Catalyzes the epimerization of various ketoses at the C(3) position. It is able to interconvert L-ribulose with high efficiency. The enzyme can also accept other ketopentoses such as D-psicose and D-tagatose with lower efficiency. L-ribulose = L-xylulose keto-D-tagatose = keto-D-sorbose D-allulose = keto-D-fructose It can also use Fe(2+). Strongly inhibited by Co(2+) and Ni(2+), and slightly inhibited by EDTA. kcat is 0.34 min(-1) for epimerase activity with D-psicose as substrate. kcat is 3.4 min(-1) for epimerase activity with D-tagatose as substrate. kcat is 8.5 min(-1) for epimerase activity with L-xylulose as substrate. kcat is 13 min(-1) for epimerase activity with L-ribulose as substrate. Optimum pH is 8. The enzyme is stable between 7 and 10. Optimum temperature is around 60 degrees Celsius. The enzyme is stable below 60 degrees Celsius. Homotetramer. Belongs to the hyi family.