Catalyzes the epimerization of various ketoses at the C(3) position. It is able to interconvert D-tagatose and D-ribulose to D-sorbose and D-xylulose, respectively. The enzyme is also able to accept other ketopentoses such as D-psicose with lower efficiency. keto-D-tagatose = keto-D-sorbose D-allulose = keto-D-fructose D-ribulose = D-xylulose Strongly inhibited (about 90% of the enzyme activity) by Ag(+), Hg(2+) and p-chloromercuribenzoic acid. Cu(2+) and Zn(2+) inhibit about 60% of the enzyme activity. Optimum pH is between 7 and 9 at 30 degrees Celsius. The enzyme is stable from 7 to 11 at 30 degrees Celsius. Optimum temperature is around 60 degrees Celsius. The enzyme is stable up to 60 degrees Celsius for 10 minutes. Homodimer. Belongs to the hyi family.