Catalyzes the reversible epimerization between 5-keto-L-gluconate (5-dehydro-L-gluconate) and D-tagaturonate, and thus probably functions in a myo-inositol degradation pathway together with IolG, IolM and IolN (PubMed:23441918). Is not active on the enantiomer 5-keto-D-gluconate (PubMed:23441918). Was also shown to be a nonphosphorylated sugar isomerase with broad substrate specificity in vitro (PubMed:28258150). Is able to catalyze the reversible C3-epimerization of L-ribulose to L-xylulose, D-ribulose to D-xylulose, D-psicose to D-fructose, and D-tagatose to D-sorbose, with a substrate preference for ketopentoses rather than ketohexoses (PubMed:28258150). Also catalyzes the aldose-ketose isomerization reaction from either D-erythrose or D-threose to D-erythrulose (PubMed:28258150). Exhibits no activity for C4-epimerization of D-tagatose to D-fructose (PubMed:28258150). 5-dehydro-L-gluconate = keto-D-tagaturonate L-ribulose = L-xylulose D-ribulose = D-xylulose keto-D-tagatose = keto-D-sorbose D-allulose = keto-D-fructose D-erythrose = D-erythrulose D-threose = D-erythrulose Binds 1 divalent metal cation per subunit. Mn(2+) is the most efficient metal cofactor, but can also use other divalent metal cations such as Ni(2+), Mg(2+), and, to a lesser extent, Co(2+) and Zn(2+), but not Ca(2+), Cu(2+), and Fe(2+). Is completely inhibited by EDTA in vitro. Optimum pH is 6.5-7.0 with L-ribulose as substrate. Optimum temperature is about 80 degrees Celsius with L-ribulose as substrate. Is hyperthermostable, with an apparent melting temperature (Tm) of 102.4 degrees Celsius. Polyol metabolism; myo-inositol metabolism. Homodimer. High thermostability and very broad substrate specificity of this enzyme make it a potential candidate for the production of rare sugars for the food and pharmaceutical industries. Belongs to the hyi family.