Sulfonium Ion Condensation: The Burden Borne by SAM Synthetase

S-Adenosylmethionine (SAM⁺) serves as the principal methylating agent in biological systems, but the thermodynamic basis of its reactivity does not seem to have been clearly established. Here, we show that methionine, methanol, and H⁺ combine to form S-methylmethionine (SMM⁺) with a temperature-independent equilibrium constant of 9.9 M^-2. The corresponding group transfer potential of SMM⁺ (its free energy of hydrolysis at pH 7) is -8.2 kcal/mol. The "energy-rich" nature of sulfonium ions is related to the extreme acidity (p K_a -5.4) of the S-protonated thioether produced by sulfonium hydrolysis, and the large negative free energy of deprotonation of that species in neutral solution (-16.7 kcal/mol). At pH 7, SAM synthetase requires the free energy released by cleavage of two bonds of ATP to reverse that process.