Recognition of Age-damaged (R,S)-Adenosyl-L-methionine by Two Methyltransferases in the Yeast Saccharomyces cerevisiae

Abstract

The biological methyl donor S-adenosylmethionine (AdoMet) can exist in two diastereoisomeric states with respect to its sulfonium ion. The S configuration, (S,S)-AdoMet, is the only form that is produced enzymatically as well as the only form used in almost all biological methylation reactions. Under physiological conditions, however, the sulfonium ion can spontaneously racemize to the R form, producing (R,S)-AdoMet. As of yet, (R,S)-AdoMet has no known physiological function and may inhibit cellular reactions. In this study, we found two Saccharomyces cerevisiae enzymes that are capable of recognizing (R,S)-AdoMet and using it to methylate homocysteine to form methionine. These enzymes are the products of the SAM4 and MHT1 genes, identified previously as homocysteine methyltransferases dependent upon AdoMet and S-methylmethionine, respectively. We found here that Sam4 recognizes both (S,S)-and (R,S)-AdoMet, but that its activity is much higher with the R,S form. Mht1 reacts with only the R,S form of AdoMet, whereas no activity is seen with the S,S form. R,S-Specific homocysteine methyltransferase activity is also shown here to occur in extracts of Arabidopsis thaliana, Drosophila melanogaster, and Caenorhabditis elegans, but has not been detected in several tissue extracts of Mus musculus. Such activity may function to prevent the accumulation of (R,S)-AdoMet in these organisms.The aging process, as well as several human diseases, has been linked to the accumulation of spontaneously damaged biomolecules. Cells have evolved several ways of dealing with these altered molecules, including degradation, excretion, and repair pathways (1-6). The balance between the formation of age-altered molecules and the pathways that limit their cellular accumulation has been described as a battle between chemistry and biochemistry, where chemistry ultimately wins (2).Although enzymes that recognize damaged DNA (3) and proteins (1, 2, 5) have been well characterized, this is not yet the case for spontaneously altered small molecules. Of the large number of metabolites that are produced and used by biological systems, many are unstable, degrading into forms that may have reduced function or that may be toxic. One pathway of small molecule degradation and cellular recognition has been described recently. Here, trans-aconitate formed spontaneously from the citric acid cycle intermediate cis-aconitate results in the inhibition of at least two steps in the cycle (7, 8). trans-Aconitate is then recognized by a specific yeast methyltransferase; the methyl ester formed has reduced inhibitory properties (9).One of the crucial small molecule metabolites in all organisms is S-adenosyl-L-methionine (AdoMet) 2 (10 -12). Second to ATP, it is probably the most widely used cofactor in nature (12, 13). Not only does it serve as the primary methyl donor, but it also functions as an amino, adenosyl, and ribosyl donor (11). It also plays a role in the formation of adenosyl radicals (14) and as a precursor of polyamines (15). AdoMet has been shown to...

Genome structure of a Saccharomyces cerevisiae strain widely used in bioethanol production

“…The Sam4p has been proposed to participate in the recycling of AdoMet from the inactive, and possibly toxic, (R,S)-AdoMet isomeric state back to the biologically essential form (S,S)-AdoMet (Vinci and Clarke 2007). Based on CGH-array and Southern blot data ( Fig.…”

Cell division, one-carbon metabolism and methionine synthesis in a metK-deficient Escherichia coli mutant, and a role for MmuM

“…(R)-SAM is produced by the spontaneous epimerization of (S)-SAM, a process which goes on continuously in solid or in liquid until an equilibrium of about 1 : 1 is reached (Vinci & Clarke, 2007). If (S)-SAM accumulates to any significant extent in the cell, and remains there unmetabolized for a significant amount of time, then (R)-SAM must also accumulate.…”

“…Yeast makes its own SAM transporter, two SAM synthases and not one but two SAM-homocysteine methyltransferases, SAM4 and MHT1, both of which use (R)-SAM as a substrate. MHT1 also uses methylmethionine and thus resembles E. coli MmuM (Vinci & Clarke, 2007) while SAM4 uses both forms of SAM, with a preference for the (R) form, but doesn't use methylmethionine. The yeast transporter assures SAM uptake.…”

Aspartate-Derived Amino Acid Biosynthesis inArabidopsis thaliana

“…However, in natural systems, biologically active (S,S)-S-adenosylmethionine can spontaneously racemize at the sulfonium ion to form toxic (R,S)-S-adenosylmethionine (de la Haba et al, 1959), which is removed via a salvage pathway. Recent evidence of homocysteine methyltransferase activity specific for (R,S)-S-adenosylmethionine in A. thaliana (Vinci and Clarke, 2007) suggests that one or more of the three A. thaliana HMT enzymes may catalyze this reaction.…”