Conversion Shift of D-Fructose to D-Psicose for Enzyme-Catalyzed Epimerization by Addition of Borate

The conversion yield of D-psicose from D-fructose by a D-psicose 3-epimerase from Agrobacterium tumefaciens increased with increasing molar ratios of borate to fructose, up to a ratio of 0.6. The formation of the psicose-borate complex was the result of the higher binding affinity of borate for psicose than for fructose. The formed psicose-borate complex did not participate in the conversion reaction, acting instead as if the product had been removed. Thus, more fructose was converted to psicose in order to restore the equilibrium. The maximum conversion yield of psicose with borate was about twofold that obtained without borate and occurred at a 0.6 molar ratio of borate to fructose. Above this ratio, the conversion yield decreased with increasing ratios, because the amount of fructose available decreased through the formation of the initial fructose-borate complex. The structures of the two sugar-borate complexes, determined by nuclear magnetic resonance spectroscopy, were α-D-psicofuranose cis-C-3,4 diol borate and β-D-fructopyranose cis-C-4,5 diol borate.