MMP 9 ( matrix metallopeptidase 9 ( gelatinase B , 92 kDa gelatinase , 92 kDa type IV collagenase ) )

MMP-9 is a Zn+2 dependent endopeptidase, synthesized and secreted in monomeric form as zymogen. The structure is almost similar to MMP2, another member of matrixmetalloproteinase family. The nascent form of the protein shows an N-terminal signal sequence ('pre' domain) that directs the protein to the endoplasmic reticulum. The pre domain is followed by a propeptide-'pro' domain that maintains enzymelatency until cleaved or disrupted, and a catalytic domain that contains the conserved zinc-binding region. A hemopexin/vitronectin-like domain is also seen, that is connected to the catalytic domain by a hinge or linker region. The hemopexin domain is involved in TIMP (Tissue Inhibitors of MetalloProteinases) binding e.g. TIMP-1 & TIMP-3, the binding of certain substrates, membrane activation, and some proteolytic activities. It also shows a series of three head-to-tail cysteine-rich repeats within its catalytic domain. These inserts resemble the collagenbinding type II repeats of fibronectin and are required to bind and cleave collagen and elastin. Like other proteolytic enzymes, MMP-9 is first synthesized as inactive proenzyme or zymogens.