Catalyzes the efficient dehydration of both L-talarate (also called L-altrarate) and galactarate to 5-keto-4-deoxy-D-glucarate (5-KDG) (PubMed:17649980, PubMed:29248502). Also catalyzes the epimerization of L-talarate to galactarate; epimerization occurs in competition with dehydration. Is required for the utilization of L-talarate as a carbon source. Also functions in galactarate utilization. Is not active on other acid sugars (PubMed:17649980). L-altrarate = 5-dehydro-4-deoxy-D-glucarate + H2O galactarate = 5-dehydro-4-deoxy-D-glucarate + H2O L-altrarate = galactarate Binds 1 Mg(2+) ion per subunit. Competitively inhibited by tartronate. kcat is 2.1 sec(-1) with L-talarate as substrate (PubMed:17649980). kcat is 3.6 sec(-1) with galactarate as substrate (PubMed:17649980). kcat is 4.8 sec(-1) with galactarate as substrate (PubMed:29248502). Homooctamer; tetramer of dimers. Cells lacking this gene lose the ability to grow on L-talarate as carbon source, and are impaired in the ability to utilize galactarate as carbon source. The enzyme product is the enantiomer of the product obtained in the galactarate dehydratase reaction catalyzed by OB2843; the enzymes thus differ in their regiochemistry of dehydration. Belongs to the mandelate racemase/muconate lactonizing enzyme family.