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. The substrate specificity decreases in the following order: D-fructose, D-tagatose, D-ribulose, D-xylulose, and D-sorbose. It shows a higher level of activity for cis ketoses than for trans-ketoses. D-allulose = keto-D-fructose Binds 1 Mn(2+) or Co(2+) ion per subunit. Inhibited by Zn(2+) and Cu(2+). kcat is 2381 min(-1) for epimerase activity with D-psicose as substrate (at pH 8 and 50 degrees Celsius). kcat is 270 min(-1) for epimerase activity with D-tagatose as substrate (at pH 8 and 50 degrees Celsius). Optimum pH is 8. Optimum temperature is 50 degrees Celsius. The activity of D-psicose 3-epimerase is very stable below 45 degrees Celsius but decreases above 50 degrees Celsius with increasing reaction time. Homotetramer. Belongs to the hyi family.