Mandelate racemase (MR) catalyzes the Mg 2+ -dependent interconversion of ( R )- and ( S )-mandelate and has been employed as a model enzyme to demonstrate that an enzyme catalyzing the deprotonation of a carbon acid substrate may be inhibited by boronic acids. We report a detailed structure–activity-based study of the ability of various boronic acid derivatives to competitively inhibit MR. 2-Naphthylboronic acid ( K i = 0.32 ± 0.01 μM), furan-3-boronic acid ( K i = 10 ± 1 μM), and thiophene-3-b