2CHNH2COOH, and its incorporation into protein via a ribosomal mechanism has earned it the label of the 21st amino acid.112 115 Assuming L configuration at the a carbon, Se-cysteine is represented by 49, R=H (Scheme 17). 

El-Begearmi, M.M., H.E. Ganther, and M.L. Sunde. 1982. Dietary interaction between methylmercury, selenium, arsenic, and sulfur amino acids in Japanese quail. Poult. Sci. 61 272-279. 

Therefore the problem of searching retardants for these chain reactions of free radicals is critical. For instance, it is known that sulfur-containing amino acid (cysteine) attracts unpaired electrons of protein [2,3], Similar properties are reported about selenium, the element of the same subgroup Vl-a of the System as sulfur [4],... 

The vast majority of research focused on selenium in biology (primarily in the fields of molecular biology, cell biology, and biochemistry) over the past 20 years has centered on identification and characterization of specific selenoproteins, or proteins that contain selenium in the form of selenocysteine. In addition, studies to determine the unique machinery necessary for incorporation of a nonstandard amino acid (L-selenocysteine) during translation also have been central to our understanding of how cells can utilize this metalloid. This process has been studied in bacterial models (primarily Escherichia colt) and more recently in mammals in vitro cell culture and animal models). In this work, we will review the biosynthesis of selenoproteins in bacterial systems, and only briefly review what is currently known about parallel pathways in mammals, since a comprehensive review in this area has been recently published. Moreover, we summarize the global picture of the nonspecific and specific use of selenium from a broader perspective, one that includes lesser known pathways for selenium utilization into modified nucleosides in tRNA and a labile selenium cofactor. We also review recent research on newly identified mammalian selenoproteins and discuss their role in mammalian cell biology. 

Once identified as a selenoprotein in this model (C. sticklandii), the need for selenium was also shown for C. sporogenes The addition of selenium to the culture medium was reported to improve the level of D-proline reductase activity as early as 1976, ° yet the first identification of the selenoprotein component of this enzyme did not occur until more recentiy in 1999 by Andreesen s group. It is quite clear now from data from these model systems, as well as from DNA sequence analysis of the grd aiiAprd operons, ° ° that Stickland reactions are common to many amino acid-fermenting clostridia. Those that are capable of proline reduction all... 

Table 12.1. The genetic code presented in this table is very nearly universal. There are isolated exceptions in the genome of mitochondria, which is described later in this chapter. Beyond that, the genetic code has been expanded to include codons for two unusual amino acids that occur in a modest number of proteins. These amino acids are selenomethionine, in which an atom of selenium replaces the sulfur atom of methionine, and pyrrolysine, a cychzed form of lysine. For details, see A. Ambrogelly, S. Palioura, and D. Soil, Nat Chem Biol 3 29-35 (2007). 

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