Catalyzes the epimerization of various ketoses at the C(3) position. It is able to interconvert D-tagatose and D-fructose to D-sorbose and D-psicose, respectively. The enzyme is also able to accept other ketopentoses such as D-ribulose and D-xylulose with lower efficiency. keto-D-tagatose = keto-D-sorbose D-allulose = keto-D-fructose D-ribulose = D-xylulose Optimum pH is 9. The enzyme is stable from 8 to 10. Optimum temperature is around 40 degrees Celsius. The enzyme is stable below 40 degrees Celsius, but decreases with increasing temperature. Homodimer. Belongs to the hyi family.