If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Synonyms: superpathway of seleno-compound detoxification, superpathway of selenium uptake and metabolism, superpathway of seleno-amino acid biosynthesis and detoxification
|Superclasses:||Biosynthesis → Amino Acids Biosynthesis → Other Amino Acid Biosynthesis|
|Degradation/Utilization/Assimilation → Inorganic Nutrients Metabolism → Selenium Metabolism → Seleno-Amino Acid Detoxification|
|Detoxification → Seleno-Amino Acid Detoxification|
Some taxa known to possess parts of the pathway include : Arabidopsis thaliana col, Astragalus, Astragalus bisulcatus, Brassica juncea, Brassica oleracea capitata, Brassica oleracea italica, Neptunia, Pisum sativum, Saccharomyces cerevisiae, Spartina, Spinacia oleracea, Sporobolus alterniflorus
Note: This is a chimeric pathway, comprising reactions from multiple organisms, and typically will not occur in its entirety in a single organism. The taxa listed here are likely to catalyze only subsets of the reactions depicted in this pathway.
Selenium is an essential micronutrient for animals and humans but has not yet been proven to be an essential micronutrient for land plants [Brown01a, Sors05]. There is evidence, however, of Chlamydomonas proteins that contains a L-selenocysteine residue suggesting that algal species might also require Se0 [Kim06c, Novoselov02]. The health requirement of Se0 for humans and animals can be met by crops and forage plants that contain selenocompounds. Helianthus annuus among the oilseed crops was shown to be a good source for this micronutrient [Terry00]. In addition, there is evidence that certain seleno-compounds, including seleno-L-methionine, have anti-carcinogenic properties [Whanger04].
Despite its importance as a micronutrient, higher concentrations of Se0 are toxic to humans, other animals, and plants largely because transporters and enzymes involved in sulfur and sulfo-compound metabolism can often erroneously substitute Se0 for sulfur and can misincorporate seleno-amino acids into proteins [Zwolak11, Ellis03, Brown81a].
In general, plants take up both selenate and selenite from soil. A series of reduction steps converts selenate to selenide / hydrogen selenide and this is assimilated into the seleno-amino acids L-selenocysteine and seleno-L-methionine [Sors09].
In Se accumulator plants, these seleno-amino acids may be modified to create organic seleno-compounds for storage. Seasonal weeds like Silene gallica and Avena sterilis ludoviciana along with perennial weeds like Cirsium arvense employ this strategy and accumulate high amounts of Se0 [Dhillon09]. Se accumulation can help to protect plants against attacks by prairie dogs, insects, and fungi (see refs in [Freeman09]).
Other plants, like Arabidopsis thaliana col generate seleno-compounds as well, but rather than storing them, they transform them into less toxic volatile compounds such as dimethyl selenide (DMSe) that can then be released from the plants [deSouza00]. This process is also called phytovolatilization [Zhou09a].
While these strategies directly benefit the plants that employ them, they can also be put to use for the phytoremediation of seleniferous soils that contain dangerously high amounts of Se0. Numerous insights into endogenous seleno-compound metabolism in plants have been gained through efforts to engineer plants that show improved selenium phytoremediation abilities [Ellis04, Dhillon09, Zhou10, Banuelos07, deSouza00].
About This Pathway
In many plant species the metabolism of selenium appears to mirror the metabolism of sulfur and often uses the same enzymes. Therefore, although this pathway has not been completely elucidated in plants, it has been built based on the comparable sulfur-based pathways, such as the reductive sulfur assimilation pathway found in plants [Sors05].
As in the sulfur reduction pathway, the ATP sulfurylase enzyme appears to catalyze the first step in selenium reduction. This claim is supported by the finding that overexpression of an Arabiopsis ATP sulfurylase promotes selenate reduction [PilonSmits99, Sors05a].
The second step in the pathway is still being investigated. There is evidence for non-enzymatic conversion of to selenite in vitro [Dilworth77], but selenite reductase activity has been observed in planta and has been measured in a number of selenium hyperaccumulators [Sors05a].
To date, there is no evidence that a sulfite reductase enzyme is needed to catalyze the third reaction in the pathway in plants. Rather, evidence collected using pea chloroplasts suggests that this reaction can proceed non-enzymatically and uses both glutathione and NADPH as reducing agents [Jablonski82].
Analyses of the overall reduction of selenate to organic selenium suggest that the first step of selenate reduction is rate-limiting [deSouza98a].
This portion of the pathway is believed to occur in the choloroplast [Ellis03].
Similar to the selenate reduction portion of the pathway, selenocysteine formation likely parallels the cysteine biosynthetic pathway in plants, and may use many of the same enzymes.The first step in cysteine biosynthesis involes a serine acetyltransferase (SAT). While there is definite evidence of enzymes capable of producing O-acetyl-L-serine in Arabidopsis and many other plant species, these enzymes have not yet been explicitly linked to selenium metabolism in planta. In fact, when SAT activity is compared across different Astragalus species that accumulate vastly different levels of selenium, no correlation is detected [Sors05a]. Selenocysteine formation is likely to be catalyzed by O-acetyl-L-serine (thiol) lyase (OAS-TL) enzymes and selenocysteine production in the presence of O-acteyl-serine has been measured in a number of plants including including Pisum sativum (pea), clover, and several Astralagus species [Ng78]. This activity could be competitively inhibited by sulfur, and conversely, cysteine biosynthesis could be competetively inhibited by selenium, arguing that these reactions are catalyzed by the same enzymes [Ng78]. Direct evidence for OAS-TL-based production of selenocysteine has only been demonstrated in the bacteria Paracoccus denitrificans to date [Burnell77], but it is likely that plant OAS-TLs catalyze this reaction, too.
L-selenocystathionine production probably depends on the activity of the cystathionine synthase enzymes that play a comparable role in sulfate metabolism. Over-expression of an Arabidopsis CGS1 enzyme in Brassica juncea appears to increase flux through this pathway and leads to higher levels of volatile selenium compound production [Van03c].
Evidence for activity of the next enzyme in the pathway, β-(seleno)cystathionine lyase can be detected in plant extracts from several species including selenium non-accumulators, such as spinach (see L-cystathione β-lyase / β L-selenocystathionase ), pea and Se accumulators, such as Astragalus bisulcatus. As seen for many other enzymes in this pathway, it is active with both the sulfur and selenium analogs [McCluskey86].
The final step toward selenomethionine biosynthesis could be catalyzed by at least three different types of enzymes if the proteins involved in L-methionine biosynthesis II also participate in selenomethionine (Se-Met) metabolism. For instance, E.coli has both cobalamin-dependent (126.96.36.199) and cobalamin-independent enzymes ( 188.8.131.52) that can catalyze the formation of Se-Met [Zhou00]. No cobalamin-independent candidates have been found in Arabidopsis. On the other hand, it does have three cobalamin-independent methionine synthases (see 2.1.14) which could also potentially accept selenohomocysteine, but their role in seleno-amino acid biosynthesis has not yet been tested [Ravanel04].
Both seleno-amino acids, L-selenocysteine and seleno-L-methionine, can be misincorporated during protein synthesis. To prevent this, selenium accumulating or hyperaccumulating plants typically transform them into non-coding seleno-amino acids, such as Se-methylselenocysteine or other related compounds. Non-accumulating plants employ a different strategy and further metabolize these seleno-amino acids into the production of volatile compounds that can be released into the atmosphere [Sors05]. Several different routes for seleno-amino acid detoxification are incorporated into this superpathway and it is unlikely that any one species employs all of them. In particular, within the plant kingdom, seleno-amino acid detoxification and volatilization II via dimethylselenoniopropanoate production, may be limited to halophytic species.
Two sub-pathways describe alternative routes that Se-non-accumulating and accumulating plants may use to further metabolize seleno-L-methionine to produce the volatile dimethyl selenide. In the hyperaccumulator Spartina species, this may occur in a four-step process that involves the formation of the intermediate compound 3-dimethylselenopropionaldehyde. The Spartina enzymes shown to function in DMSP biosynthesis [Kocsis00] may perform analogous functions in DMSeP production. However, this intermediate is not likely produced in Arabidopsis and many other plants. Therefore, a one-step variant has been proposed. Formation of dimethyl selenide may be catalyzed by a putative methylmethionine / Se-methyl-Se-methionine hydrolase in Arabidopsis thaliana and other plants, but no such enzyme has been identified or characterized yet, particularly with respect to seleno-compound metabolism [Tagmount02]. It is also possible that this reaction may happen spontaneously through chemical decomposition, but this would be expected to occur very slowly at physiological pH.
Only one sub-pathway describing the further metabolism of L-selenocysteine has been suggested so far, and it lacks some important details concerning the final steps of dimethyl diselenide production. It is possible that a series of reactions involving first a selenium methylselenocysteine selenoxide intermediate and then a methaneseleniol intermediate are required, similar to the enzymatic steps observed for dimethyldisulfide biosynthesis [Sors05].
It is also possible that some of the L-selenocysteine can be shunted toward dimethyl selenide production if enzymes such as the bacterial thiopurine methyltransferase are found to occur in plants [Ranjard02, Sors05].
Selenocysteine may also be used to form γ-L-glutamyl-Se-methyl-L-selenocysteine as this compound has been seen to accumulate in Arabidopsis over-expressing a selenocysteine methyltransferase enzyme and natively, in the hyperaccumulator Astragalus bisulcatus [Nigam69, Nigam69a, Sors05].
Subpathways: seleno-amino acid detoxification and volatilization I, seleno-amino acid biosynthesis, selenate reduction, seleno-amino acid detoxification and volatilization III, seleno-amino acid detoxification and volatilization II
Unification Links: PlantCyc:PWY-6395
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