Schematic structure of MMP-2 and MMP-9. The catalytic site contains three essential zinc ion binding sites. At the zymogen stage, a cysteine residue within the prodomain interacts with zinc to prevent substrate binding. The haemopexin domain mediates interaction with enzyme substrates. Specific to the gelatinases is the fibronectin-like domain, which further facilitates substrate binding. MMP, matrix metalloproteinase.