16
S
Sulfur
32.065
Essential: all life
Sulfur
Major functions in cells: (1)
-Proteins (Cys, Met, and Fe/S)
-Low molecular weight thiols (glutathione, mycothiol, bacillithiol,
phytochelatins)
-Biotin
-Lipoic acid
Environmental and health impacts:
-Sulfur limitation can affect plant or microbial growth
-Anthropogenic sulfur compounds contribute to acid rain
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(1) Sulfur: An Essential Macronutrient
Sulfur is required in all cells and is considered one of the macronutrients that are essential for life. Sulfur is necessary for the proteins Cys and Met, and well as Fe/S proteins. Sulfur is also found in low molecular weight thiols (glutathione, mycothiol, bacillithiol), biotin, and lipoic acid. Limitations for sulfur typically lead to changes in protein composition (the proteome) and target highly abundant proteins.
(2) Selective Reduction of Cys and Met Content in Proteins
In E. coli, upon S limitation, an abundant sulfate-binding protein is induced (Pardee, 1966). This polypeptide lacks Cys and Met, which ensures that its synthesis in response to S limitation is not impaired by restrictions on Cys and Met availability. Numerous other examples are now apparent where proteins expressed under S limitation either lack or have a reduced content of Cys and Met. Analysis of proteins annotated as having likely roles in S assimilation in both E. coli and S. cerevisiae revealed a substantial decrease (up to twofold) in the fraction of amino acids that contain S relative to the rest of the proteome (Baudouin-Cornu et al., 2001). No such difference was seen in the orthologous enzymes in humans, which was interpreted as evidence that S limitation has not been a selective pressure in mammals.
(3) Expression of S-Depleted Variants of Abundant Proteins
Expression of S-depleted protein variants or isozymes is an important mechanism of acclimation to S-limiting growth conditions. For example, in the cyanobacterium Calothrix sp. PCC7601, the phycocyanins, a key phycobilisome constituent, are encoded by three differentially expressed operons. The cpc3 operon is specifically induced under conditions of S limitation. The encoded phycocyanins have been selectively depleted of S-containing amino acids relative to the phycocyanins encoded by the cpc1 and cpc2 operons. In over 1000 amino acids of five proteins encoded by cpc3, there are only five Met initiation codons and three indispensable Cys (Mazel and Marlière, 1989). A similar response has been documented in a freshwater cyanobacterium, Fremyella diplosiphon.
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In S. cerevisiae, exposure to Cd depletes intracellular S because it induces synthesis of the Cys-containing tripeptide glutathione, which tightly binds Cd to detoxify it. When cells are treated with Cd, there is induction of isoforms of three central metabolic enzymes that are selectively depleted in S-containing amino acids (Fauchon et al., 2002). These include an alternative pyruvate decarboxylase, an enolase, and an aldehyde dehydrogenase. Several other mRNAs that were induced in Cd-exposed cells were noted to be significantly depleted in Cys- and Met-encoding capacity; these responses are dependent on Met4p, a transcriptional regulator of S assimilation. By reducing the S used in proteins, there is an increased S availability for Cd detoxification.
In the alga C. reinhardtii, several extracellular proteins and a particular isoform of chlorophyll-binding light-harvesting protein are induced by S deficiency, and these induced proteins are depleted for Cys and Met (Takahashi et al., 2001).
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Even multicellular organisms have S sparing responses. In the soybean seed, the S content of storage proteins in seed is determined by the level of S nutrition: S amino acid-rich glycinin is the major protein if S is available, but S amino acid-depleted B-conglycinin accumulates in the S-poor situation (Kim et al., 1999).
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(4) Degradation of Phycobilisomes
Photosynthetic organisms take advantage of protein degradation to release macronutrients when they find themselves S limited. In Synechococcus strain DC2, the light-harvesting phycobiliproteins may constitute 50% of total protein. Phycobilisome complexes in Synechococcus can be targeted for degradation in response to either N or S limitation (Collier and Grossman, 1992) and that degradation appears to be a highly regulated and ordered process (Grossman et al., 1993).
(5) Degradation of Sulfolipids
The S-containing lipid sulfoquinovosyl diacylglycerol (SQDG) is a major component of the thylakoid membrane in cyanobacteria and in C. reinhardtii. In response to S limitation, this lipid is degraded, providing a large percentage of the S needed for ongoing protein synthesis (Sugimoto et al., 2007).