(374ae) Evaluation of Boronic Acids for Shifting the Xylose:Xylulose Equilibrium with Immobilized Xylose Isomerase

Saccharification of lignocellulosic feedstocks to sugars can result in bimass hydrolysate that is up 30% xylose. Native S. cerevisiae (baker's yeast) is not able to ferment xylose to ethanol; this yeast can ferment xylulose, the ketose isomer of xylose, to ethanol. D-glucose isomerase (D-xylose isomerase, EC 5.3.1.5) was known to isomerize xylose to xylulose. However, the chemical equilibrium for this reaction is unfavorable for xylulose production, with an equilibrium xylulose:xylose ratio of ~15:85 at pH 7.

Boronic acids (with a general structure of R-B(OH)2, where R is an alkyl or aryl) have been known for some time to form boronate esters with diols rapidly and reversibly in aqueous solution. The affinity between different boronic acids and diols in aqueous solutions is pH dependent, with most binding at high pH values (> pH 8) and diol release at low pH (pH < 3). Borax, or sodium tetraborate, was shown to significantly improve the xylose:xylulose equilibrium in sugar solutions and in biomass hydrolysate. However, tetraborate in high concentrations can adversely impact the growth of some microbes used for the fermentation of biomass sugars.

To determine if other boronic acids may be better candidates for shifting the xylose:xylulose equilibrium with less inhibition of microbial physiology, different boronic acids and other structurally-similar compounds were investigated. The optimum pH for xylose conversion was determined for each and the optimal ratio of sugar to boronic acids was also determined. Finally growth and fermentation of yeast were tested.