Crystal structures of d-psicose 3-epimerase from Clostridium cellulolyticumH10 and its complex with ketohexose sugars

d-Psicose 3-epimerase (DPEase) is demonstrated to be useful in the bioproduction of d-psicose, a rare hexose sugar, from d-fructose, found plenty in nature. Clostridium cellulolyticumH10 has recently been identified as a DPEase that can epimerize d-fructose to yield d-psicose with a much higher conversion rate when compared with the conventionally used DTEase. In this study, the crystal structure of the C. cellulolyticumDPEase was determined. The enzyme assembles into a tetramer and each subunit shows a (β/α)8TIM barrel fold with a Mn2+metal ion in the active site. Additional crystal structures of the enzyme in complex with substrates/products (d-psicose, d-fructose, d-tagatose and d-sorbose) were also determined. From the complex structures of C. cellulolyticumDPEase with d-psicose and d-fructose, the enzyme has much more interactions with d-psicose than d-fructose by forming more hydrogen bonds between the substrate and the active site residues. Accordingly, based on these ketohexose-bound complex structures, a C3-O3 proton-exchange mechanism for the conversion between d-psicose and d-fructose is proposed here. These results provide a clear idea for the deprotonation/protonation roles of E150 and E244 in catalysis.