Insights into Biological Catalysis from Model StudiesEnzymes can achieve enormous rate enhancements, which often require >20 kcal/mol of transition state stabilization. Recent proposals to account for these large energies have invoked low-barrier hydrogen bonds [LBHBs]. It was proposed that an LBHB is formed when the pKa of a group in the enolic transition state becomes matched with that of a group on the enzyme, stabilizing the transition state by ~20 kcal/mol; in contrast, the pKa of the groups in the ground state are not matched, so that the ground state would be stabilized by only 1 to 5 kcal/mol from a normal H bond. LBHBs have been proposed to provide a large fraction of the catalytic power for many enzymes, including triose phosphate isomerase, ribonucleases, enolase, aconitase, and citrate synthase. The observation of a highly deshielded proton (dH = 18 ppm) between His57 and Asp102 in the catalytic triad of trypsin and chymotrypsin and the isotope effect on this chemical shift (dH - dD = 1.0 ppm) have been suggested as evidence for an LBHB that contributes to catalysis by serine proteases.
To test this hypothesis, the energetics of hydrogen bonds were investigated as a function of DpKa for two homologous series of compounds under nonaqueous conditions that are conducive to the formation of low-barrier hydrogen bonds. An electrostatic model of H bonding predicts that H bond strength increases linearly with increasing acidity of the donor or increasing basicity of the acceptor, reaching its maximum at DpKa = 0. Any additional energetic contribution from covalent character in an LBHB would lead to the formation of an especially strong H bond at matched pKa, resulting in a positive deviation at DpKa = 0.
A linear correlation between the increase in hydrogen bond energy and the decrease in DpKa is observed, as predicted from simple electrostatic effects on hydrogen bonding. No additional energetic contribution to the hydrogen bond is observed at DpKa = 0, however. As these results provide no indication that LBHBs provide a special energetic contribution to enzymatic catalysis, the question is raised: How do H bonds contribute to enzymatic catalysis? Suggest several alternative ways by which H bonds could contribute to enzymatic catalysis are suggested from these and other model studies, in accordance with the electrostatic model:
- A greater sensitivity of H bond strength to charge rearrangements in a low dielectric active site, relative to aqueous solution, could allow preferential transition state stabilization by enzymes.
- Enzymes may commonly use multiple interactions of moderate strength for transition state stabilization, instead of relying on a single, very strong interaction such as an LBHB.
- H bonds may also be strengthened by geometrical changes in going from the ground state to the transition state, thereby providing specific transition state stabilization.