Purification and characterization of selenocysteine beta-lyase from Citrobacter freundii

The selenophosphate synthetase family: A review

Selenophosphate synthetases use selenium and ATP to synthesize selenophosphate. This is required for biological utilization of selenium, most notably for the synthesis of the non-canonical amino acid selenocysteine (Sec). Therefore, selenophosphate synthetases underlie all functions of selenoproteins, which include redox homeostasis, protein quality control, hormone regulation, metabolism, and many others. This protein family comprises two groups, SelD/SPS2 and SPS1. The SelD/SPS2 group represent true selenophosphate synthetases, enzymes central to selenium metabolism which are present in all Sec-utilizing organisms across the tree of life. Notably, many SelD/SPS2 proteins contain Sec as catalytic residue in their N-terminal flexible selenium-binding loop, while others replace it with cysteine (Cys). The SPS1 group comprises proteins originated through gene duplications of SelD/SPS2 in metazoa in which the Sec/Cys-dependent catalysis was disrupted. SPS1 proteins do not synthesize selenophosphate and are not required for Sec synthesis. They have essential regulatory functions related to redox homeostasis and pyridoxal phosphate, which affect signaling pathways for growth and differentiation. In this review, we summarize the knowledge about the selenophosphate synthetase family acquired through decades of research, encompassing their structure, mechanism, function, and evolution.

First enzymological characterization of selenocysteine β-lyase from a lactic acid bacterium, Leuconostoc mesenteroides

We succeeded in expressing selenocysteine β-lyase (SCL) from a lactic acid bacterium, Leuconostoc mesenteroides LK-151 (Lm-SCL), in the soluble fractions of Escherichia coli Rosetta (DE3) using a novel expression vector of pET21malb constructed by ourselves that has both maltose binding protein (MBP)- and 6 × His-tag. Lm-SCL acted on L-selenocysteine, L-cysteine, and L-cysteine sulfinic acid but showed a high preference for L-selenocysteine. The k_cat and k_cat/K_m values of Lm-SCL were determined to be 108 (min^-1) and 42.0 (min^-1・mM^-1), respectively, and this was enough catalytic efficiency to suggest that Lm-SCL might also be involved in supplying elemental selenium from L-selenocysteine to selenoproteins like other SCLs. The optimum temperature and optimum pH of Lm-SCL were determined to be 37 °C and pH 6.5, respectively. Lm-SCL was stable at 37-45 °C and pH 6.5-7.5. Lm-SCL was completely inhibited by the addition of hydroxylamine, semicarbazide, and iodoacetic acid. The enzyme activity of Lm-SCL was decreased in the presence of various metal ions, especially Cu²⁺. The quaternary structure of Lm-SCL is a homodimer with a subunit molecular mass of 47.5 kDa. The similarity of the primary structure of Lm-SCL to other SCLs from Citrobacter freundii, Escherichia coli, humans, or mouse was calculated to be 47.0, 48.0, 12.5, or 24.0%, respectively. Unlike Ec-SCL, our mutational and molecular docking simulation studies revealed that C362 of Lm-SCL might also catalyze the deselenation of L-selenocysteine in addition to the desulfuration of L-cysteine.

The physiology and evolution of microbial selenium metabolism

Selenium is an essential trace element whose compounds are widely metabolized by organisms from all three domains of life. Moreover, phylogenetic evidence indicates that selenium species, along with iron, molybdenum, tungsten, and nickel, were metabolized by the last universal common ancestor of all cellular lineages, primarily for the synthesis of the 21st amino acid selenocysteine. Thus, selenium metabolism is both environmentally ubiquitous and a physiological adaptation of primordial life. Selenium metabolic reactions comprise reductive transformations both for assimilation into macromolecules and dissimilatory reduction of selenium oxyanions and elemental selenium during anaerobic respiration. This review offers a comprehensive overview of the physiology and evolution of both assimilatory and dissimilatory selenium metabolism in bacteria and archaea, highlighting mechanisms of selenium respiration. This includes a thorough discussion of our current knowledge of the physiology of selenocysteine synthesis and incorporation into proteins in bacteria obtained from structural biology. Additionally, this is the first comprehensive discussion in a review of the incorporation of selenium into the tRNA nucleoside 5-methylaminomethyl-2-selenouridine and as an inorganic cofactor in certain molybdenum hydroxylase enzymes. Throughout, conserved mechanisms and derived features of selenium metabolism in both domains are emphasized and discussed within the context of the global selenium biogeochemical cycle.

Exploring the selenium-over-sulfur substrate specificity and kinetics of a bacterial selenocysteine lyase

Selenium is a vital micronutrient in many organisms. While traces are required for microbial utilization, excess amounts are toxic; thus, selenium can be regarded as a biological double-edged sword. Selenium is chemically similar to the essential element sulfur, but curiously, evolution has selected the former over the latter for a subset of oxidoreductases. Enzymes involved in sulfur metabolism are less discriminate in terms of preventing selenium incorporation; however, its specific incorporation into selenoproteins reveals a highly discriminate process that is not completely understood. We have identified SclA, a NifS-like protein in the nosocomial pathogen, Enterococcus faecalis, and characterized its enzymatic activity and specificity for l-selenocysteine over l-cysteine. It is known that Asp-146 is required for selenocysteine specificity in the human selenocysteine lyase. Thus, using computational biology, we compared the bacterial and mammalian enzymes and identified His-100, an Asp-146 ortholog in SclA, and generated site-directed mutants in order to study the residue's potential role in the l-selenocysteine discrimination mechanism. The proteins were overexpressed, purified, and characterized for their biochemical properties. All mutants exhibited varying Michaelis-Menten behavior towards l-selenocysteine, but His-100 was not found to be essential for this activity. Additionally, l-cysteine acted as a competitive inhibitor of all enzymes with higher affinity than l-selenocysteine. Finally, we discovered that SclA exhibited low activity with l-cysteine as a poor substrate regardless of mutations. We conclude that His-100 is not required for l-selenocysteine specificity, underscoring the inherent differences in discriminatory mechanisms between bacterial NifS-like proteins and mammalian selenocysteine lyases.

Biological Chemistry of Hydrogen Selenide

There are no two main-group elements that exhibit more similar physical and chemical properties than sulfur and selenium. Nonetheless, Nature has deemed both essential for life and has found a way to exploit the subtle unique properties of selenium to include it in biochemistry despite its congener sulfur being 10,000 times more abundant. Selenium is more easily oxidized and it is kinetically more labile, so all selenium compounds could be considered to be "Reactive Selenium Compounds" relative to their sulfur analogues. What is furthermore remarkable is that one of the most reactive forms of selenium, hydrogen selenide (HSe- at physiologic pH), is proposed to be the starting point for the biosynthesis of selenium-containing molecules. This review contrasts the chemical properties of sulfur and selenium and critically assesses the role of hydrogen selenide in biological chemistry.

Selenocysteine Lyase

Selenocysteine is a naturally occurring analog of cysteine in which the sulfur atom of the latter is replaced with selenium. This seleno-amino acid occurs as a specific component of various selenoproteins and selenium-dependent enzymes. Incorporation of selenocysteine into these proteins occurs cotranslationally as directed by the UGA codon. For this process, a special tRNA having an anticodon complimentary to UGA, tRNASec, is utilized. In Escherichia coli and related bacteria, this tRNA first is amino acylated with serine, and the seryl-tRNASec is converted to selenocysteyl-tRNASec. The specific incorporation of selenocysteine into proteins directed by the UGA codon depends on the synthesis of selenocysteyl-tRNASec. Included in the selenium delivery protein category are rhodaneses that mobilize selenium from inorganic sources and NIFS-like proteins that liberate elemental selenium from selenocysteine. The NIFS protein from Azotobacter vinelandii was found to serve as an efficient catalyst in vitro for delivery of selenium from free selenocysteine to Escherichia coli selenophosphate synthetase for selenophosphate formation. The widespread distribution of selenocysteine lyase in numerous bacterial species was reported and the bacterial enzymes, like the pig liver enzyme, required pyridoxal phosphate as cofactor. Three NIFS-like genes were isolated from E. coli by Esaki and coworkers and the expressed gene products were isolated and characterized. One of these NIFS-like proteins also exhibited a high preference for selenocysteine over cysteine. M. vannielii, an anaerobic methane-producing organism, that grows in a mineral medium containing formate as sole organic carbon source, synthesizes several specific selenoenzymes required for growth and energy production under these conditions.

Selenoproteins: molecular pathways and physiological roles

Selenium is an essential micronutrient with important functions in human health and relevance to several pathophysiological conditions. The biological effects of selenium are largely mediated by selenium-containing proteins (selenoproteins) that are present in all three domains of life. Although selenoproteins represent diverse molecular pathways and biological functions, all these proteins contain at least one selenocysteine (Sec), a selenium-containing amino acid, and most serve oxidoreductase functions. Sec is cotranslationally inserted into nascent polypeptide chains in response to the UGA codon, whose normal function is to terminate translation. To decode UGA as Sec, organisms evolved the Sec insertion machinery that allows incorporation of this amino acid at specific UGA codons in a process requiring a cis-acting Sec insertion sequence (SECIS) element. Although the basic mechanisms of Sec synthesis and insertion into proteins in both prokaryotes and eukaryotes have been studied in great detail, the identity and functions of many selenoproteins remain largely unknown. In the last decade, there has been significant progress in characterizing selenoproteins and selenoproteomes and understanding their physiological functions. We discuss current knowledge about how these unique proteins perform their functions at the molecular level and highlight new insights into the roles that selenoproteins play in human health.

Kenji Soda--researching enzymes with the spirit of an alpinist

Like an alpinist continuously seeking virgin peaks to climb, Kenji Soda has investigated a variety of unique enzymes for which there was little or no information available; and by doing so he opened up a variety of new fields in enzyme science and technology. In particular, he has promoted the study of enzymes requiring vitamin B-derived cofactors such as FAD, NAD(P) and pyridoxal 5'-phosphate, shedding light on their reaction mechanisms, enzymological properties, crystal structures and potential practical applications. Highlighted in this review are the studies of enzymes acting on d-amino acids and sulphur/selenium-containing amino acids and those from thermophilic and psychrophilic bacteria.

Reaction mechanism and molecular basis for selenium/sulfur discrimination of selenocysteine lyase

Selenocysteine lyase (SCL) catalyzes the pyridoxal 5'-phosphate-dependent removal of selenium from l-selenocysteine to yield l-alanine. The enzyme is proposed to function in the recycling of the micronutrient selenium from degraded selenoproteins containing selenocysteine residue as an essential component. The enzyme exhibits strict substrate specificity toward l-selenocysteine and no activity to its cognate l-cysteine. However, it remains unclear how the enzyme distinguishes between selenocysteine and cysteine. Here, we present mechanistic studies of selenocysteine lyase from rat. ESI-MS analysis of wild-type and C375A mutant SCL revealed that the catalytic reaction proceeds via the formation of an enzyme-bound selenopersulfide intermediate on the catalytically essential Cys-375 residue. UV-visible spectrum analysis and the crystal structure of SCL complexed with l-cysteine demonstrated that the enzyme reversibly forms a nonproductive adduct with l-cysteine. Cys-375 on the flexible loop directed l-selenocysteine, but not l-cysteine, to the correct position and orientation in the active site to initiate the catalytic reaction. These findings provide, for the first time, the basis for understanding how trace amounts of a selenium-containing substrate is distinguished from excessive amounts of its cognate sulfur-containing compound in a biological system.

L-cysteine desulfidase: an [4Fe-4S] enzyme isolated from Methanocaldococcus jannaschii that catalyzes the breakdown of L-cysteine into pyruvate, ammonia, and sulfide

A [4Fe-4S] enzyme that decomposes L-cysteine to hydrogen sulfide, ammonia, and pyruvate has been isolated and characterized from Methanocaldococcus jannaschii. The sequence of the isolated enzyme demonstrated that the protein was the product of the M. jannaschii MJ1025 gene. The protein product of this gene was recombinantly produced in Escherichia coli and purified to homogeneity. Both the isolated and recombinant enzymes are devoid of pyridoxal phosphate (PLP) and are rapidly inactivated upon exposure to air. The air-inactivated enzyme is activated by reaction with Fe2+ and dithiothreitol in the absence of air. The air-inactivated enzyme contains 3 mol of iron per subunit (43 kDa, SDS gel electrophoresis), and the native enzyme has a measured molecular mass of 135 kDa (gel filtration), indicating it is a trimer. The enzyme is very specific for L-cysteine, with no activity being detected with D-cysteine, L-homocysteine, 3-mercaptopropionic acid (cysteine without the amino group), cysteamine (cysteine without the carboxylic acid), or mercaptolactate (the hydroxyl analogue of cysteine). The activity of the enzyme was stimulated by 40% when the enzyme was assayed in the presence of methyl viologen (4 mM) and inhibited by 70% when the enzyme was assayed in the presence of EDTA (7.1 mM). Preincubation of the enzyme with iodoacetamide (17 mM) completely abolishes activity. The enzymatic activity has a half-life of 8 or 12 min when the enzyme is treated at room temperature with 0.42 mM N-ethylmaleimide (NEM) or 0.42 mM iodoacetamide, respectively. MALDI analysis of the NEM-inactivated enzyme showed Cys25 as the site of alkylation. Site-directed mutagenesis of each of four of the cysteines conserved in the orthologues of the enzyme reduced the catalytic efficiency and thermal stability of the enzyme. The enzyme was found to catalyze exchange of the C-2 hydrogen of the L-cysteine with solvent. These results are consistent with three of the conserved cysteines being involved in the formation of the [4Fe-4S] center and the thiolate of Cys25 serving as a base to abstract the alpha-hydrogen in the first step of the elimination. Although the enzyme has no sequence homology to any known enzymes, including the non-PLP-dependent serine/threonine dehydratases or aconitases, the mechanisms of action of all of these enzymes are similar, in that each catalyzes an alpha,beta-elimination reaction adjacent to a carboxylate group. It is proposed that the enzyme may be responsible for the production of sulfide required for the biosynthesis of iron-sulfur centers in this archaea. A mechanism of action of the enzyme is proposed.

Selenium is mobilized in vivo from free selenocysteine and is incorporated specifically into formate dehydrogenase H and tRNA nucleosides

Selenophosphate synthetase (SPS), the selD gene product from Escherichia coli, catalyzes the biosynthesis of monoselenophosphate, AMP, and orthophosphate in a 1:1:1 ratio from selenide and ATP. It was recently demonstrated that selenium delivered from selenocysteine by an E. coli NifS-like protein could replace free selenide in the in vitro SPS assay for selenophosphate formation (G. M. Lacourciere, H. Mihara, T. Kurihara, N. Esaki, and T. C. Stadtman, J. Biol. Chem. 275:23769-23773, 2000). During growth of E. coli in the presence of 0.1 microM (75)SeO(3)(2-) and increasing amounts of L-selenocysteine, a concomitant decrease in (75)Se incorporation into formate dehydrogenase H and nucleosides of bulk tRNA was observed. This is consistent with the mobilization of selenium from L-selenocysteine in vivo and its use in selenophosphate formation. The ability of E. coli to utilize selenocysteine as a selenium source for selenophosphate biosynthesis in vivo supports the participation of the NifS-like proteins in selenium metabolism.

Utilization of selenocysteine as a source of selenium for selenophosphate biosynthesis

Selenophosphate synthetase (SPS), the selD gene product from Escherichia coli, catalyzes the biosynthesis of monoselenophosphate from selenide and ATP. Characterization of selenophosphate synthetase revealed the determined K(m) value for selenide is far above the optimal concentration needed for growth and approached levels which are toxic. Selenocysteine lyase enzymes, which decompose selenocysteine to elemental selenium (Se(0)) and alanine, were considered as candidates for the control of free selenium levels in vivo. The ability of a lyase protein to generate Se(0) in the proximity of SPS maybe an attractive solution to selenium toxicity as well as the high K(m) value for selenide. Recently, three E. coli NifS-like proteins, CsdB, CSD, and IscS, were characterized. All three proteins exhibit lyase activity on L-cysteine and L-selenocysteine and produce sulfane sulfur, S(0), or Se(0) respectively. Each lyase can effectively mobilize Se(0) from L-selenocysteine for selenophosphate biosynthesis.

Escherichia coli NifS-like proteins provide selenium in the pathway for the biosynthesis of selenophosphate

Selenophosphate synthetase (SPS), the selD gene product from Escherichia coli, catalyzes the biosynthesis of monoselenophosphate, AMP, and orthophosphate in a 1:1:1 ratio from selenide and ATP. Kinetic characterization revealed the K(m) value for selenide approached levels that are toxic to the cell. Our previous demonstration that a Se(0)-generating system consisting of l-selenocysteine and the Azotobacter vinelandii NifS protein can replace selenide for selenophosphate biosynthesis in vitro suggested a mechanism whereby cells can overcome selenide toxicity. Recently, three E. coli NifS-like proteins, CsdB, CSD, and IscS, have been overexpressed and characterized. All three enzymes act on selenocysteine and cysteine to produce Se(0) and S(0), respectively. In the present study, we demonstrate the ability of each E. coli NifS-like protein to function as a selenium delivery protein for the in vitro biosynthesis of selenophosphate by E. coli wild-type SPS. Significantly, the SPS (C17S) mutant, which is inactive in the standard in vitro assay with selenide as substrate, was found to exhibit detectable activity in the presence of CsdB, CSD, or IscS and l-selenocysteine. Taken together the ability of the NifS-like proteins to generate a selenium substrate for SPS and the activation of the SPS (C17S) mutant suggest a selenium delivery function for the proteins in vivo.

Structure of a NifS homologue: X-ray structure analysis of CsdB, an Escherichia coli counterpart of mammalian selenocysteine lyase

Escherichia coli CsdB, a NifS homologue with a high specificity for L-selenocysteine, is a pyridoxal 5'-phosphate (PLP)-dependent dimeric enzyme that belongs to aminotransferases class V in fold-type I of PLP enzymes and catalyzes the decomposition of L-selenocysteine into selenium and L-alanine. The crystal structure of the enzyme has been determined by the X-ray crystallographic method of multiple isomorphous replacement and refined to an R-factor of 18.7% at 2.8 A resolution. The subunit structure consists of three parts: a large domain of an alpha/beta-fold containing a seven-stranded beta-sheet flanked by seven helices, a small domain containing a four-stranded antiparallel beta-sheet flanked by three alpha-helices, and an N-terminal segment containing two alpha-helices. The overall fold of the subunit is similar to those of the enzymes belonging to the fold-type I family represented by aspartate aminotransferase. However, CsdB has several structural features that are not observed in other families of the enzymes. A remarkable feature is that an alpha-helix in the lobe extending from the small domain to the large domain in one subunit of the dimer interacts with a beta-hairpin loop protruding from the large domain of the other subunit. The extended lobe and the protruded beta-hairpin loop form one side of a limb of each active site in the enzyme. The most striking structural feature of CsdB lies in the location of a putative catalytic residue; the side chain of Cys364 on the extended lobe of one subunit is close enough to interact with the gamma-atom of a modeled substrate in the active site of the subunit. Moreover, His55 from the other subunit is positioned so that it interacts with the gamma- or beta-atom of the substrate and may be involved in the catalytic reaction. This is the first report on three-dimensional structures of NifS homologues.

Vitamin B6 enzymes participating in selenium amino acid metabolism

Various vitamin B6 enzymes play important roles in mammalian and microbial metabolism of selenium amino acids. Selenocysteine is synthesized from selenohomocysteine by catalysis of cystathionine beta-synthase and cystathionine gamma-lyase, which both require pyridoxal phosphate. Selenocysteine beta-lyase, a new B6-enzyme, exclusively catalyzes beta-elimination of selenocysteine, and occurs in mammalian systems and bacteria. Methionine gamma-lyase, cysteine desulfurase, cysteine sulfinate desulfinase, and D-selenocystine alpha,beta-lyase, which are B6-enzymes, act on cysteine, cysteine sulfinate, D-cystine, and their derivatives, and their selenium counterparts indiscriminately. Their reaction mechanisms are comparatively described.

Biosynthesis of selenophosphate

Selenophosphate synthetase, the product of the selD gene, produces the highly active selenium donor, monoselenophosphate, from selenide and ATP. Positional isotope exchange experiments have shown hydrolysis of ATP occurs by way of a phosphoryl-enzyme intermediate. Although, mutagenesis studies have demonstrated Cys17 in the Escherichia coli enzyme is essential for catalytic activity the nucleophile in catalysis has not been identified. Recently, selenophosphate synthetase enzymes have been identified from other organisms. The human enzyme which contains a threonine residue corresponding to Cys17 in the E. coli enzyme, has been overexpressed in E. coli. The purified enzyme shows no detectable activity in the in vitro selenophosphate synthetase assay. In contrast, when the human enzyme is expressed to complement a selD mutation in E. coli, in the presence of 75Se, incorporation of 75Se into bacterial selenoproteins is observed. The inactive purified human enzyme together with the very low determined specific activity of the E. coli enzyme (83 nmol/min/mg) suggest an essential component for the formation of selenophosphate has not been identified.

Cysteine sulfinate desulfinase, a NIFS-like protein of Escherichia coli with selenocysteine lyase and cysteine desulfurase activities. Gene cloning, purification, and characterization of a novel pyridoxal enzyme

Selenocysteine lyase (EC 4.4.1.16) exclusively decomposes selenocysteine to alanine and elemental selenium, whereas cysteine desulfurase (NIFS protein) of Azotobacter vinelandii acts indiscriminately on both cysteine and selenocysteine to produce elemental sulfur and selenium respectively, and alanine. These proteins exhibit some sequence homology. The Escherichia coli genome contains three genes with sequence homology to nifS. We have cloned the gene mapped at 63.4 min in the chromosome and have expressed, purified to homogeneity, and characterized the gene product. The enzyme comprises two identical subunits with 401 amino acid residues (Mr 43,238) and contains pyridoxal 5'-phosphate as a coenzyme. The enzyme catalyzes the removal of elemental sulfur and selenium atoms from L-cysteine, L-cystine, L-selenocysteine, and L-selenocystine to produce L-alanine. Because L-cysteine sulfinic acid was desulfinated to form L-alanine as the preferred substrate, we have named this new enzyme cysteine sulfinate desulfinase. Mutant enzymes having alanine substituted for each of the four cysteinyl residues (Cys-100, Cys-176, Cys-323, and Cys-358) were all active. Cys-358 corresponds to Cys-325 of A. vinelandii NIFS, which is conserved among all NIFS-like proteins and catalytically essential (Zheng, L., White, R. H., Cash, V. L., and Dean, D. R. (1994) Biochemistry 33, 4714-4720), is not required for cysteine sulfinate desulfinase. Thus, the enzyme is distinct from A. vinelandii NIFS in this respect.

Mechanisms of mutagenicity and toxicity of sodium selenite (Na2SeO3) in Salmonella typhimurium

The mechanisms of selenite toxicity and mutagenicity in S. typhimurium have been characterized. In contrast to previous reports, selenite toxicity was shown not to involve nonspecific incorporation into protein via the sulfur metabolic pathways. Selenite toxicity was, however, shown to involve its ability to act as an oxidizing agent, primarily through reactions with sulfhydryls. Strains which lack glutathione (GSH) are more sensitive to killing by sulfhydryl reagents. The selenite sensitivity of such a mutant was a biphasic phenomenon. The mutant was much more sensitive than a strain which contained GSH at lower selenite concentrations whereas, at higher concentrations, the mutant was much more resistant to selenite. The mechanism of selenite toxicity at lower concentrations in this mutant thus appeared to involve damage to intracellular sulfhydryls. The sensitization to higher doses of selenite by GSH could be explained by the generation of toxic oxygen species. The in vitro reactions of selenite with both cysteine and GSH readily produced H2O2 and O2-. A S. typhimurium strain which overproduces superoxide dismutase (SOD) and catalase was more resistant to high concentrations of selenite, but not killing by the lower doses. Pretreatment of cells with a nonlethal dose of selenite induced the synthesis of proteins which protected the cells from killing by H2O2 or high doses of selenite. Selenite was also a mutagen in the tester strain TA104, in which a number of other oxidizing agents have also been found to be mutagens. These results were consistent with a model in which the reactions of selenite and intracellular thiols with concomitant production of active oxygen species are the primary causal agents of selenite mutagenicity and toxicity in S. typhimurium.

Purification and characterization of Clostridium sticklandii D-selenocystine alpha, beta-lyase

We have found a novel enzyme that decomposes D-selenocystine into pyruvate, ammonia, and elemental selenium in extracts of Clostridium sticklandii and C. sporogenes. The enzyme of C. sticklandii has been purified to homogeneity. It has a molecular weight of 74,000 and consists of two subunits identical in molecular weight (35,000). Pyridoxal 5'-phosphate is required as a cofactor. In addition to D-selenocystine, D-cystine, D-lanthionine, meso-lanthionine, and D-cysteine serve as substrates. However, D-selenocysteine, D-serine, DL-selenohomocystine, and L-amino acids are inert. The enzyme also catalyzes the beta-replacement reaction between D-selenocystine and a thiol to produce S-substituted D-cysteine. L-Selenohomocysteine also can serve as a substituent donor in the beta-replacement reaction to yield selenocystathionine.

Use of selenite, selenide, and selenocysteine for the synthesis of formate dehydrogenase by a cysteine-requiring mutant ofEscherichia coli K-12

The forms of Se in the Se-dependent enzyme formate dehydrogenase is known to be selenocysteine, but the way this amino acid enters the polypeptide chain has not been established. Through the use of a cysteine-requiring mutant ofEscherichia coli K-12 that could also grow in the presence of glutathione, we were able to study the effect of selenite, selenide, andL-selenocysteine, each at a concentration of 0.1 μM, on the synthesis of formate dehydrogenase. The three forms of Se served equally well for inducing formate dehydrogenase activity, measured by dichlorophenol-indophenol reduction mediated by phenazine methosulfate. It is known that selenite can be reduced to selenide by the action of glutathione reductase, present inE. coli, and that selenocysteine is converted to elemental Se by the action of selenocysteine lyase, also present in the mutant. Elemental Se is then reduced nonenzymatically to hydrogen selenide. The conversion of both selenite and selenocysteine to selenide and the ability of each form of Se to induce the synthesis of equal levels of formate dehydrogenase suggest that the incorporation of Se into formate dehydrogenase is accomplished by a posttranslational mechanism.

Selenocysteine lyase activity in a cysteine-requiring mutant ofEscherichia coli K-12

Selenocysteine lyase activity was detected in crude extracts from a cysteine-requiring mutant ofEscherichia coli K-12. The level of activity was the same whether cells had been grown aerobically or anaerobically, with or without selenocysteine. Selenocysteine lyase catalyzes the conversion of selenocysteine to alanine and elemental Se, a reaction that is followed by a nonenzymatic reduction of the Se to hydrogen selenide. Both of these end products were identified in this study. With cysteine as the substrate, alanine and H2S were formed, but only at levels 50% less than the products formed from selenocysteine. Selenocysteine lyase has been identified in a number of mammals and bacteria; its presence in a cysK mutant ofE. coli K-12 suggests a common route whereby hydrogen selenide, derived from selenocysteine, can then be assimilated into selenoproteins.