Selenium cystein

Formate dehydrogenases are a diverse group of enzymes found in both prokaryotes and eukaryotes, capable of converting formate to CO2. Formate dehydrogenases from anaerobic microorganisms are, in most cases, Mo- or W- containing iron-sulfur proteins and additionally flavin or hemes. Selenium cysteine is a Mo- ligand. 

Crystals from selenium-methionine- and/ or selenium-cysteine-labeled proteins can be studied by multi-wavelength anomalous dispersion (MAD) phasing techniques that can facilitate the solution of an X-ray crystal structure from a single crystal form [11]. However, if in vivo expression systems are used to prepare selenium-labeled proteins, amino acid metabolism and the toxicity of Se-methionine can result in low protein yields and low incorporation rates. 

Selenium is present in foodstuffs as the compounds selenium cysteine and selenium methionine. Approximately 80% of the organic selenium present in foodstuffs seems to be absorbed by the organism. Absorption is greater in foods of plant origin compared to foods of animal origin. ... 

Organosulfur Adsorbates on Metal and Semiconductor Surfaces. Sulfur compounds (qv) and selenium compounds (qv) have a strong affinity for transition metal surfaces (206—211). The number of reported surface-active organosulfur compounds that form monolayers on gold includes di- -alkyl sulfide (212,213), di- -alkyl disulfides (108), thiophenols (214,215), mercaptopyridines (216), mercaptoanilines (217), thiophenes (217), cysteines (218,219), xanthates (220), thiocarbaminates (220), thiocarbamates (221), thioureas (222), mercaptoimidazoles (223—225), and alkaneselenoles (226) (Fig. 11). However, the most studied, and probably most understood, SAM is that of alkanethiolates on Au(lll) surfaces. 

Although the oxoacids of selenium and sulfur have comparable acid strengths (pITai 2.6 vs. p fa 1-9, respectively, for the quadrivalent species pA a 3 for both the hexavalent species), the hydride H2Se is much more acidic than H2S (pATai 3.9 vs. 7.0). Thus, while thiols such as cysteine are mainly protonated at physiological pH, selenols such as selenocysteine are predominantly dissociated under the same conditions. 

The selenium species that are drawing most attention are Se(IV) and Se(VI) in water and sediments, and the biomethylated products (dimethylselenide and dimethyldi-selenide) that are spread into the environment (Camara et al. 1995). Se-species in food (including Se-cysteine and other species in yeast) are in the limelight (Crews 1998) because of their beneficial effect on human health and their increasing use as nutraceuticals. 

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. 

Plants of the genus Allium, e.g. garlic Allium sativum), leek Allium amp-eloprasum), and onion Allium cepa), produce a bewildering variety of sulfur compounds. Selenium analogs for some of these have also been found (Section 11.1.3.6). Much work has focused on garlic (contains more than 100 such materials) and onion.56,66,67 Key compounds for formation of the Allium sulfur-containing secondary metabolites are sulfoxides of cysteine derivatives,... 

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). 

All selenium-containing proteins and enzymes in animals, microorganisms and plants incorporate selenocysteine non-specifically105 or as part of Se-dependent antioxidant enzymes such as glutathione peroxidase, (EC 1.11.1.9) which has a Se-cysteine residue in its active site.116 120 An active form of Se, selenophosphate, is produced by selenophosphate synthetase in several bacteria. This active form is required for the production of Secys-tRNA, a precursor for Se-cysteine.121 In a similar vein, a Se-containing modified-tRNA nucleoside, 5-methylaminomethyl-2-selenouridine, encodes a selenouridine synthase which replaces sulfur in tRNA with selenium.122... 

Selenium (IV) adsorbed as selenotrisulfate was then eluted from the column with either 0.1 M penicillamine or 0.1M cysteine. The eluate was then subjected to an acid digestion procedure to reduce selenium to the tetravalent state with diaminonaphthalene for fluorometric determination. Approximate agreement with the tellurium coprecipitation method was obtained. The application of both methods to the analysis of estuarine waters permitted the separate determination of both selenium (IV) and selenium (VI), since the tellurium coprecipitation methods did not differentiate between the two species. 

Probably the most effective use of XRF and TXRF continues to be in the analysis of samples of biological origin. For instance, TXRF has been used without a significant amount of sample preparation to determine the metal cofactors in enzyme complexes [86]. The protein content in a number of enzymes has been deduced through a TXRF of the sulfur content of the component methionine and cysteine [87]. It was found that for enzymes with low molecular weights and minor amounts of buffer components that a reliable determination of sulfur was possible. In other works, TXRF was used to determine trace elements in serum and homogenized brain samples [88], selenium and other trace elements in serum and urine [89], lead in whole human blood [90], and the Zn/Cu ratio in serum as a means to aid cancer diagnosis [91]. 

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],... 

To prevent membrane damage, the red cell has an armoury of antioxidants, notably a manganese or selenium containing enzyme called superoxide dismutase (SOD) and the sulfydryl compound glutathione (GSH), a tripeptide of y-glutamate, cysteine and glycine. 

Another selenium-containing molybdenum hydroxylase that has been isolated from Clostridium barkeri (identical to Eubacterium barkeri) is nicotinic acid hydroxylase (NAH). Clostridium barkeri was isolated initially as a fermentor of nicotinic acid and thus NAH is a key enzyme in the efficient fermentation of nicotinic acid as a source of carbon and energy. NAH contained selenium when purified from cells labeled with Se-selenite. However, this label was lost during denaturing gel electrophoresis and also on heating of the enzyme (Dilworth 1982). Exhaustive analysis of selenium-labeled alkylation products of NAH under various conditions revealed selenium was bound as a labile cofactor (Dilworth 1982), and not as seleno-cysteine. This report was the first to describe a selenium-dependent enzyme that did not contain selenium in the form of selenocysteine. 

NAH is composed of four subunits (SDS-PAGE) and contains a molybdenum cofactor (Dilworth 1983). Analysis of the electron paramagnetic resonance (EPR) spectra of the molybdenum center of NAH revealed a coordination of molybdenum to selenium (Gladyshev et al. 1994b). Apparently NAH is much like other selenium-dependent molybdenum hydroxylases such as XDH from C. barkeri and other purinolytic Clostridia. Whether or not the selenium is present as a ligand of molybdenum or is coordinated to molybdenum while being bound to another molecule (e.g., sulfur of cysteine) is still not known. The nature of the selenium cofactor and the mechanism of its incorporation into NAH are most likely similar to XDH and thus also require more study. 

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