Catalyzes the epimerization of L-Lys-L-Arg to L-Lys-D-Arg. Can also catalyze the epimerization of other cationic dipeptides, such as L-Arg-L-Arg, L-Lys-L-Lys and L-Lys-L-His, but with lower efficiency (in vitro). Binds 1 Mg(2+) ion per subunit. kcat is 8.4 sec(-1) for epimerization of L-Lys-L-Arg. kcat is 0.029 sec(-1) for epimerization of L-Lys-L-Lys. kcat is 0.72 sec(-1) for epimerization of L-Arg-L-Arg. Part of a large, functionally divergent protein family. Was initially predicted to have chloromuconate cycloisomerase activity, based on sequence similarity. Protein modeling and substrate docking was used to predict the substrate specificity, prior to biochemical analysis (PubMed:22392983). Belongs to the mandelate racemase/muconate lactonizing enzyme family.