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Sulfur is an essential element in biological systems, contributing to synthesizing key biomolecules, including amino acids such as cysteine and methionine, and cofactors such as coenzyme A and biotin. Microorganisms primarily assimilate sulfur as sulfate (SO₄²⁻) from the environment, which must undergo a series of biochemical transformations before it can be incorporated into cellular components. As sulfate is highly oxidized, it must undergo assimilatory sulfate reduction to...
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After folding, the ER assesses the quality of secretory and membrane proteins. The correctly folded proteins are cleared by the calnexin cycle for transport to their final destination, while misfolded proteins are held back in the ER lumen. The ER chaperones attempt to unfold and refold the misfolded proteins but sometimes fail to achieve the correct native conformation. Such terminally misfolded proteins are then exported to the cytosol by ER-associated degradation or ERAD pathway for...
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Structural insights into the catalytic mechanism of Escherichia coli selenophosphate synthetase.

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Selenocysteine Lyase.

Thressa C Stadtman

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    Selenocysteine incorporation into proteins relies on specific tRNA and selenium delivery proteins. Bacteria like E. coli utilize NIFS-like proteins and selenocysteine lyase for selenium metabolism and selenoprotein synthesis.

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    Area of Science:

    • Biochemistry
    • Molecular Biology
    • Microbiology

    Background:

    • Selenocysteine, a cysteine analog with selenium, is vital for selenoproteins and selenium-dependent enzymes.
    • Its incorporation into proteins is directed by the UGA codon via a specialized tRNA (tRNASec).
    • In bacteria like Escherichia coli, this process involves seryl-tRNASec conversion to selenocysteyl-tRNASec.

    Purpose of the Study:

    • To investigate the mechanisms of selenocysteine incorporation and selenium metabolism in bacteria.
    • To characterize selenium delivery proteins and their roles in selenoprotein synthesis.
    • To explore the function of NIFS-like proteins and selenocysteine lyase in bacterial selenium pathways.

    Main Methods:

    • Investigated the synthesis of selenocysteyl-tRNASec in Escherichia coli.
    • Characterized selenium delivery proteins, including rhodaneses and NIFS-like proteins.
    • Isolated and analyzed NIFS-like genes and their expressed products from E. coli.
    • Studied the enzymatic activity of bacterial selenocysteine lyase, noting cofactor requirements.
    • Examined selenoenzyme synthesis in Methanococcus vannielii.

    Main Results:

    • NIFS protein from Azotobacter vinelandii efficiently catalyzes selenium delivery from selenocysteine to E. coli selenophosphate synthetase.
    • Bacterial selenocysteine lyases, like the pig liver enzyme, require pyridoxal phosphate as a cofactor.
    • Three NIFS-like genes from E. coli were isolated, and their products characterized.
    • One E. coli NIFS-like protein showed a strong preference for selenocysteine over cysteine.
    • Methanococcus vannielii synthesizes essential selenoenzymes for growth on formate.

    Conclusions:

    • The synthesis of selenocysteyl-tRNASec is crucial for selenocysteine incorporation into proteins.
    • NIFS-like proteins and selenocysteine lyase play significant roles in bacterial selenium metabolism.
    • Bacterial selenium pathways are diverse and essential for the function of selenoenzymes in various microorganisms.