S-Adenosylethionine

The enzyme shows a high substrate specificity for AdoMet, and affinity to the substrate is also high with a Km ranging from 12 to 60 pM, and the pH optimum is between 8.5 and 9.5. Interestingly, S-adenosylethionine shows some activity as a substrate. The enzyme reaction is competitively inhibited by AVG and AOA, which are inhibitors of pyridoxal phosphate-linked enzymes. 

Not only is S-adenosylethionine inactive as an ethyl donor for transfer RNA (Ortwerth and Novelli, 1969), but this ethyl analog also inhibits the methylation of transfer RNA by S-adenosylmethionine (Moore and Smith, 1969). The impression that S-adenosylethionine is a rather poor substrate for many methyltransferases is supported by the observation... 

Interestingly ethionine is converted to S-adenosylethionine by this enzyme. Since the ethyl group is only slowly transferred to other substances, however, ethionine can deplete cells of ATP. Indeed, the cytotoxic effects of ethionine are thought to result from depletion of cellular ATP (see also Sect. B. Para. 8.1.9). 

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