α2-macroglobulin

a plasma protein that inhibits a wide variety of proteolytic enzymes including trypsin, plasmin, thrombin, kallikrein, and chymotrypsin. It is a tetramer of four identical subunits, M_r between 650,000 and 735,000, and acts by means of a bait region that can entrap proteinases of very different substrate specificities as well as catalytic mechanisms, reducing the accessibility of the proteinase functional sites, particularly to large molecules, but not completely inactivating them. Also written alpha₂-macroglobulin.