Investigating the role of a backbone to substrate hydrogen bond in OMP decarboxylase using a site-specific amide to ester substitution

Hydrogen bonds between backbone amide groups of enzymes and their substrates are often observed, but their importance in substrate binding and/or catalysis is not easy to investigate experimentally. We describe the generation and kinetic characterization of a backbone amide to ester substitution in the orotidine 5′-monophosphate (OMP) decarboxylase from Methanobacter thermoautotrophicum (MtOMPDC) to determine the importance of a backbone amide-substrate hydrogen bond. The MtOMPDC-catalyzed reaction is characterized by a rate enhancement (∼10¹⁷) that is among the largest for enzyme-catalyzed reactions. The reaction proceeds through a vinyl anion intermediate that may be stabilized by hydrogen bonding interaction between the backbone amide of a conserved active site serine residue (Ser-127) and oxygen (O4) of the pyrimidine moiety and/or electrostatic interactions with the conserved general acidic lysine (Lys-72). In vitro translation in conjunction with amber suppression using an orthogonal amber tRNA charged with L-glycerate (^HOS) was used to generate the ester backbone substitution (S127^HOS). With 5-fluoro OMP (FOMP) as substrate, the amide to ester substitution increased the value of K_m by ∼1.5-fold and decreased the value of k_cat by ∼50-fold. We conclude that (i) the hydrogen bond between the backbone amide of Ser-127 and O4 of the pyrimidine moiety contributes a modest factor (∼10²) to the 10¹⁷ rate enhancement and (ii) the stabilization of the anionic intermediate is accomplished by electrostatic interactions, including its proximity of Lys-72. These conclusions are in good agreement with predictions obtained from hybrid quantum mechanical/molecular mechanical calculations.