Crystal structures of d-psicose 3-epimerase from Clostridium cellulolyticum H10 and its complex with ketohexose sugars

摘要

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 cellulolyticum H10 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. cellulolyticum DPEase was determined. The enzyme assembles into a tetramer and each subunit shows a (β/α)₈ TIM barrel fold with a Mn^{2+ 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. cellulolyticum DPEase 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.}