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Cysteine and S-sulfocysteine biosynthesis in phototrophic bacteria
Cys is not synthesized by mammals and therefore is 1 of several of their essential amino acids. It is 1 of the 20 commonly used amino acids in protein synthesis and its genetic codewords are UGU/UGC. Although Cys and Sec use a different genetic language and have different biosynthetic mechanisms, they are structurally similar and the replacement of sulfur with selenium in methionine (Met) synthesis has been reported wherein selenomethionine can be inserted into protein in place of methionine (,47). Cys in place of Sec in normally selenium containing proteins has also been reported in selenium deficient rodents (), but the specific mechanism of how this occurred was not addressed. We examined whether SPS2 would engage sulfide in place of selenide in generating thiophosphate that would be used as an active sulfur donor in making Cys attached to tRNA[Ser]Sec (reaction catalyzed by SecS), whereby replacing Sec with Cys in normally selenium containing proteins. The replacement of Cys for Sec on tRNA[Ser]Sec and the incorporation of Cys into protein by this pathway is discussed below.
Although earlier, conflicting reports suggested that SPS1 was either the mammalian enzyme responsible for making the active selenium donor (–) or was involved in recycling selenium in Sec biosynthesis (), our in vitro studies demonstrated that SPS2 synthesizes selenophosphate and SPS1 has another role (). To resolve the discrepancies between the earlier studies (, ) and ours (), and to elucidate the roles of SPS1 and SPS2 intracellularly, these proteins were individually knocked down using RNAi technology and the consequences of their resulting loss examined (). Selenoprotein biosynthesis was abolished in NIH 3T3 cells by loss of SPS2 expression, but loss of SPS1 had no effect. Complementation experiments were also carried out wherein selenoprotein synthesis was restored in SPS2 deficient NIH 3T3 cells with either mSPS2(Cys) or SelD but not with mSPS1. These studies unequivocally established that SPS2, which synthesizes selenophosphate in vivo, is essential to selenoprotein synthesis and that SPS1 most likely has another role in cells other than Sec or selenoprotein synthesis (, ). It should also be noted that sps1 is an essential gene in Drosophila development () and has been shown to have a role in preventing megamitochondrial formation (). Interestingly, SPS1 has been retained in insects such as the red flour beetle and silkworm that have lost all selenoprotein synthetic machinery, providing further evidence that sps1 must have a role independent of Sec and selenoprotein biosynthesis ().
carries out the chain elongation steps of fatty acid biosynthesis
Using the nucleotide and protein sequences of CβS and CS orthologs from plants, bacteria, yeast, and parasitic protozoa as queries, a search of T. rangeli genome and transcriptome databases allowed the identification of genes encoding CβS and a partial gene sequence for CS. Additionally, the T. rangeli genome contains a single copy of the cystathionine γ-lyase (CGL) gene of the RTS pathway but lacks the genes encoding serine acetyltransferase (SAT) present in the de novo biosynthetic pathway of other trypanosomatids. The sequences for CβS and CS were then back-searched using the SWISSPROT and NCBI databases, which confirmed the identity of both genes. These results suggest that, as in other trypanosomatids, T. rangeli possesses genes coding for the enzymes involved in these two cysteine biosynthetic routes: CβS in the transsulfuration pathway and CS in the de novo biosynthesis pathway.
It requires 7 rounds of this pathway to degrade palmitate (a C16 fatty acid).
A graphic chart of these important metabolic steps may be found in the web site.
Transsulfuration is an active pathway for cysteine biosynthesis …
The biosynthesis of amino acids involves several biochemical pathways in which amino acids are assembled from other precursors. The biosynthesis of amino acids is distinct from that involving lipids or carbohydrates because it includes the use of nitrogen.
Even though the biosynthesis of Sec was established in eubacteria in the early 1990s (–), only in the last several years was the complete biosynthetic pathway of this selenium-containing amino acid determined in eukaryotes and archaea (). Very recently, it was also shown that cysteine (Cys) can be synthesized de novo by replacing sulfide with selenide in the Sec biosynthetic pathway, forming Cys on tRNA[Ser]Sec (). The biosynthesis of Sec and the novel pathway of Cys biosynthesis are the subjects of this review.
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novo pathway for cysteine biosynthesis is found ..
We found significantly lower levels of total thiol content in T. rangeli compared to T. cruzi epimastigotes. Based on the fact that cysteine forms the basic building block of all thiol antioxidants , one possible explanation for the lowest thiol levels observed may be because T. rangeli only uses the RTS pathway as a cysteine biosynthesis source. Another important aspect is related to the fact that exogenous organic sulfur-containing amino acids can be supplied by transporters [,,]. However, such a mechanism and its possible influence on the total thiol levels in T. rangeli remain to be explored.
Two pathways for cysteine biosynthesis in ..
The presence of a truncated CS gene as revealed by high-quality sequencing (Phred ≥50), and the absence of CS activity in both epimastigote and trypomastigote extracts suggests that the de novo cysteine biosynthetic pathway is absent or not functional in T. rangeli. Nevertheless, T. rangeli possesses a functional RTS pathway, a characteristic shared with T. brucei, for which only CβS activity has been reported in bloodstream trypomastigote extracts but at a very low level . This result indicates that similarities in the metabolism of sulfur-containing amino acids exist between T. rangeli and T. brucei, another parasite that does not possess an intracellular mammalian host stage. Such findings may suggest that the extracellular stage of the life cycle of parasitic protozoa and the RTS biosynthetic pathway are causally connected.
Pentose Phosphate Pathway; Cysteine Biosynthesis; ..
Our results indicate that RTS appears to be the only pathway for cysteine biosynthesis in T. rangeli. At the genomic level, T. rangeli contains single copies of genes coding for the CβS and CGL (cystathionine γ-lyase) enzymes of the RTS pathway but lacks genes encoding a protein of the cysteine de novo biosynthetic pathway (SAT). Additionally, a partial gene sequence for CS was found that has an A-G nucleotide transition at position 470, which generates a stop codon (TAG) (data not shown); thus, the truncated protein encoded lacks two of the four lysine residues required for CS activity.
Tuscany Diet - Biochemistry and Nutrition
Cys, an essential amino acid in mammals, was also found to be synthesized de novo by replacing Sec in the synthesis of selenium containing proteins. The precise means of how this synthesis occurs was determined in vitro and the presence of Cys in place of Sec was shown to occur in vivo in both cells in culture and in mice. In addition to establishing the pathway of Sec biosynthesis, the replacement of Sec with Cys provides unique possibilities for regulating the expression of selenoproteins and their functions as well as elucidating the biological roles of dietary selenium.
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