Comparative Analysis of Two Different Amide-to-Ester Bond Mutations in the β-Sheet of 4-Oxalocrotonate Tautomerase

Publisher Website
Google Scholar
Abstract
Here we describe the total chemical synthesis and biophysical characterization of two backbone-modified, ester bond-containing analogues of the homohexameric enzyme 4-oxalocrotonate tautomerase (4OT). The amide-to-ester bond mutations in the two analogues in this study, (OI2)4OT and (OI7)4OT, were designed to effectively delete specific backbone−backbone hydrogen bonds in the β-sheet region of the native 4OT hexamer. The (OI2)4OT and (OI7)4OT analogues each contained one ester bond per monomer that effectively deleted 12 backbone−backbone hydrogen bonds per hexamer. The structural properties of each analogue were characterized by size-exclusion chromatography (SEC), far-UV CD spectroscopy, and catalytic activity measurements, and they were found to be very similar to the structural properties of the wild-type enzyme. The results of equilibrium unfolding studies revealed that the (OI2)4OT and (OI7)4OT analogues were stabilized by 47.7 ± 2.5 and 45.0 ± 2.5 kcal/mol, respectively, under standard state conditions (1 M hexamer) as compared to a value of 69.6 ± 3.3 kcal/mol for the wild-type control. Our results suggest that the two different, but structurally similar, backbone−backbone hydrogen bonds deleted in (OI2)4OT and (OI7)4OT make nearly equivalent contributions to the thermodynamic stability of the 4OT hexamer.