On the Mechanism of the Pentose Phosphate Epimerases

The mechanism of the l-ribulose 5-phosphate 4-epimerase has been examined. In agreement with previous observations (Mcdonough, M. W., and Wood, M. A. (1961) J. Biol. Chem. 236, 1220) no exchange of hydrogen with H2O is observed. Suitable double label experiments indicate that hydrogen transfer in the reaction is intramolecular. The data for kinetics of the exchange of the hydrogen at C-3 of substrate catalyzed by a d-ribulose 5-phosphate 3-epimerase have been examined. The data indicate that the enzyme must contain two hydrogen donors (acceptors) which interact with the hydrogen at carbon 3 of d-ribulose 5-phosphate and d-xylulose 5-phosphate, respectively.