Involved in the biosynthesis of D-psicose. Catalyzes the reversible epimerization of D-fructose at the C3 position to yield D-psicose. The enzyme is highly specific for D-psicose and shows very low activity with D-tagatose. D-allulose = keto-D-fructose Binds 1 Mn(2+) or Co(2+) ion per subunit. Completely inhibited by EDTA and partially inhibited by Zn(2+), Mg(2+) and Cu(2+). kcat is 49 sec(-1) for epimerase activity with D-psicose as substrate (at pH 7 and 55 degrees Celsius). kcat is 4.7 sec(-1) for epimerase activity with D-tagatose as substrate (at pH 7 and 55 degrees Celsius). Optimum pH is 7. Optimum temperature is 55 degrees Celsius. Homotetramer. Belongs to the hyi family.