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complement c3b in complex with factor h domains 1-4
The proteinase-binding alpha-macroglobulins (A2M)  are large glycoproteins found in the plasma of vertebrates, in the hemolymph of some invertebrates and in reptilian and avian egg white. A2M-like proteins are able to inhibit all four classes of proteinases by a 'trapping' mechanism. They have 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, thus trapping the proteinase. The entrapped enzyme remains active against low molecular weight substrates, whilst its activity toward larger substrates is greatly reduced, due to steric hindrance. Following cleavage in the bait region, a thiol ester bond, formed between the side chains of a cysteine and a glutamine, is cleaved and mediates the covalent binding of the A2M-like protein to the proteinase. This family includes the N-terminal region of the alpha-2-macroglobulin family.