The role of Bronsted base basicity in estimating carbon acidity at enzyme active sites: a caveat

Many enzymes catalyze the abstraction of a proton from a carbon acid substrate to initiate a variety of reactions; however, the development of a complete quantitative description of enzyme-catalyzed heterolytic cleavage of a C-H bond remains a challenge to enzymologists. To determine the pKC-Ha value for such substrates bound at the active site, recent studies have estimated the equilibrium for formation of the deprotonated intermediate at the active site, however, accurate knowledge of the pKBH+a of the conjugate acid of the Bronsted base catalyst (BH+) is also required. Herein, it is shown that using the value of pKBH+a of the enzyme-substrate complex can underestimate the value of pKC-Ha by an amount between zero and p delta, where p delta is the change in basicity of BH+ upon going from the enzyme-substrate complex to the enzyme-intermediate complex.