Chiral Instability at Sulfur of S-Adenosylmethionine

S-Adenosylmethionine, generated enzymically in chirally pure form (S configuration at sulfur), undergoes simultaneous irreversible conversion to 5′-deoxy-5′-(methylthio)adenosine and homoserine with a rate constant of 6 × 10⁻⁶ s⁻¹ at pH 7.5 and 37 °C and reversible conversion to an enzymically inactive stereoisomer (R configuration at sulfur) with a forward rate constant of 8 × 10⁻⁶ s⁻¹ at pH 7.5 and 37 °C. These forms of instability require small turnover times and/or stabilization through macromolecular binding for S-adenosylmethionine, if organisms that utilize it are to avoid losses of metabolic energy.