Methionine Salvage Enzyme Uses a Unique Mechanism to Overcome a Challenging Aldose-Ketose Isomerization

The enzyme-catalyzed interconversion of aldoses and ketoses historically involves one of two mechanisms, both of which require an aldehyde form of the substrate. Methylthio-d-ribose-1-phosphate (MTR1P) isomerase (MtnA), which functions in the methionine salvage pathway, poses a challenge to this canon because its substrate cannot readily access such a form. MtnA must catalyze the opening of the ribofuranose ring and hydrogen transfer between C-2 and C-1. Primary ²H and ¹³C kinetic isotope effects measured at these positions indicate that hydrogen transfer limits k_cat/K_M. Inverse kinetic solvent viscosity and solvent kinetic isotope effects measured for this constant implicate a protein conformational change after substrate binding and the participation of Cys160 as the catalytic base responsible for shuttling the proton between C-2 and C-1. Supported by QM/MM calculations, an E1 elimination-tautomerization sequence is most consistent with these findings and represents a third mechanism for enzymatic aldose-ketose isomerization.