Catalyzes the conversion of L-arabinose to L-ribulose. In vitro, converts D-galactose into D-tagatose. beta-L-arabinopyranose = L-ribulose Binds 1 Mn(2+) ion per subunit. Inhibited by copper. Optimum pH is 7.5-8.0. Optimum temperature is 80 degrees Celsius. Carbohydrate degradation; L-arabinose degradation via L-ribulose; D-xylulose 5-phosphate from L-arabinose (bacterial route): step 1/3. Homotetramer. Belongs to the arabinose isomerase family.