Matrix Metalloproteinase (MMP)

Matrix metalloproteinases are a family of zinc-dependent endopeptidases, more than twenty in humans, that degrade the components of the extracellular matrix. The collagenases MMP-1, MMP-8 and MMP-13 are unusual in being able to cleave the intact triple helix of fibrillar collagen, cutting at a single site, after which the fragments unwind and become substrate for other enzymes. Gelatinases MMP-2 and MMP-9 clear those fragments and basement membrane collagen. All are secreted as inactive zymogens held shut by a cysteine-zinc interaction, activated by proteolysis, and then restrained by four tissue inhibitors of metalloproteinases, the TIMPs. What determines tissue fate is the ratio, not the absolute level.

The ratio shows up in disease. Osteoarthritic cartilage over-expresses MMP-13 against type II collagen while aggrecanases strip proteoglycan. The therapeutic history is a caution: broad-spectrum MMP inhibitors were taken into large oncology trials in the 1990s and failed, producing a dose-limiting musculoskeletal syndrome of joint pain and stiffness without a survival benefit.

That failure is the point. MMPs are not a damage pathway to be switched off. Angiogenesis, cell migration, growth-factor release and bone remodelling all require controlled proteolysis, so inhibiting the family broadly interferes with repair as much as with degradation.

The common error on spec sheets is reading a reduction in MMP expression as a benefit in itself. Gelatin zymography and messenger RNA counts measure enzyme present, not enzyme active, and neither accounts for TIMP. A compound described as MMP-lowering in cell culture has demonstrated a biochemical effect and nothing about tissue outcome.