Catalyzes the reversible C-3 epimerization of several ketoses. Shows the highest enzymatic activity for the epimerization of L-ribulose to L-xylulose. Is also able to convert D-allulose (also known as D-psicose) to D-fructose and, to a lesser extent, L-tagatose to L-sorbose, D-ribulose to D-xylulose, L-allulose to L-fructose and D-tagatose to D-sorbose. L-ribulose = L-xylulose D-allulose = keto-D-fructose keto-L-tagatose = keto-L-sorbose D-ribulose = D-xylulose L-allulose = keto-L-fructose keto-D-tagatose = keto-D-sorbose Binds 1 divalent metal cation per subunit. Seems able to use Mg(2+), Mn(2+) or Co(2+). kcat is 5664 min(-1) for the epimerization of D-allulose. kcat is 2509 min(-1) for the epimerization of D-fructose. Optimum pH is 7.5-8.0. Is stable from pH 6.0-11.0. Optimum temperature is 70 degrees Celsius. Is highly stable at 55 degrees Celsius. Homotetramer. This enzyme is an ideal candidate for the industrial production of D-allulose, which has the potential to be used not only as a food ingredient but also as a growth regulator and/or agricultural chemical. Belongs to the hyi family.