Is able to inhibit all four classes of proteinases by a unique 'trapping' mechanism. This protein has a peptide stretch, called the 'bait region' which contains specific cleavage sites for different proteinases. When a proteinase cleaves the bait region, a conformational change is induced in the protein which traps the proteinase. The entrapped enzyme remains active against low molecular weight substrates (activity against high molecular weight substrates is greatly reduced). Homotetramer, which consists of two pairs of disulfide-linked chains. Lacks the thioester bond found in other members of this family. Glycosylated; contains 56 glucosamine units per subunit. Belongs to the protease inhibitor I39 (alpha-2-macroglobulin) family.