An activation mechanism for matrix metalloproteinase family zymogens (named in reference to the zipper-type mechanism), in which the thiol group of a cysteine residue in the N-terminal propart binds to and blocks the active-site Zn atom so that activation occurs when the thiol group is itself blocked, e.g. by N-ethylmaleimide or a heavy metal, or when the propart is proteolytically excised.