Reference: Cai H, et al. (2001) Identification of the gene and characterization of the activity of the trans-aconitate methyltransferase from Saccharomyces cerevisiae. Biochemistry 40(45):13699-709

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Abstract


We have identified the yeast open reading frame YER175c as the gene encoding the trans-aconitate methyltransferase of Saccharomyces cerevisiae. Extracts of a yeast strain with a disrupted YER175c gene demonstrate a complete loss of activity toward the methyl-accepting substrates trans-aconitate, cis-aconitate, DL-isocitrate, and citrate. Reintroduction of the YER175c gene on a plasmid results in an overexpression of the activity toward each of these methyl-accepting substrates. We now designate this gene TMT1 for trans-aconitate methyltransferase. We examined the methyl-accepting substrate specificity of this enzyme in extracts from overproducing cells. We found that trans-aconitate was the best substrate with a K(m) of 0.66 mM. Other substrates were recognized much more poorly, including cis-aconitate with a K(m) of 74 mM and the decarboxylation product itaconate with a K(m) of 44 mM. The ratio of the maximal velocity to the K(m) of these substrates was only 0.24% and 0.9% that of trans-aconitate; for other substrates including citrate and other tricarboxylate and dicarboxylate derivatives, this ratio ranged from 0.0003% to 0.062% that of trans-aconitate. We then asked if any of these compounds were present endogenously in yeast extracts. We were able to identify trans-aconitate 5-methyl ester as well as additional unidentified radiolabeled products when S-adenosyl-L-[methyl-(3)H]methionine was mixed with TMT1(+) extracts (but not with tmt1(-) extracts), suggesting that there may be additional substrates for this enzyme. We showed that the product 5-methyl ester of trans-aconitate is not readily metabolized in yeast extracts. Finally, we demonstrated that the activity of the yeast trans-aconitate methyltransferase is localized in the cytosol and increases markedly as cells undergo the metabolic transition at the diauxic shift.

Reference Type
Journal Article
Authors
Cai H, Dumlao D, Katz JE, Clarke S
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