Matrix metalloproteinase 9 （MMP-9）, also known as 92 kDa type IV collagenase, 92 kDa gelatinase, gelatinase B （GELB）, or CLG4B, is secreted by neutrophils, macrophages, and various transformed cells. It is the most complex member of the MMP family in terms of domain structure and activity regulation.
Structurally, MMP-9 can be divided into five distinct domains: a prodomain that is cleaved upon activation, a gelatin-binding domain consisting of three contiguous fibronectin type II units, a catalytic domain with the zinc-binding site, a proline-rich linker region, and a carboxyl-terminal hemopexin-like domain. This enzyme degrades various substrates, including gelatin, collagen types IV and V, and elastin.
MMP-9 is implicated in a variety of autoimmune diseases, such as systemic lupus erythematosus, rheumatoid arthritis, and multiple sclerosis, and is considered a potential therapeutic target