Isomerization of aldo-disaccharides to keto-disaccharides in arginine solution and phosphate buffer under subcritical conditions

Maltose, cellobiose, and lactose dissolved in 0.01 mol/L arginine solution or 0.01 mol/L phosphate buffer (pH 7.0) at 0.2 mol/L were reacted batchwise at 110 °C. The yields of the corresponding keto-disaccharides were 23.4% and 17.6% for maltose, 19.3% and 13.0% for cellobiose, and 20.0% and 13.7% for lactose in arginine solution at 30 min and phosphate buffer at 150 min, respectively. All the disaccharides were hydrolyzed in the range of 0.5 to 3.5%, the degree of hydrolysis being greater in arginine solution than in phosphate buffer. The monosaccharides produced by hydrolysis were slightly isomerized to other monosaccharides. A decrease in pH and an increase in absorbance at 280 and 420 nm continued even at the almost constant yields of the main products. The browning was particularly pronounced in the latter half of the reaction, suggesting a change of the by-products to substances with high absorption coefficient.

Rational Design of Loop Dynamics for a Barrel-Shaped Enzyme by Introducing Disulfide Bonds

Loop dynamics redesign is an important strategy to manipulate protein function. Cellobiose 2-epimerase (CE) and other members of its superfamily are widely used for diverse industrial applications. The structural feature of the loops connecting barrel helices contributes greatly to the differences in their functional characteristics. Inspired by the in-silico mutation with molecular dynamics (MD) simulation analysis, we propose a strategy for identifying disulfide bond mutation candidates based on the prediction of protein flexibility and residue-residue interaction. The most beneficial mutant with the newly introduced disulfide bond would simultaneously improve both its thermostability and its reaction propensity to the targeting isomerization product. The ratio of the isomerization/epimerization catalytic rate was improved from 4:103 to 9:22. MD simulation and binding free energy calculations were applied to provide insights into molecular recognition upon mutations. The comparative analysis of enzyme/substrate binding modes indicates that the altered catalytic reaction pathway is due to less efficient binding of the native product. The key residue responsible for the observed phenotype was identified by energy decomposition and was further confirmed by the mutation experiment. The rational design of the key loop region might be a promising strategy to alter the catalytic behavior of all (α/α)6-barrel-like proteins.