Selenocysteine

A very interesting aspect of biological selenium compounds is the occurrence of selenocysteine in a protein component (selenoprotein A) of glycine reductase, in formate dehydrogenase and in glutathione peroxidase (see below). As pointed out by Stadt-man , the incorporation of this rather unusual amino acid into proteins in a very specific way poses something of a mystery and at present the mechanism by which this occurs is not known. 

---

Selenium. Selenium, thought to be widely distributed throughout body tissues, is present mostly as selenocysteine in selenoproteins or as selenomethionine (113,114). Animal experiments suggest that greater concentrations are in the kidney, Hver, and pancreas and lesser amounts are in the lungs, heart, spleen, skin, brain, and carcass (115). 

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. 

In addition to the twenty amino acids commonly found in proteins, two others—selenocysteine and pyrrolysine—are found in some organisms, and more than 700 nonprotein amino acids are also found in nature. y-Amino-butyric acid (GABA), for instance, is found in the brain and acts as a neurotransmitter homocysteine is found in blood and is linked to coronary heart disease and thyroxine is found in the thyroid gland, where it acts as a hormone. 

Se-p-D-Ribopyranosyl-D-selenocysteine or (S)-2-amino-2-carboxyethyl 1-seleno-p-D-ribopyranoside or 3-(P-D-ribopyranosylseleno)-D-alanine... 

Figure 28-12. Selenocysteine (top) and the reaction catalyzed by selenophosphate synthetase (bottom).

Selenocysteine, an essential active site residue in several mammahan enzymes, arises by co-translational insertion of a previously modified tRNA. 

Nordberg J et ai Mammalian thioredoxin reductase is irreversibly inhibited by dinitrohaiobenzenes by alkylation of both the redox active selenocysteine and its neighboring cysteine residue.) Biol Chem 1998 273 10835. 

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. 

Boyington JC, VN Gladyshev, SV Khangulov, TC Stadtman, PD Sun (1997) Crystal structure of formate dehydrogenase H catalysis involving Mo, molybdopterin, selenocysteine, and an Fe4S4 cluster. Science 275 1305-1308. 

Glutathione-peroxidase (GSH-Pxase) is an enzyme found in erythroqrtes and other tissues that has an essential selenocysteine residue involved in the catalytic decomposition of reactive oxygen species. In the erythrocyte, hydrogen peroxide is the principle reactive oxygen species available. 

The final detailed example concerns the NifS CsdB protein. This enzyme belongs to a pyridoxal S -phosphalc (PLP)-dependent family of enzymes [75, 77], These enzymes react with L-cysteine and L-selenocysteine to generate L-alanine and... 

Nickel-selenium coordination compounds have received attention in recent years, because a unique Ni-selenocysteine interaction was revealed in the active site of [FeNiSe]-hydrogenases.1083,1084 Of particular interest in this regard are mixed CO/selenolate complexes. Distorted square planar (393) was prepared from [CpNi(CO)]2, PhSeSePh, and [Fe(CO)3(SePh)3] and provides the first example of CO bound to a square planar Ni11 center in thiolate/selenolate environment.1085,1086 Upon addition of RSSR, species of the series [Ni(CO)(SR) (SePh)3 ] are formed. //(CO) ranges from 2,023 cm-1 to 2,043 cm-1 and is regarded as a spectroscopic reference for the CO binding site in [NiFeSe] hydrogenases. 

A number of zinc selenium complexes have now been characterized, with particular interest in the formation of zinc selenide semiconductors and quantum dots. In many cases analogous structures to those observed with thiol or thiolates are recorded. 77Se NMR is frequently used in characterization, and comparison with the sulfur equivalent is relevant. Zinc selenium compounds are of particular interest as precursors for metal/selenide materials and their relevance as models for selenocysteine-containing metalloproteins. 

In the structure of the [NiFeSe] enzyme from Desulfomicrobium baculatum, one of the four cysteines involved in Ni coordination, one sulfur cysteine is replaced by a selenocysteine (Scheme 77b).409 419... 

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... 

Selenomethionine group had altered immune function, altered serum enzyme activities, and elevated concentrations of selenium in liver (4 times control values) and breast muscle (14 times). Sodium selenite-treated birds had normal immune function and selenium tissue burdens however, serum enzyme activity was disrupted in the 3.5 mg/L group Adults normal. Impaired reproduction (reduced survival of ducklings, increased developmental abnormalities) for selenomethionine occurs between 4 and 8 mg/kg ration selenocysteine did not impair reproduction at 16 mg Se/kg ration... 

Clostridium stricklandii, selenocysteine produced by, 22 89 Clostridium thermosaccharolyticum,... 

---