Catalyzes the epimerization of L-Lys-L-Arg to L-Lys-D-Arg (in vitro). Catalyzes the epimerization of positively charged dipeptides, with a preference for substrates with a basic amino acid in the second position. Has epimerase activity with L-Lys-L-Lys, L-Arg-L-Arg, L-Val-L-Arg, L-Val-L-Lys and L-Ala-L-Arg (in vitro). Binds 1 Mg(2+) ion per subunit. Part of a large, functionally divergent protein family. Protein modeling and substrate docking was used to predict the substrate specificity, prior to biochemical analysis. Belongs to the mandelate racemase/muconate lactonizing enzyme family.