Involved in peptidolytic degradation of cyclic heptapeptide hepatotoxin microcystin (MC) (PubMed:11769251,PubMed:8899999). Cleaves both linear MC and the tetrapeptide degradation product of MC (PubMed:11769251). Cleaves the Adda-Glu peptide bond of linear MC heptapeptides (PubMed:22591122). Binds 1 zinc ion per subunit. Inhibited by the metal chelators EDTA and 1,10-phenanthroline. Belongs to the peptidase M81 family. MC degradation was originally described as sequential, three-step process catalyzed by enzymes subsequently named as MlrA, MlrB and MlrC, where MlrC catalyzes hydrolysis of tetrapeptides generated by MlrB (PubMed:8899999). It is shown later that purified MlrC hydrolyzes also linear MC (PubMed:11769251,PubMed:22591122) so it is not clear if MlrB is indispensable in this pathway.