Matrix Metalloproteinases (MMPs)
The zinc-powered scissors that shred your collagen. They built your skin and they age it.
Grounded in the research on Matrix metalloproteinase (Wikipedia)
What they actually are
Matrix metalloproteinases (MMPs) are a family of zinc-dependent enzymes that cut up the extracellular matrix: collagen, elastin, fibronectin, the literal scaffolding holding your skin and connective tissue together. Humans have 23 of them. They were discovered in 1962 by Jerome Gross and Charles Lapiere, who watched a tadpole tail dissolve during metamorphosis and asked what was eating the collagen. The answer was an enzyme they named collagenase (now MMP-1). They are 'matrixins,' a subfamily of the metzincin superfamily, and they need a zinc ion in the active site to work. No zinc, no cut.
The mechanism: zinc and a catalytic water
Each MMP has a catalytic zinc ion held by three histidines in a conserved HEXXHXXGXXH motif. The zinc activates a water molecule that hydrolyzes the peptide bond in collagen, snipping the protein chain. They are made as inactive zymogens (proMMPs) kept off by a 'cysteine switch': a cysteine in the prodomain binds the zinc and blocks it until the prodomain is cleaved off. Once active, they're held in check by TIMPs, tissue inhibitors of metalloproteinases (four of them, TIMP-1 to 4) that jam the active site 1-to-1. MMP activity is the balance of MMP vs TIMP. Tip that balance and tissue starts breaking down.