Identification of the gene and characterization of the activity of the trans-aconitate methyltransferase from Saccharomyces cerevisiae.

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 Km of 0.66 mM. Other substrates were recognized much more poorly, including cis-aconitate with a Km of 74 mM and the decarboxylation product itaconate with a Km of 44 mM. The ratio of the maximal velocity to the Km 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-3H]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.

Distinct reactions catalyzed by bacterial and yeast trans-aconitate methyltransferases.

The trans-aconitate methyltransferase from the bacterium Escherichia coli catalyzes the monomethyl esterification of trans-aconitate and related compounds. Using two-dimensional (1)H/(13)C nuclear magnetic resonance spectroscopy, we show that the methylation is specific to one of the three carboxyl groups and further demonstrate that the product is the 6-methyl ester of trans-aconitate (E-3-carboxy-2-pentenedioate 6-methyl ester). A similar enzymatic activity is present in the yeast Saccharomyces cerevisiae. Although we find that yeast trans-aconitate methyltransferase also catalyzes the monomethyl esterification of trans-aconitate, we identify that the methylation product of yeast is the 5-methyl ester (E-3-carboxyl-2-pentenedioate 5-methyl ester). The difference in the reaction catalyzed by the two enzymes may explain why a close homologue of the E. coli methyltransferase gene is not found in the yeast genome and furthermore suggests that these two enzymes may play distinct roles. However, we demonstrate here that the conversion of trans-aconitate to each of these products can mitigate its inhibitory effect on aconitase, a key enzyme of the citric acid cycle, suggesting that these methyltransferases may achieve the same physiological function with distinct chemistries.