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Biosynthesis of Phosphoserine in the Methanococcales

Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06520,¹ Department of Chemistry and Biochemistry and Institute for Cellular and Molecular Biology, The University of Texas at Austin, Austin, Texas 78712²

Received 11 August 2006/ Accepted 16 October 2006

Methanococcus maripaludis and Methanocaldococcus jannaschii produce cysteine for protein synthesis using a tRNA-dependent pathway. These methanogens charge tRNA^Cys with L-phosphoserine, which is also an intermediate in the predicted pathways for serine and cystathionine biosynthesis. To establish the mode of phosphoserine production in Methanococcales, cell extracts of M. maripaludis were shown to have phosphoglycerate dehydrogenase and phosphoserine aminotransferase activities. The heterologously expressed and purified phosphoglycerate dehydrogenase from M. maripaludis had enzymological properties similar to those of its bacterial homologs but was poorly inhibited by serine. While bacterial enzymes are inhibited by micromolar concentrations of serine bound to an allosteric site, the low sensitivity of the archaeal protein to serine is consistent with phosphoserine's position as a branch point in several pathways. A broad-specificity class V aspartate aminotransferase from M. jannaschii converted the phosphohydroxypyruvate product to phosphoserine. This enzyme catalyzed the transamination of aspartate, glutamate, phosphoserine, alanine, and cysteate. The M. maripaludis homolog complemented a serC mutation in the Escherichia coli phosphoserine aminotransferase. All methanogenic archaea apparently share this pathway, providing sufficient phosphoserine for the tRNA-dependent cysteine biosynthetic pathway.

Published ahead of print on 27 October 2006.

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