Basic domain structures of the matrix metalloproteinases (MMPs). MMPs consist of: a propeptide (grey), which maintains the enzymes in a latent state; a catalytic domain (blue) with the active site and the catalytic zinc (Zn) (red); and, with the exception of the matrilysins, a COOH-terminal domain (C) (yellow) with homology to the serum protein hemopexin. The latter two domains are connected by a linker peptide. Gelatinases have an insert of three fibronectin type II repeats (turquoise) in the catalytic domain, which is involved in substrate recognition. Membrane-type MMPs contain a transmembrane domain (black) and a cytoplasmic tail (green) at the COOH terminus, which anchors these enzymes in the cell membrane.